Introduction

Even though the appreciation and awareness of geoheritage is growing worldwide, as demonstrated by both a surge of thematic publications in the last decade or so, including handbooks and manuals, and the implementation of a range of practical solutions at global and national levels, it is probably fair to say that in many places geoheritage values are still insufficiently accounted for. They may be known to narrow groups of geoscience specialists, but much less so to the local people, landowners, site managers, and the like. This comment particularly applies to localities, which have been long functioning as tourist spots, but for reasons different than geoheritage and geodiversity, which remain “hidden” or “invisible” values. Nonetheless, such poorly appreciated geoheritage may be of special scientific significance and may also have considerable potential for general Earth science education. Relevant examples of such localities include scenic spots, enjoyed for striking shapes or colors of rock outcrops, waterfalls and natural bathing pools, caves, canyons, and panoramic viewing points (Goudie 2002). All these attributes are the outcome of local geological and geomorphological history, which however may be scarcely communicated to the visitors or not communicated at all. In other localities, tourist interest is centered around historical sites representing cultural heritage, whereas the geoheritage component remains overlooked or, again, is rather weakly exposed. This imbalance and opportunities to enhance visitors’ experience through the promotion of geoheritage are increasingly recognized, in both urban and countryside context (Reynard et al. 2017; Gordon 2018; Chylińska and Kołodziejczyk 2018). In the former, the focus is mainly on the geoheritage value of building stones (De Wever et al. 2017; Lezzerini et al. 2019; Santi et al. 2021; Kubalíková and Zapletalová 2021), but tracing natural landform change due to urban growth is also increasingly popular (Pica et al. 2016, 2018; Tičar et al. 2017). In the countryside context, relationships between specific location of cultural heritage sites, land use patterns, and historical events on one hand, and geological and geomorphological setting on the other hand are examined and explained (Migoń and Latocha 2008; Lugeri et al. 2011; Knight and Harrison 2013; Gordon 2018).

It has been noted for some time that the above comments regarding undervalued geoheritage apply, somewhat paradoxically, to many localities of outstanding cultural value, including many recognized as globally significant heritage of mankind—World Heritage properties (Alexandrowicz et al. 2009; Migoń 2014, 2018; Reynard and Giusti 2018). These situations may be further resolved into two families of cases. One includes sites, whose geoheritage and geodiversity values are well appreciated by geoscientists, but little of this awareness is conveyed beyond the professional group and transmitted to other stakeholders and visitors. A good example is the array of impressive granite-gneiss domes of Rio de Janeiro, which constitute a globally unique geomorphological landscape (Fernandes et al. 2010; Silva et al. 2015), recently included in the IUGS-endorsed list of 100 Global Geosites (Mansur and Valeriano 2022). However, the World Heritage inscription of Carioca Landscapes of Rio de Janeiro was exclusively based on cultural criteria, even though it was acknowledged that “Its exceptionally dramatic landscape is punctuated by a series of forested mountains that tower over the city, rising to the uppermost peak of the Tijuca massif at 1,021 m high, and cascading down to the coast where the steep cone shapes of Sugar Loaf (Pão de Açúcar), Urca, Cara de Cão and Corcovado frame the wide sweeps of Guanabara Bay that shelters Rio de Janeiro from the Atlantic Ocean” (http://whc.unesco.org/en/list/1100; access date: 2022-04-13). Nonetheless, nothing in this statement emphasizes the outstanding value of the topography. The other group contains properties, which are either insufficiently researched or have hardly been subject to evaluation in terms of geoheritage, and certainly not in the global context. Consequently, their international significance is poorly known. It needs to be noted, though, that the list of scholarly geoheritage publications about such properties is steadily growing (e.g., Sheth et al. 2017; Reynard and Estoppey 2021).

In this paper, we intend to contribute to better understanding of one such locality, which is both a nationally popular tourist spot and a World Heritage property, recently inscribed using solely cultural criteria. It is a tabular hill of Jabal Al-Qarah, located within the Al-Ahsa Oasis in the eastern part of Saudi Arabia. Not only it is an impressive, geomorphologically diverse landmark, already known for its extensive cave system (Hötzl et al. 1978; Hussain et al. 2006), but we will argue that examination of its morphology may significantly contribute to a recently explored research avenue that seeks to evaluate the role of subsurface processes in the evolution of sedimentary tablelands (Young et al. 2009; Duszyński et al. 2016; Wray and Sauro 2017; Migoń and Duszyński 2022a, 2022b; Sauro and Mecchia 2022). Hence, its geoheritage significance is much more important than one may think.

