Introduction

The Kaliningrad region, the westernmost region of Russia, is located along the coast of the Baltic Sea. The territory borders two countries—Lithuania and Poland. Despite its small size, 15,100 km2 (the region is the smallest administrative unit of the Russian Federation), this is a territory with an appealing geological history and unique natural heritage. The colourful landscapes had been frequented by visitors for centuries. Yet, geologically and geomorphically interesting sites, some world-unique, remain uncovered by national and international tourists, despite their attractiveness and scientific relevance. Some of these places are routinely on the present visitors’ itineraries and the Kaliningrad Region’s travel programmes; however, the overall public consciousness of their natural values is rather poor.

Initial studies and pilot scientific explorations performed during the 19th century disclosed a complex geological and geomorphological history of the Kaliningrad Region (formerly East Prussia), spanning over the past millions of years (see below). Mainly, the rich palaeontological records from the Sambia Peninsula became Europe re-known. Exposure of affluent fossiliferous (Eocene to Pleistocene) formations along the cost inspired interest in the pre-WW2 time, as well as in the modern scientific community. Particularly opulent and world-famous amber beds had become the focus of close investigations, hand-in-hand with their active industrial exploitation (Grigelis 2001; Kharin and Eroshenko 2017; Sivkov and Zhamoida 2017; Krek et al. 2018).

The present-day exposure and proliferation of ancient fossils, having direct linkage to the regional geoheritage concept, has been fostered by active sea erosion of unconsolidated geological formations constructing prominent coastal cliffs (Khorikov et al. 2018; Ryabchuk et al. 2021). A spectrum of geological, geographical, palaeontological, biological, geo-environmental and other sciences help to uncover records on the past landscapes and life forms once occupying the present NE Baltic coast area (e.g. Kolesnik 2017; Mikhnevich 2017; Romanova et al. 2017; Rozhnov 2019; Mychko et al. 2021; Napreenko et al. 2021). Apart from stratified formations. and enclosed palaeoecological and palaeo-climatic archives, the modern topography sceneries and geomorphic sites provide witness to complex surficial geology evolution, resulting in the present mosaic relief counting regionally unique geo-forms. Identification and documentation of these places bring along the need for their protection as well as publicity within the currently inaugurating geo-tourism.

The concept of geoheritage is relatively new in Russia (Mikhailenko et al. 2021). Until now, there were no detailed physical geography studies in the Kaliningrad Region explicitly focusing on the regional geo-sites that would cope with the conducted geoheritage and geo-tourism studies in the neighbouring part of northern and eastern Poland (Jamorska et al. 2020; Żyto 2020; Migoń 2021; Skibiński et al. 2021), the Russian mainland (e.g. Marshall and Siveter 2018; Ruban 2021) or in other Baltic states (Satkūnas and Nicius 2007; Willman et al. 2009; Raukas and Stankowski 2010; Skridlaite et al. 2018; Porshnov et al. 2019; Strzelecki et al. 2022). Geological knowledge transfer and geo-conservation, balancing the present economic development, constitute applied fundamental aspects of the geoheritage research projects (e.g. Tuuling et al. 2011).

This study provides an overview of the present geoheritage of the Kaliningrad Region in a broader geographic frame of the SE Baltic Sea area and the European geoheritage context. The most prominent and scientifically relevant physiographic features are discussed in the context of pristine relief preservation as well as the promotion of regionally sustainable tourism. The selected geo-sites underscore the complex natural history of the study area, starting with the Precambrian orogeny to the Late Pleistocene glaciations. The existing natural and cultural (including industrial) landscape elements add to an overall image of the modern geography of this picturesque, geomorphic region.

Geological Research History

The history of geological and palaeontological investigations of East Prussia (since 1945, the Kaliningrad Region) encompasses more than two centuries. The field studies can be divided into three periods: East Prussian (until 1945), Soviet (1945–1991) and Russian (since 1991). The first detailed mapping of the former German lands was performed by Gustav Zaddach, professor of zoology at the Königsberg University. He mapped Cenozoic deposits of the Samland (Sambia) Peninsula and assembled an extraordinary palaeontological collection from the local, rich fossiliferous formations (Zaddach 1860, 1868). At about the same time, the works of Gottlieb Berendt on the geology of the Curonian Lagoon and its shores were published (Berendt 1869). The regional geological history and geomorphology along the coast of the peninsula were further completed by the geologists Ernst Schellvin (Schellwien 1905) and Alexander Tornquist, the latter in his supreme work ‘Geology of East Prussia’ (Tornquist 1910). Numerous studies by German geologists are devoted to Quaternary geology, palaeogeography and neo-tectonics. The most significant is the monograph of Hans Hess von Wichdorf’s ‘Geology of the Curonian Spit’ (Wichdorff 1919).

Close attention was dedicated to the Baltic amber deposits, which are the richest in the world. Inclusions of plant and insect fossils and their taxonomy in the 37 million-year-old pine resin were described in special paleontological works by the German botanist Heinrich Göppert and the palaeontologist Georg Berendt (Göppert and Berendt 1845). Fossiliferous lignite formations with a rich taxonomic spectrum of ancient plants were presented by the Swiss palaeontologist Oswald Heer—being one of the founders of modern palaeobotany (von Heer 1869). East Prussia’s fossiliferous formations were extensively studied by Fritz Noetling, focusing on Palaeogene bedrock fossils (Noetling 1885, 1888) and Lower Palaeozoic and Mesozoic fossils from the land’s Pleistocene glacial material. Finally, among other outstanding scientists, the palaeontologist Josef Pompecki studied trilobites from local glacial geological bodies (Pompeckj 1890).

Investigations in structural and historical geology intensified after the WW2 war when the historical region of East Prussia became a part of the Soviet Russian Republic. This period is characterised by large-scale geological surveys and raw material prospecting. The purely empirical and fundamental studies of a broad research spectrum (e.g. Budancev and Sveshnikova 1959; Baltakis 1966) were supplemented by applied research with linkage to the re-establishing as well as the newly developing industry of the Kaliningrad Region. Since the 1960s, the study of the Earth-crust structure began, the impetus for which was its regional oil and gas potential (Otmas et al. 2018). Drilled wells made it possible to explore deposits previously inaccessible to the pioneering scientists. The Cenozoic stratigraphy of the region became the focus of investigations of Soviet geologists (Baltakis 1970; Dodonov 1971; Grigelis et al. 1971; Katinas 1971), followed by more recent works (e.g. Grigelis 1996; Mychko 2022). Along with the fundamental fieldwork, intensive exploitation of amber deposits took place over the decades, leaving behind an industrial landscape at the main mining sites.

The start of the Russian period of the geology and palaeontology studies of the Kaliningrad Region meant a significant reduction of funding and fieldwork activities. Only small-scale local research, mostly related to amber-bearing deposits, was carried out (Grigelis 2001). At the same time, some summarising works appeared on the geological history of the Baltic States (Paškevičius 1999) and the geology stratigraphy of the Kaliningrad Region (Zagorodnykh et al. 2001). Other publications deal with the Palaeogene (Grigelis 1996; Standke 1998; Aleksandrova and Zaporozets 2008a, 2008b) and the Jurassic-Palaeogene evolution; the regional palaeography based on foraminifera (Lukashina 2006, 2010); tectonics (Otmas et al. 2006); glacial geology (Biske et al. 2012); and mineral resources of the area (Zagorodnykh and Kunaeva 2005; Kharin 2009).

