Interpretation of the macrofossil and sedimentary sequences
Trench F. Zone F1: 11,190–10,730 to 10,690–10,340 cal bp. Sedimentation began in standing water, with Phragmites, probably P. australis (Haslam 1972), growing at the site. A range of emergent and aquatic taxa were present in the wider area (notably Nymphaea, Potamogeton spp., Carex (including C. paniculata and C. rostrata), Menyanthes, and Typha spp.), though given the tendency of plant material to accumulate at lake edges these were not necessarily growing locally (e.g. Birks 1973; Zhao et al. 2006; Koff and Vandel 2008). The local growth of trees (B. pubescens, B. pendula, P. tremula, and possibly Salix spp.) at the shore is demonstrated by large Betula and Salix/Populus stems and branches within the sediments.
Zone F2: 10,690–10,340 to 10,530–10,270 cal bp. The decrease in the quantity and range of macrofossils reflects a reduction in the volume of water reaching the sediments (see also Dark 1998a, p 131), which is tentatively interpreted as a shift to a seasonally flooded environment. Cladium and Typha spp. began to grow locally, probably in response to shallower conditions. Corylus, Salix/Populus, and Viburnum spp. were present on the adjacent dry ground.
Zone F3: 10,530–10,270 to 10,110–9,770 cal bp. The absence of aquatic material and the decline in other taxa mark the point where the deposits formed beyond the reach of the lake. The presence of a tree stump indicates the in-situ growth of Salix/Populus. Emergent vegetation (Cladium, Carex spp. and Typha spp.) was present nearby. Peat formation was interrupted at 10,170–9,890 cal bp by the deposition of a layer of silty sand.
Zone F4a: 10,110–9,770 to 8,620–7,450 cal bp. From the composition of the sediments and the macrofossil assemblage, herbaceous plants (including fen taxa Thalictrum and Eupatorium) had colonised the area as organic sedimentation resumed. Quasi-terrestrial conditions were present locally but it was sufficiently wet to support Menyanthes, at least for part of the time. The first band of humified peat reflects a drop in the local water table, causing the existing deposit to dry out and humify. This started at 9,590–9,480 cal bp with the deposition model indicating a hiatus in peat formation of 70–350 years, before organic sedimentation resumed at 9,440–9,150 cal bp.
Zone F4b: 8,620–7,450 to 7,420–7,090 cal bp. The increasing quantity of macrofossils suggests that the surface of the peat became slightly wetter (improving preservation) though the local vegetation remained unchanged. A subsequent fall in the water-table is marked by the second band of humified peat, which again caused the existing deposits to dry out. This began at 7,560–7,420 cal bp, with a hiatus in peat formation of 10–230 years, before organic sedimentation resumed at 7,430–7,250 cal bp.
F5: 7,420–7,090 to 6,970–6,440 cal bp. Alnus, Urtica, Carex spp. and possibly Corylus colonised the local area. The presence of Alnus cones and wood in the sediment indicating in-situ growth of alder.
Zone A1: 11,690–10,460 to 11,120–10,360 cal bp. Open water was present at the sampling point with beds of Characeae present locally.
Zone A2: 11,120–10,360 to 10,160–9,690 cal bp. The shift to organic sedimentation reflects a decrease in water depth. Macrofossil records indicate the local presence of aquatic (Potamogeton spp. and Nymphaea) and emergent (S. lacustris, Typha, and Carex) taxa.
Zone A3: 10,160–9,690 to 9,400–9,000 cal bp. The absence of aquatic macrofossils in the basal samples reflects a temporary fall in lake level leaving the deposits above lake-water level. Based on the deposition model this event had a duration of up to 370 years. Cladium and Carex spp. colonised the local area before a rise in lake level (modelled at 10,100–9,500 cal bp) submerged the sampling point, and aquatic and deep-water emergent taxa (Potamogeton spp. and S. lacustris) become re-established. The water-depth gradually shallowed in the upper half of the zone and a Cladium swamp formed at the sampling point.
Zone A4a: start: 9,400–9,000 cal bp; end: uncertain age. The consistent absence of aquatic material indicates that the deposits began to form beyond the extent of the lake. Herbaceous plants, including Hydrocotyle vulgaris, and also ferns were growing locally.
Zone A4b: Undated. Conditions became wetter leading to increased preservation of plant macrofossils, though the deposits remained terrestrial in character. Eupatorium, Urtica and Mentha grew locally.
A5: Undated. Alnus, Betula spp. and Carex spp. colonised the area. As with trench F the presence of Alnus cones and wood in the deposits indicate in-situ growth of alder.
B1: Undated. Open water was present at the sampling point with Characeae and Myriophyllum spicatum present in the surrounding area.
B2: start: uncertain age; end: 8,980–8,640 cal bp. The change to organic sediments marks a shift to shallower water. Aquatic and emergent plants (notably Nymphaea, Typha spp., S. lacustris and Carex spp.) grew locally.
B3: 8,980–8,640 to 8,520–8,340 cal bp. The absence of aquatic macrofossils marks the point where the deposits began to form beyond the reach of the lake. Cladium and Juncus spp. were growing locally.