Study Site

Location and General Setting

In Arabic, the term “Jabal” means hill or mountain. Jabal Al-Qarah is located in the Eastern Province of Saudi Arabia, c. 130 km to the south-west of Dammam and 10 km to the north-east of the city center of Al-Hofuf (Fig. 1). In fact, it lies amidst a densely built-up and highly populated urban area inhabited by more than 1.15 million people (SGAS n.d.), which has developed within the Al-Ahsa oasis, claimed to be the largest desert oasis in the world. The contemporary climate is hyper-arid, with a mean annual temperature of 27 °C, absolute highs up to 50 °C, and mean annual rainfall of c. 80 mm, almost exclusively between December and April (Climate data for Saudi Arabia, http://jrcc.sa/reports_files/1985_2010Reports/Ahsa.RTF; accessed 2022-04-13).

Fig. 1
figure 1

Location map of Jabal Al-Qarah Mountain in Saudi Arabia. Note the densely built-up area next to the hill and the presence of two minor residual hills to the north and north-west, labeled as a butte and pinnacle

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Jabal Al-Qarah is a flat-topped hill, slightly elongated, nearly 2 km long from north to south and up to 1.4 km wide. Its outline is compact, lacking longer promontories or embayments penetrating into the interior of the elevation. The highest spot is 205 m asl, whereas the surrounding flatland lies at c. 140 m asl, making a height difference less than 70 m. However, the outer slopes of Jabal Al-Qarah are steep, cliffed, or boulder-covered along much of the perimeter. They are also strongly dissected by a network of ravines, so that access to the top surface is not straightforward. One technical dirt road runs to the plateau from the north-east, serving the telecommunication facilities.

In the immediate vicinity of Jabal Al-Qarah, there are several other elevations, but much smaller and lower than the main one (Fig. 1). Towards the north, separated by a 400-m-wide gap, is a circular hill that measures approximately 400 m × 300 m and is c. 25 m high. Another one of even smaller dimensions is located to the north-west from the main hill. It has a roughly circular base and the diameter of 100 m. Both are surrounded by urbanization.

World Heritage

Jabal Al-Qarah is located within the boundaries of the recently inscribed World Heritage property “Al-Ahsa Oasis, an Evolving Cultural Landscape.” The inscription took place in 2018 and was based on fulfilling three criteria designed for cultural sites. These are criteria no. (iii), emphasizing organic evolution of the cultural landscape and continuity of agricultural tradition; no. (iv), highlighting diversity of the landscape and the vestiges of architectural monuments from different historical periods; and no. (v) that underlines Al-Ahsa as an outstanding example of traditional human settlement in arid lands (http://whc.unesco.org/en/list/1563/; access date 2022-04-13). The property covers 8544 ha and is internally divided into twelve component parts of various sizes. Jabal Al-Qarah is part of the largest of them, identified as the Eastern Oasis.

Although the existence of Jabal Al-Qarah is repeatedly mentioned in the nomination document and there are a few references to its caves (Al-Ahsa Oasis 2017), no attempts to evaluate natural features from the broader geoheritage perspective were apparently undertaken. The focus of the nomination was the irrigation-based agricultural landscape that has developed in the middle of an extremely dry desert due to abundant water resources, which however had to be carefully managed to ensure sustainability of the settlement. Moreover, the property includes archaeological sites, traditional village layouts, and elements of historical architecture.

Geology

Jabal Al-Qarah is part of the Shedgum plateau that presents rugged topographic surfaces between two smooth flat areas, the Jafurah sand desert to the east and the Rubayda (Dahna) sand desert to the west (Saner et al. 2005). Tectonically, the study area is part of the Arabian platform which includes all terrain from Kuwait to the south of Qatar Arch, characterized by very low dipping (1–2°) of the sedimentary succession in the north-easterly direction.

The sediments of Jabal Al-Qarah are mainly of Miocene (Aquitanian-Messinian) age. During that time, the Arabian platform, as part of the Arabian plate, was separated from Africa due to the onset of the opening of the Red Sea rift and drifted toward the Eurasian plate. Particularly, in the Middle Miocene, the Arabian platform was invaded by remarkable transgression that reached to the Al-Summan plateau (west of the Shedgum plateau) and created continental, transitional to shallow marine depositional settings, giving rise to the Hadruk, Dam, and Hofuf formations (Powers et al. 1966).