Renewed research efforts over the past twenty years integrate modern systematic investigations related to the geological and relief development of the region. Thematically, these works include Silurian fossil remains (Suyarkova 2017), Cretaceous and Palaeogene palynology (Aleksandrova and Zaporozhets, 2008a-b; Iakovleva et al. 2021; Kuzmina et al. 2023), amber inclusions and fossils from amber-bearing layers (Alekseev 2013, 2017; Sadowski et al. 2016, 2017; Mychko 2018; Brunke et al. 2019; Manukyan 2016, 2019; Mychko and Tarasenko 2020; Mychko et al. 2021), Pleistocene mammoth fauna (Martynovich and Grishanov 2015, 2016), the Sambia Peninsula Holocene vegetation history (Arslanov et al. 2011), as well as seismicity and geo-dynamics of the SE Baltic coast (Assinovskaya and Ovsov 2008; Assinovskaya 2011; Babakov 2018), coastal geomorphology (Dorokhov et al. 2022) and aquatic and terrestrial palaeoecology of the Kaliningrad Region (Gritsenko et al. 2018) among other scientific themes. Finally, regional historical geology and palaeontology have been most recently presented in an overview summary monograph (Mychko 2022).

Regional Geography: Geology and Geo-Environments

The study area encompasses the south-eastern continental part of the Baltic Sea area, bordering NE Poland and southern Lithuania from the South and Northeast, respectively (Fig. 1). The broader territory is part of a large and orogenically stable Earth’s crust—the East European/Russian Platform. This major European geological structure is surficially represented by a vast plain, a western extension of which creates the Sambia Peninsula. Precambrian crystalline rocks form the platform’s massive basement, which is deformed by complex structural elements—anticlises and synclises. The latter are regionally represented by the major Baltic Synclise, running across the SE Baltic coast (Mychko 2022). This structural unit is bounded in the NW by the crystalline basement of the Baltic Shield, in the East by outcrops of the Latvian Saddle, in the Southeast by the Mazurian-Belarusian Anticlise; in the Southwest, it is separated from the Pomeranian Depression by the platform’s marginal Trans-European Suture Zone—Tornquist Zone (Pira et al. 2003).

Fig. 1
figure 1

Study area. (A) The Kaliningrad Region (the Russian Federation) on the map of Europe. (B) Geographic location of the discussed geo-sites: Natural geo-sites: 1 –‘Krant Ground’ section, the Filino Bay; 2 – ‘Brown Coal Formation’ near Otradnoye Village; 3 – clays of the Baltic Glacial Lake near Kulikovo Village; 4 – fossil gyttja and peat section near Lesnoy Village; 5 – Ice Age erratics; 6 – Grand Curonian Sand Dune Field; 7 – Vistula Spit; 8 – Vishtynetskoye Lake; 9 – Vishtynetskaya Upland glacial relief. Cultural geo-sites: 1 – Palaeolithic sites on the Sheshupe River; 2 – Mesolithic sites on the Vishtynets Upland; 3 – Bronze Age burial mounds (Otradnoye–Svetlogorsk); 4 – ruins of the Königsberg Castle and Cathedral (13th–14th century); 5 – the Middle Ages Teutonic castles and temples; a – Bal’ga Castle, b – Brandenburg Castle, c – Preußisch Eylau Castle, d – Friedland Temple, e – Gerdauen Castle, f – Tapiau Castle, g – Insterburg and Georgenburg Castles, h – Neuhausen Castle, i – Schaaken Castle, j – Labiau Castle, k – Tilsit Castle, l – Ragnit Castle. Anthropogenic (industrial) geo-sites: 1 – Sinyavino amber pit; 2 – Primorsky amber pit; 3 –Palmnikensky amber pit; 4 – fossiliferous sand and gravel pits; 5 – Talpaki-Chernyakhovsk gravel pit with Pleistocene flora and fauna occurrences

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The geological arrangement of the Polish-Lithuanian Depression and the Baltic Synclise includes suites of tectonically deformed rocks and complex structural elements (bends, floors, etc.). The earliest stage of the regional Platform’s development created the Archaean-Proterozoic crystalline basement composed of diverse igneous and metamorphic rocks. This base is overlain by a massive sedimentary cover with a total thickness of 1.4 km in the eastern part and 3 km in its western part (Zagorodnykh et al. 2001). The platform cover includes deposits of all geological ages from Palaeozoic to Cenozoic, with the thickest in its SW part due to the basement lowering by 8 km (Ūsaityt 2000). The Caledonian Orogeny of the Early–Middle Palaeozoic (490–390 Ma) generated Cambrian, Ordovician, Silurian and Early Devonian deposits. These litho-complexes reflect the complete transgressive-regressive tectonic-sedimentary cycle of the Baltic Synclise formation, with a unidirectional SW stratigraphic subsidence (Kirikov 2011).

The Late Palaeozoic Hercynian Orogeny is associated with two separate structural stages, the Lower Hercynian and the Upper Hercynian, represented by a complex of regressive Devonian and Permian formations, respectively. The subsequent Cimmerian folding stage (~210–100 Ma) generated thick transgressive terrigenous and carbonate sedimentary bodies. Oil and gas reservoirs are locally trapped off-shore in these formations (Otmas et al. 2018). The regional deposits of the Polish-Lithuanian Depression are completed by the formations of the Alpine structural stage, acting from Early Cretaceous (~102 Ma) to Pliocene (~5–3 Ma) over most of the territory, including the Baltic shelf (Kirikov 2011). These formations constitute up to 60 m high, steep coastal cliffs in the western part of the Sambia Peninsula, currently intensively undercut by sea wave actions (Babakov 2018). Enclosed Palaeocene and Eocene siliciclastic deposits document numerous sedimentary discontinuities and erosional surfaces in the lithified fossiliferous horizons (Kasiński et al. 2020).

These ancient structural units are topped by geologically recent sedimentary deposits. The Pre-Quaternary relief in the SE Baltic region and the adjoining Baltic Sea bottom is characterised by buried palaeo-valleys and sedimentary formations of various geneses, lithologies and thicknesses (Ūsaityt 2000). The terrestrial part of the present peninsula was physiographially characterised by denudation uplands (Miocene and Pliocene palaeo-relief relics) and erosional landforms (palaeo-incisions), reaching a depth up to 240 m, length 1–10 km and width of 0.5–2 km (Mikhnevich 2015).

The Quaternary landscape was shaped by Pleistocene glaciations and interglacial marine transgressions. The latest (MIS 2) Fennoscandian ice-lobe advance reached the SE Baltic area about 20,000 yr ago (Lambeck et al. 2010; Makkaveyev et al. 2020; Korsakova et al. 2022), followed by ice stagnation and final melt within the eastern part of the Kaliningrad area at ~11,700 yr BP. Glacial flows override and modelled the former non-glaciated plains, transporting cubic kilometres of rocks and other unconsolidated deposits over several hundreds of kilometres. The pre-Quaternary relief with a topographic gradient higher than the present one was significantly smoothed. Mass proglacial, glacio-fluvial and glacial (till) accumulations blanket the present landscape. This is characterised by ice-lodgement and ice-retreat landforms, defining the modern topography. Although glaciers have repeatedly advanced over this territory, the last Valdai Glaciation (MIS 2) played a key role in the formation of the present relief.