Two main lithostratigraphic units were recognized in the wider Jabal Al-Qarah area, namely Dam and Hofuf formations (Fig. 2). The Early Miocene Dam Formation consists of limestone, marl, and claystone with minor intercalations of sandstone, chalky limestone, and coquina. The Hofuf Formation is divided into four main litostratigraphic units, including basal marls and conglomerates, followed by calcareous sandstones, a thick unit of intercalating sandstones and mudstones, capped by conglomerates containing mainly limestone clasts (Powers et al. 1966). Within Jabal Al-Qarah only the upper parts of the Hofuf Formation are exposed, which consist of sandy limestone overlain by an approximately 75-m-thick sequence of light gray calcareous sandstone, with reddish marl/silty marl interbeds (Hussain et al. 2006). The latter include one at the base of the hill and the level of the main cave entrance, whereas the other one is approximately in the middle part of the sandstone unit. The topmost part of the succession at Jabal Al-Qarah is made of a thin limestone bed, up to 2 m in thickness (Fig. 2). Lateral and vertical changes within the sedimentary succession result in the differential weathering impact, accounting for the stepped nature of slopes, the occurrence of overhangs, and rock columns with wide caps (hoodoo rocks). The Hofuf Formation at Jabal Al-Qarah shows a prominent, quasi-regular jointing pattern. It consists of two principal joint sets, trending WSW–ENE and N–S to NNW–SSE (Hussain et al. 2006).

Fig. 2
figure 2

Generalized lithological column of Dam and Hofuf formations of Miocene age (modified after Powers et al. (1966) and Saner et al. (2005)). Note that this is an attempt of regional synthesis, not the stratigraphic log of Jabal Al-Qarah. Therefore, the relationships of hillslope profile of the mesa (thick orange line) and the cave extent (vertical bar) to lithology are only approximate

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Landform Diversity

Surface Landforms

The general appearance of Jabal Al-Qarah is one of a tabular mesa, a landform typical for sedimentary tablelands, where the dip of the successive strata is negligible. However, in contrast to many mesas, which have planar upper surfaces and smooth, concave outer slopes (Migoń and Duszyński 2022a), the morphology of Jabal Al-Qarah is much more complex, even though at a broad scale it may be divided into two units: an undulated top surface and dissected outer slopes (Figs. 3 and 4).

Fig. 3
figure 3

General subdivision of the mesa into undulating plateau (top) surface (A) and strongly dissected outer slopes (B). Codes (a), (b), and (c) refer to piles of boulders in the peripheral parts of the mesa (see Fig. 10). White rectangles show location of close-up images (Figs. 6 and 11) and open triangles show direction and width of viewing (Figs. 4, 5, and 9). Source: Google Earth imagery

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Fig. 4
figure 4

Undulating plateau surface with shallow troughs and low elevations, terminated against a heavily jointed cliffed escarpment, with numerous erosional incisions in the outer slopes (foreground) (photo by Subhajit Sen)

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The top surface represents an undulating plateau, with numerous shallow troughs of different sizes, joining into an intricate pattern of drainage lines, resembling dendritic or, locally, trellis. The main troughs show preferential NNW–SSE strike, turning N–S in the northern part of the mesa, but the tributaries, typically more incised, show various orientations (Fig. 3). The length of the largest drainage systems exceeds 300 m. Most troughs open towards the rim of the plateau and change into more incised, V-shaped valleys in downstream direction, and then into deep gullies and canyons within the outer slopes of the mesa (Fig. 5). Some, however, are drained internally and terminate at open clefts, which provide connection to the system of subterranean slots and galleries (see below) (Fig. 6). Faint remnants of troughs in the downstream direction from sinks suggest that interception of drainage and its diversion into underground occurred as a later phase of drainage system development.

Fig. 5
figure 5

Transition from an undulating plateau surface to the heavily dissected outer slopes. Shallow troughs on the plateau become more incised downstream, then giving way to deep gullies in the marginal parts of the mesa (foreground) (photo by Subhajit Sen)

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Fig. 6
figure 6

Examples of runoff pathways on the plateau (blue tracks) which end in sinks (yellow circles). Continuation of drainage occurs underground, through the maze cave system, with some corridors open to the sky (to the right). Source: Google Earth imagery

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A very specific part of plateau morphology of Jabal Al-Qarah is its eastern section, where numerous deep clefts and slots open to the surface occur within an area of approximately 6 ha, measuring 315 m × 250 m (Figs. 6 and 7). The pattern is distinctly rectangular, consisting of main clefts striking WSW–ENE and perpendicular clefts striking NNW–SSE, typically shorter. A few minor systems of open slots occur further to the south and north, close to the plateau margin, whereas they are apparently absent along the western margin of the plateau. The reasons why the deep clefts are developed so well here and almost absent elsewhere are not entirely clear. Certainly, this is not due to differential incision, as the surface around the hill is almost level, at the same altitude all around. For the same reason, preferential gravitational opening of fractures is difficult to accept as an explanation and fractures both parallel and perpendicular to the margin of the mesa have been exploited. One possible explanation assumes that caprock in this part of the mesa is stronger than elsewhere and inhibits valley deepening, otherwise occurring within the top surface (Fig. 5). Therefore, as illustrated by Fig. 6, shallow surface drainage is directed underground, towards the section with slots and caves.

Fig. 7
figure 7

Deep open clefts in the eastern part of the plateau, which connect with the maze cave system inside the mesa (photos by Subhajit Sen)

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The outer slopes show stepped topography, reflecting variability in rock strength within the succession of the Hofuf Formation due to the interlamination lithology. More resistant units (limestone, sandstone, conglomerate) support cliffs, which locally are up to 40 m high, whereas intervening softer units (siltstone, shale, and marl) underlie slope sections of lower inclination and benches (Hussain et al. 2006). In addition, the outer slopes are heavily dissected, with structure-controlled ravines and canyons up to 18 m deep and 350 m long. The separating ridges and spurs are locally heavily fragmented themselves, divided into free-standing blocks and columns along joints perpendicular to the strike of the canyons. Some of these columns are molded into hoodoo shapes, with more resistant beds of sandstones or limestones forming the caps (Fig. 8). Hoodoos occur either in isolation or in rows, carved from narrow ridges between drainage lines. Spectacular examples of the latter are present in the south-eastern part of the mesa, where multiple ridges capped by a more resistant sandstone bed extend finger-like into the foreland, some as long as 200 m (Fig. 9).

Fig. 8
figure 8

Fragmentation of the outer slopes of the mesa by joint action of weathering and erosion, near the entrance to the cave system. Notice a hoodoo rock in the middle part of the image and the cluster of buildings next to the rock formations (photo by Subhajit Sen)

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Fig. 9
figure 9

Rows of hoodoo rocks carved from a narrow ridge in the heavily dissected, south-eastern corner of Jabal Al-Qarah. The image also illustrates the proximity of buildings to the scenic landforms (photo by Subhajit Sen)

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Striking geomorphic features along several sections of the mesa margin are clusters of large boulders derived from more resistant sandstone beds. They occur in three geomorphic contexts (Figs. 4 and 10): (a) thick piles at the periphery of the mesa, mantling the mesa/plain junction. These boulder belts may as much as 80 m wide, but typically less, 30–50 m; (b) outlying piles of boulders, disconnected from the marginal slope; (c) blocky fills of valleys and canyons dissecting the outer slopes. Transitions between these settings occur and, for example, in the northern part of Jabal Al-Qarah, the blocky belt widens in sections, where plateau valleys descend towards the surrounding plain and type (a) merges with type (c) (Fig. 11). Block clusters of type (c) occur in various places at Jabal Al-Qarah, whereas types (a) and (b) are largely restricted to the northern and western margin.

Fig. 10
figure 10

Different geomorphic settings of large block piles in the marginal parts of the mesa. a Along the perimeter. b Outlying blocks in two piles (1 and 2). c Canyon fills (photos by Subhajit Sen)

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Fig. 11
figure 11

Blocky apron along the northern margin of Jabal Al-Qarah, extending into valleys debouching from the plateau (arrows). Source: Google Earth imagery