The post-glacial physiographic development displays a continuing glacio-isostatic rebound in the northern part of the Kaliningrad Region, while the southern part is experiencing a slow submerging. Terraces of different stages of the Holocene marine formations of the Baltic Sea have an evident regional trend uplifting towards the North. The terraces display local deformations (Bitinas and Damušytė 2007). Apart from the Earth-crust rebound, the region is exposed to deep seismicity risks (Ulomov et al. 2008). A specific regional landscape element is represented by the Curonian Split—a sandy depositional landform that encloses the Curonian Lagoon, the most extensive lagoon of the Baltic Sea. Coastal and seabed erosion along the split provides evidence of very dynamic erosional processes (Zhamoida et al. 2009), a shifting sand dune lowering due to intense wind activity under a deficit of new aeolian sedimentary load (Povilanskas 2009). Finally, historical and present industrial activities have contributed significantly to re-shaping the pristine regional geo-environments.

Research Methods

The methodology of the study is based on the recent fieldwork within the coastal and inland study area of the Kaliningrad Region. These efforts included detailed hard and soft data assembling and a formal classification. The former work entails mineralogy, petrology, palaeontology and archaeology record collecting from the principal geo-sites and their stratigraphic and topographic documentation; the latter comprehends a geographic/GIS geo-sites’ mapping completed by cartographic and photographic documentation of the particular geoheritage loci.

The field data were completed by the geological materials and fossils from the existing collections of the Shirshov Institute of Oceanology, the Amber Museum and the Museum of the World Ocean, Kaliningrad. The outlined regional Earth sciences’ history followed a review of the particular literature on the subject. The study area reveals a wide geomorphic variety of landforms and geo-sites and the associated natural habitats. The classification of geo-sites follows the concept of J. Chlachula (2020).

Geo-sites Classification

Natural Geo-sites

‘Krant Ground’ Section

Along the northern coast of the Sambia Peninsula between the villages of Filino and Primory’e, there is the picturesque Filinsky Bay (Fig. 1B). The surroundings of the bay became famous for their appealing landscapes. A particularly beautiful example is the high Zipfelberg dune, located on compact red sandstones, with a nearby steep cliff, Zaddahkuppe. On both sides of the bay shaped by gentle terraced slopes, there are high, steep cliffs of the Palaeogene and Neogene bedrock formed by iron-mineral-consolidated, cross-stratified sands, silts and clays, topped by thick, mixed glacial deposits of the Last Glacial moraine. A series of terraced palaeo-cliffs recorded below the sea level at the 26–52 m depth document a progressing submergence of the peninsula’s shoreline during the Baltic Ice Lake transgressions (Dorokhov et al. 2022).

The Palaeogene deposits of this exclusive section are represented by prominent red ferruginous sands and cemented sandstones locally called the ‘Krant Ground’ (Fig. 2a). A total thickness of this facies is ~8–9 m, at places interbedded by green loose sands and sands of the ‘Brown Coal Formation’ (see below). The red sand unit is exposed in the lowest part of the cliffs, retaining a sub-horizontal stratification, dipping ~10° east towards the town of Svetlogorsk. The ferruginous sands and sandstones of the ‘Krant Ground’ were formed under shallow coastal conditions of an ancient sea-shelf along a northern margin of the Danish-Polish Strait during the second half of the Priabonian Age of Late Eocene (~34 million years ago) (Iakovleva et al. 2021). The incorporated rocks contain a very rich complex of fossils represented by serpulids, scaphopods, gastropods, bivalves, bryozoans, sea urchins and others (Fig. 2b, d; Fig. 7f–h). These Eocene marine beds are topped by Oligocene lake beds and fluvial sands (Fig. 2c), including freshwater fossils, which indicate a coastal regression and a shift to terrestrial settings. This outcrop—a unique geo-site of the Cenozoic geological history—is the only accessible locality of the Palaeogene fossils in the Kaliningrad Region and also along the entire coast of the present Baltic Sea (Mychko et al. 2021; Mychko 2022). Regrettably, this site is subject to active coastal erosion, causing its gradual destruction.

Fig. 2
figure 2

Natural pre-Quaternary (Cenozoic) geo-sites of the Kaliningrad Region (the Sambia Peninsula). (a) Eocene marine formations with ferruginous sands and concretions (‘Krant Ground’ deposits of Zipfel’berg section, Primory’e). (b) Oyster shell and a shell cast from the fossiliferous ‘Krant Ground’ deposits (a) (Primory’e). (c) Major geological disconformity (arrow) between the Eocene marine beds and Oligocene riverine facies, documenting a drying-up of the Baltic palaeo-sea basin and establishment of a terrestrial environment at ~34 million year ago (Donskoye). (d) Shell (Ostrea) from the Eocene fossiliferous beds of ‘Krant Ground’ (Donskoye). (e) Oligocene-Miocene deposits of the ‘Brown Coal Formation’ (Otradnoye). (f) Fossil leaf imprint (8 cm) from the ‘Brown Coal Formation’, Miocene. Photographs by the authors (a–d, JC; e–f, EM)

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‘Brown Coal Formation’ Section

Stratigraphically above the ‘Krant Ground’, there are deposits of the ‘Brown Coal Formation’, properly geologically called the Kurshskaya and Rantavian Formations (Kuzmina et al. 2023). These massive dark brown beds (Fig. 2e), Oligocene–Neogene in age, are exposed along the sea-shore from the village of Primory’e to the town of Svetlogork (Fig. 1B). The lower and central clayey beds of this lignite formation contain numerous well-preserved fossil plant macro-remains (Figs. 2f and 7n). These deposits were formed under continental conditions after an episode of drying-up of the Danish–Polish Strait at the Eocene–Oligocene boundary. The sealed macrofossils are represented by numerous taxa—cypresses with Taxodium, poplars, palms and others (von Heer 1869; Budancev and Sveshnikova 1959; Mychko 2022). In addition to imprints of leaves (phytoleim), fossil seeds, conifer cones and lignitized tree trunks are found.

The fossil flora represents its northernmost geographic and age-corresponding distribution in Europe. It differs markedly from the same-age palaeo-plant taxa around the world. The layers with the fossil plants are confined to the ‘Lower’ and ‘Middle’ Clays of the Kurshskaya Formation and represent a most significant and rich palaeontological locality. Analogous beds of the same geological history, but with a less representative flora, are exposed at Cape Rozeve on the Baltic coast in Poland.

Ice Age Erratics

Within the investigated territory, glacial relief forms are well apparent and widespread over the present landscape. The thickness of these glacial surficial features, best exemplified by terminal moraines, can reach more than 200 m. These landforms create an elevated topography (150–200 m asl.) dissected by small streams. Ice-flow patterns of the expanding Scandinavian Ice Sheet and subsequent recessional ice-marginal positions during the Last Glacial Stage (MIS 2) in the adjoining Lithuania demonstrate the existence of large ice streams 100–300 km in length, forming end moraines ~3000 years after the LGM (Kalm 2012). A similar glacial chronology and ice dynamic is also presumed for the Kaliningrad Region. Coastal erosion and inland fluvial processes lead to a significant and enduring distortion of these glacigenic forms. On the Sambia Peninsula, in particular, the glacial tills construct most interesting sections and spectacular geo-sites (Fig. 3a). The cliffs are formed by a sequence of glacial till facies separated by a massive glacio-fluvial gravel bed with small clastic sediment fractions washed away (Fig. 3a). The close-to-vertical cliffs occasionally show a dislocated structure by a post-glacial neo-tectonics (Fig. 3b) due to the soft-sediment overburden (Dodonov 1971; Biske et al. 2012). The released coarse stony materials (cobbles and boulders) accumulate at the foot of the cliffs (Fig. 3c). The glacial erratics are represented by diverse rocks brought-in by the ice-sheet from Scandinavia and the Baltic coast and sea-bottom original geological occurrences. The visually attractive rocks are of magmatic, metamorphic and sedimentary lithologies of a broad geological age—Precambrian, Cambrian, Ordovician, Silurian, Devonian, Jurassic and Cretaceous (Fig. 3c). Diverse fossils of animals and plants are frequently in these clastic formations, such as Ordovician trilobites, Jurassic ammonites, Cretaceous belemnites and Silurian brachiopods (Fig. 6o). Many of these fossils are exceptional in the world’s palaeontological records; they mostly originate and were released from the inaccessible bottom of the former Baltic Sea. Hundreds of new fossil species and genera have been described since the end of the 19th century from the Ice Age rock clasts of the region (Jentzsch 1892; Mychko 2022). Isolated glacial erratics in the form of large solitary rocks occur throughout the country (Fig. 3d).