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Cave Systems

Cave systems inside Jabal Al-Qarah have been long known, with the Ghar An Nashab I cave (Cave of the Archer) being the largest and most publicized (Figs. 12 and 13). The cave was mapped in detail by Hötzl et al. (1978), who provided the figure of 746 m as the total length of cave corridors. They also mentioned the northern part of this system, named as Ghar An Nashab II cave, but did not provide respective cartography. However, they indicate the cumulative length of the entire system as more than 1.5 km. The cave map was more recently reproduced by Hussain et al. (2006) and is presented here in a simplified version (Fig. 12). The map shows that the system consists of long, straight, narrow, but tall corridors, intersecting one another at right angles. Locally, the corridors widen into minor chambers. In the Ghar An Nashab I cave, the entrance part is made of one large, wide corridor of ENE–WSW strike, with a few short (up to 20 m) branches, but in the inner part four roughly parallel galleries occur, ended in impenetrable slots. Sandstone boulders lie on the floors of some corridors and chambers. A distinctive feature of cave galleries is their vertical extension up to the surface of the mesa, with numerous openings allowing daylight to come into the cave interior (Fig. 13a, b). Hussain et al. (2006) observed that speleothems are absent in the caves, whereas Hötzl et al. (1978) emphasized episodic runoff from the mesa top into the cave and the presence of alluvial veneer on the floors (Fig. 13f).

Fig. 12
figure 12

Simplified ground plan of the Ghar An Nashab I cave (after Hussain et al. 2006). Note protracted straight sections of cave passages which demonstrate adjustment to dominant fractures

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Fig. 13
figure 13

Interior of the Ghar An Nashab I cave. a, b Views towards the roofs of galleries. c Vertical walls of a gallery. d Upward tapering of a corridor and continuation as a narrow fissure. e Narrowing of a gallery. f Near-entrance section of the cave, with evidence of water flow (photos by Subhajit Sen)

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The lithological succession within the mesa locally comprises a layer of caliche (hardened limestone) (Fig. 14a), which is harder than the sandstone and marl layers below. Therefore, picturesque hoodoo rocks form at the surface, whereas underground the caliche layers inhibit lateral growth of cave corridors and account for pointed cross-sections of cave galleries, tapering upward. If these galleries are sufficiently wide, vertical support for the roof is reduced and collapses from the ceiling occur, producing boulders present on the cave floor. At the same time, the key role in guiding the spatial pattern of cave passages is played by criss-crossing vertical fractures (Fig. 12).

Fig. 14
figure 14

Lithological and structural control on the morphology of the cave. Caliche layer that forms a cap of rock pinnacle at the surface (a) provides a barrier for upward growth of a cave passage (b). If the passage is wide enough, roof is no longer supported and cave ceiling collapses occur, supplying rock blocks to the cave floor (c). Both b and c show how vertical fractures control the strike and shape of cave passages (photos by Subhajit Sen)

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Geomorphological Evolution and Geoheritage Significance

The mesa of Jabal Al-Qarah is an erosional outlier separated from the retreating eastern escarpment of the Shedgum Plateau, currently located some 10 km to the west. In close vicinity, there are other remnants of the plateau such as the butte to the north and the pinnacle to the north-west (Fig. 1). However, what makes the mesa potentially highly significant are its caves and hillslope morphological features.

Hötzl et al. (1978) strongly argued for marine origin of the caves and other geomorphic features of the hill, linking them with the marine transgression at the end of the Pliocene. They interpreted mid-slope benches as successive levels of abrasion platforms, whereas canyons penetrating into the hillslopes were considered coastal inlets and named as “wave-cut gorges.” Cave corridors were claimed to be shaped by incoming waves, exploiting lines of structural weakness, although it was also admitted that further modifications due to weathering and episodic runoff in subaerial conditions occurred. Finally, blocky piles and mantles around the hill were interpreted as products of submarine rockslides. However, subsequent research did not confirm the presumed extent of marine transgression and Hussain et al. (2006, p. 17) made it clear saying that “as the highest Quaternary Sea level in the area is less than 100 m (…), the role of marine erosion in forming the caves is unlikely.” Consequently, other landforms which used to be considered littoral would have a different origin too.

We propose that the suite of subaerial and subterranean landforms present at Jabal Al-Qarah can be explained through the allied concepts of subsurface erosion and in situ cliff disintegration. The former occurs in soft to medium-strong, jointed clastic rocks, taking advantage of a well-developed joint pattern that allows for easy infiltration of meteoric and surface waters underground (Bruthans et al. 2012a; Duszyński et al. 2016). If the rock is soft, individual grains can be easily detached and evacuated by subterranean flows, leading to widening and upward growth of underground channels. It is important to note that the mechanisms of enlarging the conduits are not karstic, although rock mass weakening may involve dissolution of cement (Sauro 2014; Wray and Sauro 2017). In the latter concept, it is argued that for cliffed escarpments, where mechanisms of subsurface erosion operate, rock falls and catastrophic rock-slope failures are processes of minor to negligible importance (may not occur at all). Instead, ongoing removal of material by subterranean flows results in breakdown of jointed rock mass into separate joint-bound compartments, which are subject to minor displacements and subsidence into ever bigger voids (Duszyński and Migoń 2015). Over time, a rocky escarpment turns into an irregular strip of loose boulders, stacked one upon another. Continuing erosion of underlying strata, whether mechanical or chemical, results in further subsidence and disintegration of the former caprock, so that caprock blocks may take a position at significantly lower altitude than dictated by the regional geological structure. In an advanced phase of this long-term process, irregular boulder clusters or dispersed large boulders are the only vestiges of once existing mesas and buttes (Migoń et al. 2020; Migoń and Parzóch 2021).

Referring to Jabal Al-Qarah specifically, its geological structure involving relatively soft and well-jointed rock clearly facilitates the mechanisms described above. Runoff from the plateau surface, even if originally subaerial, disappears underground using numerous joint-related slots and shafts developed at joint intersections. Then, it works its way towards mesa margins, enlarging the joints and turning them into narrow corridors. If roofs become unstable, collapses occur producing residual boulders on the floor and causing upward extension of the corridors (see Bruthans et al. 2012a, 2012b), as can be observed in the Ghar An Nashab I cave. Narrow slots emerging from rock walls, some partially filled with stacked boulders, and boulder-filled ravines are compatible with the model and have close counterparts in the area of Broumovské stěny in Czechia, where the concept of subsurface erosion was applied to explain the origin of canyon networks in sandstones (Duszyński et al. 2018). However, the underground flows can also be diffuse, resulting in seepage at the boundaries of beds of different permeability. If this occurs, the overlying strata will gradually lose support, break down into smaller compartments, and become displaced downslope. With time, a blocky apron develops and if the rock involved is relatively strong, such aprons may be quite durable. Selective weathering along lithological contacts produces the same result, although probably over a longer timescale. Blocky aprons at Jabal Al-Qarah appear to owe their origin to such long-term operation of weathering and seepage that cause removal of softer material from beneath. Two outlying piles of boulders at the western footslope of the mesa (Fig. 10b) illustrate two very late phases of this evolutionary pathway. The outlier no. 1 is already separated from the escarpment, but the soft bed below the hard cap is still partially preserved, supporting it in topographically higher position. In the outlier no. 2, the soft material is nearly completely eroded away and the chaotic pile of boulders lies at lower elevation. Ongoing subsurface erosion and rock strength contrasts explain the widening of the blocky apron and its extension into the valleys, as it is the runoff paths where the process is expected to act most efficiently. These specific features of block distribution are difficult to reconcile with the concept of submarine rockslides, suggested for the locality by Hötzl et al. (1978).

An argument for the role of subterranean flows also requires consideration of timing of these processes. Nowadays, the environment in this part of the Arabian Peninsula is very arid, although episodic rainfall events may occur and it is about these events that Hötzl et al. (1978, p. 66) remarked “All the recent episodic surface runoff disappears into shafts.” However, the Quaternary history of the peninsula involved alternating dry and semi-humid periods (Al-Sayari and Zötl 1978; Grainger 2007; Preusser 2009), and it is the latter that witnessed much more efficient activity of groundwater in the development of landforms (Edgell 1990; Weijermars 1999). Thus, the most efficient development of the underground drainage and cave extension are likely to be linked with these more humid intervals.

Thus, Jabal Al-Qarah emerges as an important locality that can help to validate the concepts developed in a different geographical setting (Central Europe), typified by much more humid climate, especially in contemporary conditions, but not so much different in terms of geological structure and geomorphic context (sedimentary tableland in a predominantly clastic succession). It demonstrates the role of subsurface runoff in the evolution of residual hills in sedimentary tablelands and adds to the diversity of evolutionary pathways experienced in such areas (Migoń et al. 2020, 2023; Migoń and Duszyński 2022a). It also shows that significant geomorphic role of groundwater is not restricted to karst terrains, but may apply to other settings as well if there is adequate lithological and structural preconditioning such as the presence of a regular system of fractures and soft rocks, easy to erode even by limited subterranean flows, especially under more resistant cap that would slow down surface lowering (also Laity and Malin 1985; Higgins and Osterkamp 1990; Bruthans et al. 2012a, 2012b; Duszyński et al. 2016). Finally, it provides an excellent example of a non-karstic cave, which developed through long-term erosional widening of joints, accomplished by joint action of weathering and runoff. It thus differs from most crevice caves in sandstones, which develop through gravity-driven joint opening, typically in association with mass movements (Margielewski and Urban 2003, 2017; Lenart et al. 2014).