Fig. 3
figure 3

Natural Quaternary geo-sites of the Kaliningrad Region. (a) Glacigenic sequence of the Last Glacial (Vaidal/MIS 2) moraine with glacio-fluvial gravel facies within the till, topped by massive erratic boulders (Svetlogorsk, Sambia Peninsula). (b) Glacio-tectonic deformation of soft glacial clays/till (Svetlogorsk). (c) Glacial boulders washed from the Vaidal moraine (Cape Taran, Sambia Peninsula). (d) Scandinavian glacial granite erratic (Nikol’skoe, Sheshupe Valley). (e) Freshwater gyttja from a base of the stratified organic complex of the Baltic Lake littoral zone (Lesnoy). (f) Holocene fossil soil (a former land surface) within the coastal sand dune field of the Curonian Spit (Lesnoy). Photographs by the authors (a–d, f, JC; e, EM)

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Curonian Spit

The Curonian Spit, divided into the Kaliningrad’s and the Lithuanian sectors (Fig. 1B), is a world-unique geo-form and natural landscape of exceptional aesthetic value. It is the largest accumulative sandy formation in the SE part of the Baltic Sea, along with the sand bars of the Hel Peninsula of the adjoining Polish Baltic coast. The total length of the spit is about 98 km (Fig. 4b); its widest land section reaches 3.5 km. The southern (Russian) part of the spit (~50 km long) is protected in the frame of the Curonian Spit National Park, re-known for its biodiversity and the major active sand dune field (Fig. 4a). The spit is separated from the mainland by a large freshwater lagoon—the Curonian Lagoon, which connects to the Baltic Sea through a narrow strait at Klaipeda, Lithuania.

Fig. 4
figure 4

Natural Quaternary geo-sites of the Kaliningrad Region. (a) The Curionian sand dune field (the Efa Dune) with the Curonian Lagoon (Morskoye). (b) The Curonian Spit along the Baltic Sea shore. (c) The Baltic Glacial Lake formation (Kulikovo). (d) Fore-dunes of the Vistula Spit. (e) The Last Glacial Lake Vishtynetskoye filling a thermokarst/permafrost-degradation depression on the Vishtynetsk Highlands. (f) Final Pleistocene terraces of the Sheshupe River valley with a Palaeolithic site complex. Photographs by the authors (a–b, JC; c–f, EM)

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The geological and geomorphic structure of the Curonian Spit attests to a complex evolution. Its base is built by the Cretaceous bedrock overlain by Quaternary facies—massive glacial till (moraine) deposits and glacio-fluvial sandy-gravelly facies of a total thickness of 80–100 m. The uppermost post-glacial part of the stratigraphic sequence consists of variegated strata of different sedimentary structures and lithologies. These intercalated units suggest changing past sedimentary environments, including lacustrine (sands, clay, sapropel), alluvial (sands, sandy clays, silts), lagoonal (marls, sapropels), palustrine (peat, gyttja) (Fig. 3e), marine (sands) and aeolian (sands), with the latter forming most of the present surface cover (Fig. 3f). The age of the buried organic layers is mid-to-late Holocene. The terrestrial aeolian and marine beach sands shape the modern relief of the Curonian Spit. During its formation, apart from polygenetic sands, mass volumes of sandy alluvia of the large Neman River palaeo-delta were transported (Napreenko et al. 2019, 2021). The Curonian Spit National Park (est. in 1987) was declared a UNESCO World Heritage Site, encompassing a total area of 6621 ha.

Lesnoy Section

The Lesnoy stratigraphic section—a coastal geo-site—is a most interesting Quaternary geological profile found at the village Lesnoy (Fig. 1B). The site closely documents the post-glacial formation stage of the Curonian Spit. The Lesnoye outcrop is found exposed in its upper part, ~2 m above the present sea level. The basal stratigraphic units are overlain by a layer of silt mixed with a mollusc detritus (0.3 m thick) and fossil peat (0.2 m), which are covered by aeolian sands and loose surficial deposits (1–2 m) fixed by a modern soil. The origin, geomorphological features, structural characteristics and incorporated fossil records of the Pleistocene base of the spit and the Holocene cover strata were the subject of research already in the early 20th century (Wichdorff 1919).

The age of the basal peat layer is Early Boreal; the underlying silt, rich in mollusc remains, is late Last Glacial. The upper part of the prominent sequence of fossil peat units topped by palaeosols (Fig. 3f) has been 14C-dated to 1600–300 yr BP (Chlachula et al. 2023). The section corroborates the Holocene stratigraphy records of coastal peat lands from the raised bog site Svinoye in the southern part of the Curonian Spit, dated to 6300 cal. yr BP–present time (Napreenko et al. 2019). Fossil peat layers and palaeosol of the corresponding chronologies from the Łeba sand barrier, southern Baltic coast (Rotnicki et al. 2009), suggest the final stages of sea-level lowering. The sealed molluscs are mostly represented by shells of gastropod pond snails, Stagnicola palustris. This type of small-sized gastropod is not typical for fresh water bodies and is likely to live in brackish, saline lagoons (Mychko 2022). Radiocarbon dating of the lagoon’s seabed organic units indicates an eastward, 0.4–0.5 m/year migration of the spit near the Lesnoye site over the last 5000–6000 year period (Zhamoida et al. 2009). Fossil flora occurs in peats near Lesnoy in the form of macro-remains of plants and pollen. These remains were studied in some detail already by H. Preuss (1910).

Grand Curonian Dune Ridge

The Grand Curonian Dune Ridge on the Curonian Spit is another significant geomorphic site within the South–East Baltic area (Fig. 1). The original material sources of the dune ridge, largely formed during the early post-glacial, were loose sandy deposits of mixed provenance, both sub-aquatic and terrestrial, which accumulated along the Baltic coast.

The modern appearance of the Curonian Spit was formed in the post-Littorina time as a result of the consolidation of the local sandy bodies into a single lithodynamic system with prevailing transportation of clastic material along the shore (Sergeev 2015). The local pattern of current geomorphic processes is exemplified in 14 specific dune sectors along the entire dune field, up to 200 m asl high in the central part of the Curonian Peninsula (Fig. 4a). The predominant shape of the dunes shows a constant trend is flattening of the highest shifting/mobile dune ridges and lowering of the average height of the moving dunes’ crests, with a minimum loss of sedimentary loads and stable volumes of sand in the major dune bodies over the past 50 years (Povilanskas 2009). The isolated shifting dunes of the Curonian dune field contain smaller volumes of the aeolian sediment and are more susceptible to gradual degradation. Studies from the northern (Lithuanian) part of the Curonian spit revealed landward migration of the barrier spit since the mid-Holocene, corresponding to ~600 m for the 200-year period of the 16th–18th centuries, i.e., ~3 m/year (Buynevich et al. 2011). The significance of this geo-site is underscored by its aesthetic value and high touristic attractiveness (Urbis et al. 2019).

Vistula Spit

The Vistula (Baltic) Spit is another large-scale sand-depositional geo-site, exceptional on the entire Baltic Sea shore. Similar in origin and morphology to the Curonian Spit, the Vistula Spit is located in the SW coastal area of the Kaliningrad Region and the adjoining part of Poland (Fig. 1B). It separates the Vistula (Kaliningrad) Bay from the Gdansk Bay. The length of this coastal depositional landform is 65 km; the width is 300–1800 m in the middle and southern parts and 8–9 km in the northern part. The spit is formed by sandy beaches and builds partially forested dunes (Fig. 4d).

The origin of the Vistula Spit is similar to that of the Curonian Spit. A major difference is their size and the parameters of the specific geomorphic elements. During the formation of the Curonian Spit, large amounts of sandy alluvial sediments of the forming post-glacial Neman River delta were used. The main volume of sandy loads brought by the Vistula River concentrated outside the Gulf of Gdansk, constructing the Hel Spit and the Łeba Spit–lagoon system (Ejtminowicz 1982; Rotnicki 1995; Rotnicki et al. 2009). A later evolution of these two accumulative forms has much in common. The main increase in the costal barrier size occurred in sea-regression periods as a result of the formation of wide coastal plains, the so-called palve—regressive terraces structured by a series of low fore-dunes. The most favourable conditions for the Vistula Spit development date to the mid-Holocene, when the sea-level rise decreased and the amount of sand in the coastal zone reached optimal volumes (Badyukova et al. 2014). In addition to the aeolian sand cover on the Vistula Spit, which has a different geological age, there are fossiliferous clays with clusters of large freshwater shells: Viviparus viviparus, Unio pictorum, Bithynia tentaculata and other sub-recent mollusc species, freshwater and brackish-water diatoms (Bitinas et al. 2008). The SE basin of the Baltic Sea is a key part of studies of the development and geo-dynamics of the coastal zone structural geomorphic units (Gelumbauskaitė 2009).

Baltic Glacial Lake Formation

The Baltic Glacial Lake of the Last Ice Age (24,000–12,000 yr BP) formed at the final recessional deglaciation stage at the Pleistocene-Holocene boundary and Early Holocene (Andrén et al. 2011). Proglacial lacustrine basins flooding the territory of the present Kaliningrad Region document periodic damming and drainages of accumulated ablation waters. The initial ice-sheet retreat along the present SE Baltic coast started at ~13,500 cal yr BP (Gelumbauskaitė 2009). The lowering stages of the Baltic Lake’s existence are seen in the terraced landforms, suggesting sub-glacial spillways and periodic water-level drops. The last lake-water rise occurred during the final major cooling, generating the Fennoscandian ice re-advance during the Younger Dryas (12,900–11,700 yr BP), followed by the final stage of the Baltic Ice Lake drainage at ~10,300 yr BP (Björck 1995). Holocene wave-cut cliffs (the submerged coastlines) along the Sambia Peninsula document the sequenced stages of the post-glacial lake drainage (Sivkov et al. 2011).

Organic formations related to the Baltic Glacial Lake are quite widespread in the Kaliningrad Region, both on land and the continental shelf. The most attractive sections are found in the village of Kulikovo (Fig. 1B). There, the lacustrine facies are represented by variegated brownish-grey, banded clays and sandy clays, or dark bluish-grey, thinly layered strata (Fig. 4c). These deposits are rich in a shell detritus, in particular the Sphaerium corneum bivalves (Mychko 2022), Pisidium sp., Planorbis sp., Stagnicola sp., Valvata sp. and other molluscs.

Vishtynetskoye Lake

The glacial Lake Vishtynetskoye is a freshwater basin in the eastern part of the study area along the border limits of Poland and Lithuania (Fig. 1B). The lake, also called the ‘European Baikal’, is the largest, deepest and cleanest lake in the Kaliningrad Region. Located on the Vishtynetskaya Upland, the basin (with a surface-water elevation of 172 m asl.) is confined by the northern slopes of the Baltic Ridge, representing a margin of a terminal moraine (Orlenok et al. 2000). The deep-water lake, accommodated in six large depressions (max. depth 54 m), is fed by twelve rivers and streams draining the uplands (Fig. 4e). The lacustrine basin was originally formed by glacio-fluvial flows filling thermokarst-generated depressions during the South Lithuanian phase of the Pomeranian stage of the Valdai Glaciation. Its age is almost ten thousand years older than the Baltic Sea (Baltic Glacial Lake), with an estimated age of ~22,000–25,000 yr BP (Orlenok et al. 2000). Lake shores represent a favourite place for eco-tourism; the surrounding hills of the Vishtynetskaya Upland inspire the interest of landscape scientists.

The stratigraphic sequence of lacustrine deposits from the nearby Kamyshovoye Lake in the eastern part of the Kaliningrad Region (Fig. 1B) produced the most detailed palaeoenvironmental sedimentary records on the post-glacial climate history of the area subjected to a glacio-isostatic rebound over the period between 15,200 and 6,500 cal. yr BP (Kublitskiy et al. 2020; Druzhinina et al. 2020).

Cultural Geo-sites

Prehistoric Occupation Sites

Numerous archaeological monuments and single artefacts were found in the Kaliningrad Region, representing a long-settled cultural area of the south-eastern Baltic territory. These prehistoric and historical loci document linkages of ancient communities to the post-glacial and modern landscapes, and environmental adaptations of people in the SE Baltic area over time. Occasionally, anthropogenic transformations of past landscapes lead to the formation of interesting geo-sites with cultural geoheritage connotations.

The earliest cultural records with contextual connection to the regional palaeo-landforms are associated with Palaeolithic habitation of the eastern Ice Age periglacial plains. The discovered Pleistocene sites concentrate in the Sheshupe River valley (Fig. 4f) along the boarder with Lithuania (Fig. 1B). The potentially earliest locality, Ryadino, is reported as early as 50,000 years old (Druzhinina et al. 2016). Although the site’s geo-context and age need further verification, they might implicate settlements prior to the Last Glacial ice advance over the East Baltic territory. The richest collection of lithic items (>1150 pieces), represented by diverse flint items—cores, flakes, knife-like blades, scrapers, and other stone-tool types—originates from the Final Palaeolithic site Nikolskoe IV (Druzhinina 2010). Other post-LGM (<18,000 years old) sites are found in the villages of Sholokhovo (Timofeev 1983) and Lesnoye (Druzhinina 2010). Most of the earlier (pre-Last Glacial) archaeological monuments were clearly obliterated by glacial erosion associated with the repeated Scandinavian ice-sheet expansion over north-central Europe. Stone Age sites with marine-recourse-based adaptations are presently positioned along the SE Baltic coast on elevated grounds due to the isostatic land uplift (Muru 2017). Some cultural loci are subjected to distortion due to acting geomorphic processes. Preserved inland archaeological records, along with associated palaeoenvironmental proxy archives, testify to early economy activities on the Vishtynets Upland from the Mesolithic until the Middle Ages (Druzhinina 2022; Druzhinina et al. 2023).