Besides the above, the geoheritage significance of Jabal Al-Qarah is further enhanced by bizarre medium-size and minor landforms such as different varieties of hoodoo rocks, occurring in rows or in isolation, small rock bridges and tunnels, polygonal cracks, arcades along horizontal discontinuities, and overhangs (rock shelters). For all these reasons, also recognizing accessibility and popularity of the site among visitors, Jabal Al-Qarah may qualify as a geosite sensu Reynard (2004), i.e., as a place which helps to better understand geological past and geomorphological processes that shape the surface of the Earth. Whatever the formal designation would be, however, the development of the locality as geosite would require addressing various geoconservation and geotourism organization challenges, as reviewed in the following sections.

Geoconservation Challenges

Geoconservation challenges at Jabal Al-Qarah are of two kinds, related to the protection of geoheritage values of the hill as a landform and to the management of the caves. A general framework is provided by the management plan developed for the World Heritage property (Al-Ahsa Oasis 2017), although this is focused on cultural values and the integrity of cultural landscapes rather than on geology and geomorphology. Nevertheless, the plan sets limitations upon the spatial expansion of built-up areas and the hill is among areas not planned for urban development. In fact, the natural conditions themselves provide a serious limit to development. Thus, both Jabal Al-Qarah and two minor satellite hills in its vicinity will remain as little altered natural features within an otherwise densely built environment. However, there remains a problem of urban development around the perimeter of the hill, especially in the vicinity of the caves and the hoodoos (Figs. 8 and 9), which may obstruct the view and compromise the scenic qualities of Jabal Al-Qarah. This threat has been identified in the World Heritage nomination document, stating that “The development plan for Al-Qarah, aiming at transforming the site into a major tourist attraction, is not entirely compatible with the conservation strategy proposed in the nomination. It has been agreed that the private developer will adapt the original plans to limit their impact on the natural scenery” (Al-Ahsa Oasis 2017: p. 283). Nevertheless, a large group of buildings exists near the entrance to the caves, reducing the natural beauty of the cliffed hillside. There are also several places around the perimeter of the mesa, in the south and north, where groups of houses stand in direct contact with the cliffs (Figs. 3 and 9). Telecommunication facilities on the top of Jabal Al-Qarah are also an intrusive element in the landscape.

Access to the hilltop is currently unrestricted but there are no specifically designated trails either. People climbing Jabal Al-Qarah use many footpaths that cross the upper surface of the mesa, whereas only a few steep paths negotiate cliffed hillsides. The most convenient access path is the dirt track serving the hilltop telecommunication facilities. While making the plateau more accessible in the future, safety issues need to be considered since the stability of cliffs in a generally weak substratum is poor and open cracks crossing parts of the plateau may cause risk of accidents for careless visitors.

At present, a part of the cave system is open to the public and this one has electricity and camera surveillance, whereas ceiling openings provide additional light. The remaining part of the labyrinth lacks facilities allowing for visits, contains very narrow and unlit passages, and is closed to tourists. Sustainable use of the caves is another challenge, especially given their growing popularity as a tourist spot, whereas passages are narrow, and the host rock is generally soft and erodible. However, speleothems—most fragile components of the subterranean karst environment requiring specific conservation measures—are lacking. Rare storm phenomena pose a threat since runoff from the hilltop surface may be intercepted by deep cracks and fissures, leading into the cave galleries. Weakening of bedrock may result in wall and roof collapses, creating risk to visitors and limiting accessibility, but on the other hand these need to be regarded as key natural processes shaping the caves. Therefore, the cave may be temporarily closed to visitors. Parts of the cave system outside the current tourist path should be preferentially left undeveloped.

Finally, perhaps the most important challenge is to persuade local authorities, various stakeholders, and local people about the unique geoscientific value of the locality, which is much more than a scenic spot and site of cultural interest. This awareness was apparently lacking during the preparation of the World Heritage nomination dossier for the Al-Ahsa Oasis, but redressing the balance seems both urgent and appropriate.