Historic Architectonical Monuments

Many historic monuments connected with the pre-modern colonisation of these territories and (re-)established settlements are located on the outstanding landscape forms, which attracted settlement. Among the most prominent geo-sites, there is the Königsberg Cathedral (14th century) on Kant Island on the Pregolya River (a relict of the Last Ice Age glacio-fluvial terrace), the ruins of the Königsberg Castle (13th century) on a hill (moraine) on the right bank of the Pregolya River (Fig. 5a) and other Kaliningrad’s monuments found on distinct geomorphic locations. Other Middle Ages’ sites include temples, such as the Friedland Temple (14th century), and the Teutonic castles—the Bal’ga Castle, Brandenburg, Neuhausen, Schaaken, Labiau and Tapiau Castles (13th century); the Preußisch Eylau, Gerdauen, Insterburg and Georgenburg Castles (14th century); and the Tilsit and Ragnit Castles (15th century). These fortified sites, including larger fortresses as well as smaller mansions, erected along the main rivers and elevated grounds of the former glacial and post-glacial landscapes, served for defence and administrative control of the local surroundings (Turnbull 2011). For the construction, coarse glacial clastic materials and erratics were extensively used (Fig. 5b). Applications of such building materials along with the monuments’ geomorphic loci, often of complex geological histories, underscore, in complexity, the lasting geoheritage value of these places.

Fig. 5
figure 5

Cultural geo-sites (a, b) and modern industrial geo-sites (c–f) of the Kaliningrad Region. (a) Königsberg castle ruins built of glacial rock materials (13th century). (b) Teutonic castle Insterburg (14th century) elevated above the Instruch River with a fundament constructed of glacial erratic granite boulders. (c) Sinyavino amber mine. (d) Primorsky amber pit (Yantarny). (e) Amber-bearing layer ‘Blue Ground’ (Primorsky amber pit, Yantarny). (f) Extracted glacio-fluvial deposit within a lodgment till (the Vaidal/MIS 2 moraine), Lublino gravel pit. Photographs by the authors (a, JC; b–f, EM)

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These and other historical architectonical monuments display a close linkage to landscape and represent the most interesting and valuable element of the cultural landscape, integrating the former geomorphic site predispositions of the natural (post-glacial) relief. Regrettably, some of them are now in ruins and/or their original geological bedrock and geomorphological background has been disturbed by warfare and other human actions of the past centuries, but also natural processes, including erosion, gravity-forces and seismicity. Protection, geo-conservation and restoration of these cultural geo-sites is, therefore, most acute.

Modern Anthropogenic (Industrial) Geo-sites

Amber Quarries

The Kaliningrad Region is world-famous for its rich amber deposits (Zhamoida et al. 2017). The local amber—the fossilised natural resin of ancient pine-trees—is associated with Eocene deposits rich in amber reserves. Geologically, most of these fossiliferous bodies are found on the Sambia Peninsula and its continental slope (Sivkov and Zhamoida 2017). The Baltic amber has been a subject of trade since prehistory due to its physical attractiveness and value (Beck 1985; Bellintani 2014). Industrially, this organic mineral (succinite) has been mined in the former Eastern Prussia since the 19th century and (re)gained a major appreciation in modern jewellery. In the western part of the peninsula, there is a flooded quarry, Sinyavino Amber, mined from 1913 to 1972 (Fig. 5c).

Currently, there are two major amber-mining facilities in the western part of the Sambia Peninsula—Palmnikensky and the only operating Primorsky (Fig. 5d), both of the JSC ‘Kaliningrad Amber Plant’. The cumulative amount of the mineral from both quarries extracted and processed is ~300 t/year; the total amber reserves’ estimate is 116,000 tons. The amber geological provenience in the Primorsky mine pit is confined to sands and silts of the so-called ‘Blue Ground’ at the 55 m depth; an average thickness of the mineral placers is 5.5 m (Fig. 5e). The deep open quarry exposed complete suites of Cenozoic deposits. The lower part of the sequence is represented by the Late Eocene Prussian and Palve Formations, consisting of variegated layers of marine origin—‘Blue Ground’ (i.e., the principal amber-bearing unit), ‘Quicksand’, ‘White Wall’ and ‘Green Wall’. The overlying Kurshskaya and Rantavian Formations are built with interstratified sands, lacustrine and alluvial clays. The upper part of the cover deposits is geomorphologically represented by Pleistocene glacial moraines structured by logdement and melt-out tills (Fig. 5f). Occasionally, the coarse glacigenic deposits include glacially eroded clastics of various geological ages incorporating fossilized pre-Cenozoic organisms (Fig. 6).

Fig. 6
figure 6

Palaeontological geoheritage of the Kaliningrad region. Glacial clastic deposits of the Precambrian, Palaeozoic and Mesozoic rock formations sealing ancient fossils: (a) Åland quartz porphyry, Precambrian. (b) Iotnic (Jotnischer) sandstone, Precambrian; (с) Kalmarsund sandstone boulder, Lower Cambrian; (d) shell of the cephalopod ‘Endoceras’ sp., Ordovician; (e) trilobite carapaces (Asaphus laevissimus), Ordovician; (f) shell of brachiopod (Spirifer (Delthyris) elevates), Silurian; (g) single coral (Rugosa), Silurian; (h) scale imprint of the fish Holoptychius? sp., Devonian; (i) imprint of graptolite (Monograptus sp.), Silurian; (j) shell of ammonite (Quenstedtoceras vertumnum), Jurassic; (k) shell of ammonite (Cardioceras (Maltoniceras) schellwieni), Jurassic; (l) shell of bivalve (Trigonia sp.), Jurassic; (n) belemnite rostrum (Belemnitella sp.), Cretaceous; (o) plesiosaur neck vertebra (Plesiosaurus helmersenii), Cretaceous. Collections repository: (a–d, f–n) Atlantic Branch of P.P. Shirshov Institute of Oceanology, Kaliningrad; (e) Museum of the World Ocean Kaliningrad; (o) Kaliningrad Regional Museum of History and Arts. Scale 1 cm. Photographs by EM

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The local Baltic amber often contains nice inclusions of fossil flora and fauna of ancient coniferous forests that once flourished within the territory of the present Baltic Sea (e.g. Weitschat 1997; Weitschat and Wichard 2002; Wichard et al. 2009; Weitschat and Wichard 2010; Sadowski et al. 2019). Arthropods (arachnoids, insects, centipedes, crustaceans) are the most common life forms (Fig. 7j–m). Less often are encountered molluscs and small vertebrates, such as lizards, along with a variety of macro-botanical remains. Most exceptionally, fossil soft tissues of organisms are found. The newly described palaeontological species underscore the immense taxonomic variety of the local amber formations (Mychko 2022).

Fig. 7
figure 7

Some fossils of the Cenozoic of the Kaliningrad region: (a) crab carapace (Coeloma balticum), Upper Eocene, ‘Blue Ground’; (b) oyster shell (Ostrea sp.), Upper Eocene, ‘Blue Ground’; (c) core of gastropod shell, Upper Eocene, ‘Blue Ground’; (d) shark tooth (Carcharocles sp.), Upper Eocene, ‘Blue Ground’; (e) colony of the bryozoan (Onychocellidae gen. et sp. indet.), Upper Eocene, ‘Krant Ground’; (f) core of upper valve of oyster (Ostrea (Cubitostrea) ventilabrum), Upper Eocene, Krant Ground»’; (g) core of left valve of scallop (Pecten (Flabellipecten) incurvatus), Upper Eocene, ‘Krant Ground’; (h) core of sea urchin test (Laevipatagus bigibbus), Upper Eocene, ‘Krant Ground’; (i) pine cone (Pinus sp.) in amber, Upper Eocene; (j) isopod crustacean in amber, Upper Eocene; (k) centipede (Chilopoda) in amber, Upper Eocene; (l) pseudoscorpion in amber, Upper Eocene; (m) ichneumonid rider (Pherhombus antennalis) in amber, Upper Eocene; (n) taxodium imprints (Taxodium balticum), Lower Oligocene; (o) woolly rhinoceros tooth (Coelodonta antiquitatis), Upper Pleistocene; (p) caballoid horse astragalus (Equus cf. caballus), Upper Pleistocene; (q) woolly mammoth tooth (Mammuthus primigenius), Upper Pleistocene. Collections repository: (a) Steinmann Institute for Geology, Mineralogy and Palaeontology, Bonn, Germany; (b) Amber Combine, Yantarny, Kaliningrad region, photo by EM; (e–h, n) Atlantic Branch of P.P. Shirshov Institute of Oceanology, Kaliningrad, photo by EM; (i, l, m) Kaliningrad Amber Museum, Photographs by A. Manukyan; (k) collection of B. Zhukovsky, scale 1 cm; A. Manukyan; (b–d, j, o–q), Museum of the World Ocean Kaliningrad

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Amber is also found scattered along the marine shore and on the Baltic Sea shelf. The mineral is washed out by currents and waves from the bottom of the sea close to the coast, where the ‘Blue Ground’ deposits are exposed at a shallow depth. Especially after storms, the mineral is widely distributed on the beaches from the Vistula Spit to the Curonian Spit. Due to their unique nature, all amber exploitation and natural places are classified within the national geoheritage concept as principal world-unique geo-sites.

Glacial Deposit Quarries

The SE Baltic Coat area experienced major Pleistocene glaciations during the past hundreds of thousands of years. The regional geological history predetermined the extensive distribution of clastic deposits of the glacial and glacio-fluvial genesis. These mass accumulations represent rich reserves of construction materials (sand, gravel and boulders), which are used for the economic development of the Kaliningrad Region.

There are many quarries extracting these raw materials from the original glacial or glacigenic landforms, including moraines, eskers, sander channels and proglacial lakes (Fig. 5f). Structurally, these formations also include massive rock deposits similar to those found in the natural outcrops of the Sambia Peninsula coast (see Natural Geo-Sites).

Apart from the newly created geo-forms, the industrial quarries represent sites of high potential for finding Pleistocene megafauna remains within the framework of both scientific research and geo-tourism (Fig. 7o–q). In addition to the various minerals and fossils found in the clastic rocks (see above), relics of the Ice Age flora, including fossil wood, are commonly encountered in the sand/gravel pits (Fig. 5f). The present findings concentrate between the Talpaki village and the Chernyakhovsk town. Identification of the fossil-bearing facies is currently problematic since the present gravel extraction is underwater, and the geological outcrops are not available for a close study. Yet, with respect to the existing radiocarbon dates ~40,000 yr BP made on the Pleistocene fauna and flora (Kuzmin et al. 2016), the provenience of these fossils from the MIS 3 interstadial beds of ancient alluvial deposits (the Ratneno Formation) stratigraphically below the Kurshkaya moraine of the Valdai Glaciation (MIS 2) is most likely.

The retrieved palaeontological assemblages comprise the skeletal remains of the megafauna, typical of Late Pleistocene Northern Eurasia, including woolly mammoth (Mammuthus primigenius), woolly rhinoceros (Coelodonta antiquitatis), aurochs (Bos primigenius), primitive bison (Bison priscus), horse (Equus ferus), bighorn deer (Megaloceros giganteus), cave lion (Pantera leo spelaea) and others (Mychko 2022). Together with these animal remains, wood of sub-fossil preservation seldom occurs.

Environmental Impact on the Baltic Coast Geo-sites

The SE Baltic is geomorphologically an unstable area due to the acting past and present natural forces, such as sea and wind erosion, and inland neo-tectonics. Some of the identified geo-sites of the Kaliningrad Region are subjected to physical actions that (may) cause their gradual destruction. This particularly concerns the area of the Sambia Peninsula, which is increasingly exposed to progressing erosional processes by sea wave dynamics and storminess, causing a dramatic retreat of the sea shorelines. The principal storm surge factor has risen significantly in the SE Baltic area due to climate extremes over the past decades (Rozynski and Pruszak 2010; Cerkowniak et al. 2015). These on-shore events lead to the undercutting of the basement of the fossiliferous cliffs, best-observed in the Filinsky Bay with the ‘Krant Ground’ outcrops (Fig. 2a–d), as well as in the area of the Svetlogorsk town and the Donskoye village. The section of the sea coast near Filinsky Bay is being destroyed due to sea erosion at a rate of ~0.6–1.5 m/year (Ryabkova and Levchenkov 2016). A similar adverse environmental feedback to the modern shoreline zone has been detected along the Polish and Lithuanian coasts (e.g. Uścinowicz et al. 2004; Žilinskas 2005).

Even more threatened are the low-laying geo-sites, such as those on the Curonian Spit (Fig. 4b). The SE Baltic sea level documented along the Lithuanian coast (the Klaipeda Strait) as a result of global warming has increased by 14 cm over the past 100 years (Dailidienė et al. 2006). Most susceptible to coastal erosion are the narrow sandy barriers protecting the raised sandy coastlines, easily broken by storm surges. These actions also cause a reduction of beaches all around the coastal area (Łabuz 2015, 2016). The overall budget of the coastal sediment loads has dropped significantly, demanding protective, but rather costly anti-erosional mitigation measures. Extremely erosive storm surges, generated by atmospheric anomalies, foster the destruction of the coastal dunes in the Polish Baltic sector, where the erosion rate accounts for >100,000 m3 per 1 km (Tylkowski 2017). Such geo-hazards largely affect the Russian coast (Spiridonov et al. 2011).

On-land wind activity is another principal factor disturbing the stability of geo-sites constructed of loose sandy deposits. The isolated shifting dunes of the Grand Curonian Dune Ridge, built just by small volumes of sand, suffer from accelerated degradation. These unconsolidated landforms lack a new sand supply because of encirclement by a forest. The average annual deflation rate in the Lithuanian sector of the split is estimated to be ~0.3% of their total volume (Povilanskas 2009). Approximate calculations from the Russian part of the sand dune field are missing, but are assumed to be comparable. On the opposite, the large mobile dunes of the Curonian dune field seem to be less endangered by erosion because of a relatively rapid (2–3 m/year on average) migration of the 10–50 m high dunes non-stabilised by vegetation cover. Similar rates of dune migration with a speed of 3–10 m per year were detected at the Slovinski National Park in North Poland (Michalowska and Glowienka 2008; Koprowski et al. 2010). Aside from implications to the relief evolution of the adjoining sea and lake coasts, reconstructions of the past and present dune dynamics should be integrated into the coastal development, environmental protection and management strategies.