Opportunities for Geotourism

The Vision 2030 for the Kingdom of Saudi Arabia highlights the development of tourism and various localities are being identified and developed across the country as potential sites of tourist interest. This creates an opportunity to develop geotourism and promote the natural heritage, as complementary asset to the much more recognized cultural heritage. Indeed, the geoheritage and geotourism potential of the country is slowly being realized (Moufti and Németh 2013; Németh and Moufti 2017; Al Mohaya and Elassal 2023). Thus, despite very limited appreciation of geoheritage of Jabal Al-Qarah at present, the locality appears suitable to be developed for geotourism, in addition to its cultural and scenic values. External conditions facilitating such a strategy include good accessibility, the presence of good infrastructure in the city of Al-Hofuf, and history-rooted attractiveness of the locality. Thus, possible concerns that geology-based attractions alone may not provide a sufficient basis for the development do not apply here. The designation as a World Heritage property boosts tourism and provides opportunities to diversify the tourist product.

At the moment, interpretation facilities focused specifically on geoheritage are non-existent, but both the cave experience can be enhanced by adding research-based explanation of the origin of underground passages, as well as surface thematic trails supported by either panels or downloadable web-resources can be designed. Regarding the latter, two options are possible. First, the perimeter road around the hill, more than 6 km long in total, provides an opportunity to erect observation sites at key spots, where different geological sections of the sedimentary succession and evidence of mass movements of different types can be examined. This self-driving, biking, or pedestrian route can be complemented by a medium-length walking trail to the summit surface of the mesa. Its added value in respect to the perimeter trail and the cave visit would principally result from two aspects. First, the summit trail would show landform features not visible from the footslope observation sites, such as ephemeral channels gathering storm runoff, closed hollows (sinkholes), and, above all, open cracks which connect with cave galleries inside the rock massif. Second, the bare top surface of Jabal Al-Qarah allows one for having wide panoramic views of the surroundings, including further sites of geoheritage interest. In particular, a good view of two minor residual hills, the butte to the north and the pinnacle to the north-west, along with a more distant view of plateau edge to the west, offers a template to explain the evolution of residual hills by long-term dissection and scarp retreat, from plateau through mesa, butte, to the solitary pinnacle as a penultimate stage. Although such a scenario may be an oversimplification of the reality at a specific place (see Migoń et al. 2019, 2020), it may still serve as a useful educational tool.

Conclusions

Jabal Al-Qarah in eastern Saudi Arabia is a unique, but apparently an overlooked site of geoheritage interest, despite its recent emergence as a tourist destination, especially after the Al-Ahsa Oasis, within which it is located, has become World Heritage cultural property (in 2018). In a way, it illustrates regrettably frequent cases, when geoheritage values are nearly completely ignored while evaluating global significance of sites of cultural heritage. Jabal Al-Qarah is a mesa (tabular hill) separated from the parent plateau by long-term dissection and escarpment retreat. It is built primarily of calcareous sandstones, with mudstone and marl interbeds, and is quasi-regularly jointed. This geology, set in an arid climatic context, preconditioned the development of a peculiar system of caves, which owe their origin not to dissolution processes (although these may play a subordinate part), but rather to mechanical erosion and mass movement along joints, which account for their widening and local opening towards the upper surface of the mesa. The labyrinthine network of underground galleries is complemented by surface features indicative of drainage interception by subsurface conduits (dry valleys, open cracks, sinkholes). Thus, Jabal Al-Qarah significantly contributes to the better understanding of the development of both tabular relief and non-karstic caves, fitting the recent line of research in sedimentary tablelands aimed at investigating surface–subsurface connectivity in non-calcareous settings (Bruthans et al. 2012a, 2012b; Duszyński et al. 2016; Wray and Sauro 2017; Migoń et al. 2020; Sauro and Mecchia 2022). Future investigations have the potential to make Jabal Al-Qarah a key site of global relevance and significance in this respect.

Besides scientific values, Jabal Al-Qarah has considerable potential to be developed as a geotourist destination. Good access and renowned cultural heritage of the Al-Ahsa Oasis facilitate tourism, providing an opportunity to make the local tourist product more diverse and emphasize natural underpinning of the cultural development of the oasis. Both traditional and modern means in geo-education can be considered and implemented.