A major hazard to locally exposed geo-sites, mainly in the cliff forms, is the present seismic activity. The latest earthquake series at a magnitude M 4.3–5.3 occurred in 2004 (Ulomov et al. 2008). The events occurred under the central-northern part of the Sambia Penninsula at 16 km and 20 km depth due to a lateral strike slip of the Fennoscandian Shield (Wiejacz and Debski 2006). Another natural risk factor is the post-glacial earth-crust rebound over the once ice-loaded geographical space. Neo-tectonics has been repeatedly recorded within the broader territory. Although these unpredictable actions may cause relatively little material loss, they distort the original geological and geo-archaeological strata and release the enclosed fossils or cultural remains (e.g. Druzhinina et al. 2017). The analogous past endogene deformations are preserved in the form of glacio-tectonic facies of ice-compressed soft sedimentary beds (Dodonov 1971; Narloch et al. 2021). A glacially induced faulting and compression, active during the last 15,000 years (Steffen et al. 2019), are apparent in the mapped Sambia Peninsula geo-site sections (Fig. 3b).

Following the last deglaciation, the shorelines of the south-eastern Baltic Sea have experienced significant geomorphic changes due to the marine/lake transgressions and regressions induced by melting of the continental ice sheets, repeated damming and drainages of the large proglacial basins and the regional isostatic land uplift following the ice-retreat. A number of the Stone Age (Palaeolithic–Mesolithic) sites are therefore located several tens of metres above the present sea level (Muru 2017). Documentation and reconstruction of ancient settlement patterns of the Final Pleistocene and Early Holocene coastal occupation contribute to the rescue of the regional cultural geoheritage.

Finally, particularly, the near-coastal locations are heavily affected by modern anthropogenic activities associated with the development of new resorts and residential places. Often, these construction sites are found nearby the coastal cliffs, threatening their stability, in addition to natural erosion. The Kaliningrad Region is also rich in some mineral deposits, such as Mesozoic marls, limestone and phosphorites, and Cenozoic brown coal, aggregates, amber and peat, among others, which substantiate their active exploitation (Zhamoida et al. 2017). Glacial deposits then constitute readily available construction materials. All these industrial activities potentially endanger geoheritage sites, both known as well as potential.

Geoheritage Conservation and Geo-tourism Perspectives

The diversity of geo-sites and attractive landforms of the study area calls for its preservation. Reconnaissance, documentation and publicity of unique geo-sites and projected geo-parks provide stimuli for their registration in the national and international Baltic coast nature heritage programmes, implementing proper geo-environmental conservation policies. Interdisciplinary studies using GIS, geological, archaeological, historical and heritage approaches, along with new technologies of the geomorphosites identification and field-data processing, can better identify the site-specific conservation demands and needs (e.g. Warowna et al. 2013; Dedkov 2020).

The most prominent and threatened geo-sites of international significance—the ‘Krant Ground’ site, the Curonian Spit and the Curonian Dunes, among others—call for urgent actions to secure their preservation. The ‘Krant Ground’ site, in particular, is subjected to the most active coastal erosion due to dominant west-oriented wave streams, causing a gradual destruction of this unique locality. A status of a natural geoheritage monument of the Baltic coast significance will warrant future protection and add to its physical consolidation. In the case of the Curonian Spit and the dune field, except for the implications to the relief evolution of the adjoining sea and lake coasts, reconstructions of the past and present sand body dynamics should be integrated into the coastal development, environmental protection and landscape management strategies.

The inclusion of the geoheritage sites into the European Geopark network (Geopark 2021) will add to international awareness of the geoheritage of the Kaliningrad Region.

The possible conflict between the geo-sites conservation and economic use may be settled by improved field knowledge and a geological data transfer to developers. A better common consciousness of the general public of the geoheritage importance will contribute to uncovering the natural history of the region. Geoheritage trails, education programmes and a passion for palaeontological fossils’ search along the coast are just some of the fundamental aspects and activities addressing the present and new visitors coming to these places. These field activities and attractions are properly completed by permanent expositions and museums, such as the Museum of the World Ocean and the Amber Museum in Kaliningrad; the Amber Exposition and the field platform at Yantarny, offering a panorama view of the amber exploitation in the Primorskoye amber pit; the natural exposition of the Curoninan Split National Park and Nature Museum complex in Lesnoy, among other public facilities.

A separate issue is geoheritage relevance to regional tourism. Geo-tourism, as tourism of geology and landscapes (Dewar 2011), may play a major role it its future development. Sustainable geo-tourism, based on land conservation and recreation, is still marginally practised, if at all, in the Kaliningrad Region, compared to the neighbouring Baltic counties—Poland or Lithuania (e.g. Alexandrowicz 2004; Satkūnas and Nicius 2007; Łach 2017; Wendt 2020; Golubina and Kruzhalin 2021). Geo-tourism, along with eco-tourism and cognitive and scientific tourism, constitutes an integral part of the modern global tourism industry (Dowling and Newsome 2018). The Baltic Sea region is one of the most developed places of active international cooperation (Klemeshev et al. 2017). The broader region has been a place of intense political, economic and cultural contacts since the early Middle Ages. Modern cross-border tourism has gained on its importance (Carié 2019; Cerić and Więckowski 2020). The Curonian Spit, as one of the principal Baltic UNESCO Heritage Sites, is a subject of the Lithuania—Russia Cross-border Programme, promoting sand spit protection as well as the trans-border eco-tourism (Anokhin et al. 2021). A potential threat to similarly naturally fragile places is mass tourism centred at some spatially concise locations (Widawski and Jary 2019).

Geo-archaeological and historical monuments within the geo-tourism framework may add to travel itinerates (Banerjea et al. 2019). Rocky cliffs and landforms constitute equally relevant geoheritage categories inspiring visitor’s interest, with high educational value (e.g. Górska-Zabielska and Zabielski 2019; Górska-Zabielska et al. 2020). Historical sites, such as the Teutonic castles, geomorphically bound to the outstanding landforms, should be key spots of the Warmia-Kaliningrad trans-border geo-tourism (Żęgota 2021). Overall, an integration of the Kaliningrad Region’s geo-tourism into the international tourism framework of the broader Baltic area has the best premises for re-establishment of the once-flourishing foreign tourism to the coastal area of the former Eastern Prussia. The local landform variety with well-stratified cliffs, prominent fossils’ occurrences and rich palaeontological sites can play a significant role in the process of discovering the Past Amber Land (Mychko 2022).

Conclusion

The Kaliningrad Region is a geomorphologically and geologically rather diverse territory. The mapped SE Baltic coastal and inland geo-sites, either the spectacular fossiliferous sections, the amber mines or the world-unique sand spit forms, illustrate a long and complex geological history and relief evolution, with the most diverse stratigraphies, lithologies and the associated records of past environments and ancient life forms. A state declaration of some of these unique places as national monuments, along with the establishment of geo-parks on the Sambia Peninsula, the adjoining Curonian Spit and other geomorphically appealing places, will significantly contribute to knowledge and appreciation of the exquisite geoheritage of the Kaliningrad Region on both national and international levels. Independently, effective multi-facetted mitigation approaches should be implemented to provide background and tools for the preservation of the local geo-sites. Geo-conservation of the most threatened places, particularly those exposed to the coastal erosion along the Sambia Peninsula, may have just limited effects. Many of the coastal sea cliff-loci, enclosing unique stratigraphic sequences and fossil records, are pre-destined to gradual destruction. Yet, the application of adequate strategies may reduce harm to other geo-sites of the Kaliningrad Region affected by negative anthropogenic factors which may be subject to some control (amber and gravel mining, construction activities, etc.).