B4a: start: 8,520–8,340 cal bp; end: age uncertain. The deposits remained terrestrialised. Eupatorium, Juncus spp. and ferns were growing locally.
B4b: Undated. Eupatorium, Alnus and Betula spp. grew locally or in the surrounding area. Failure to record Alnus cones or wood suggests that alder was not present at the sampling point.
C1: Undated. Open water was present at the sampling point with Characeae and Myriophyllum growing locally.
C2: start: uncertain age; end: 8,040–7,880 cal bp. The switch to detrital muds indicates a shallowing of the water, emergent and aquatic plants (notably Nymphaea, Potamogeton spp., S. lacustris, and Carex spp.) colonise the local area. Eupatorium seeds reflect the presence of fen nearby.
C3: 8,040–7,880 to 7,830–7,660 cal bp. The decline in the quantity of aquatic macrofossils suggests that the water depth at the sampling point continued to fall. Carex spp., Juncus spp. and Eupatorium grew locally.
C4: start: 7,830–7,660 cal bp; end: age uncertain. The absence of aquatic material marks the point where the deposits formed beyond the extent of the lake. Herbaceous plants, possibly including Cladium and Juncus spp., were present locally.
Trenches H and I
Based on the character of the basal organic deposits and the lack of aquatic plant macrofossils, deposition began in a terrestrial fen environment with Eupatorium and Thalictrum growing locally and Betula spp. present in the surrounding area. The deposits subsequently dried out creating a band of dark humified peat. This corresponds stratigraphically with the second band of humified peat recorded in trench F (see ESM Table 2). The start of this event (in trench F) dates to 7,560–7,420 cal bp. Fen environments returned before shrubs or trees (including Alnus, and Betula spp.) colonised the area resulting in the layer of wood peat. This transition to Carr was dated in trench F to 7,420–7,090 cal bp, and is likely to be broadly contemporary in trenches H and I.
Correlating the records
A radiocarbon chronology for the sequence of environmental change recorded in the plant macrofossil and sedimentary profiles is shown in Fig. 8. In addition, the increase in fen taxa in the upper part of trench F and core A, which reflects a shift to a slightly wetter depositional environment (zones F4b and A4b, respectively), is assumed to be broadly contemporary.
Evidence for hydrological changes
Two forms of hydrological change have been documented in this study; the gradual shallowing of the lake due to ongoing sedimentation in the basin, and more rapid fluctuations in lake-level/water-table. The chronological relationship between these changes is shown in Figs. 8, 9.
At a general level, the data show the linear development of the lake environments. At each of the sampling points the plant macrofossil assemblage reflects an overall trend from wetter to drier conditions, with the macroremains of aquatic and emergent taxa giving way to macroremains from plants characteristic of wet or boggy ground, and fen or carr environments. With the exception of the earliest fluctuation in lake level recorded in core A (see below), once conditions at each sampling point were such as to be beyond the reach of the lake they remained terrestrialised. Similarly, the chronological relationship between the profiles shows that open water and aquatic environments were succeeded by increasingly shallow and ultimately terrestrial conditions within the basin over time (see Fig. 9).
This broad pattern was interrupted by changes in lake or local water-table levels. The most significant was the fluctuation in lake level recorded in core A. This began at 10,160–9,690 cal bp with a fall in lake level that left the deposits around the sampling point beyond the reach of the lake for up to 370 years. This is contemporary with the deposition of the sand layer in trench F (Figs. 8, 9). Assuming a causal as well as chronological relationship, the sand may have derived from erosion of sediments on the higher ground above the lake edge during this drier phase. However, there is no evidence for a shallowing of the water in cores B or C at this time (Fig. 8) indicating that the fall in lake level was not large enough to alter the environment in the deeper parts of the basin.
The subsequent rise in lake level in core A at 10,100–9,500 cal bp occurs within the same modelled age range as the transition from carr to fen in trench F (10,110–9,770 cal bp, zone F3–F4a) (Fig. 8). The correlation of these two events suggests that the rising level of the lake caused the deposits at the lake edge to become too wet to support the local growth of trees and thus resulted in fen development. This development is also seen elsewhere within the basin. In trench F the transition from carr to fen coincided with a switch from wood peat to a coarse herbaceous detritus (ESM Table 2a), and a similar stratigraphic horizon was recorded at Seamer Carr (Cloutman 1988b) and other parts of the basin (Paul Lane, personal communication).
The remaining events had a lesser impact on the wider environment. A transition to a slightly wetter environment was inferred from an increase in the quantity of macrofossils recorded in trench F and core A (zones F4b and A4b). The macrofossils show no evidence for a significant change in the local vegetation, and the effects were probably limited to a localised shift to wetter ground conditions that increased macrofossil survival. Two fluctuations in the local water-table were also identified from the sedimentary and chronological record in trench F. In both cases these were represented by a layer of heavily humified peat and a hiatus in organic sedimentation at the interface between the humified layer and the overlying deposit. These represent periods when the surface of the peat dried out causing the extant deposits to humify and peat formation to cease, before a subsequent shift to wetter conditions caused organic sedimentation to resume. There is no evidence for a comparable event in cores A–C, suggesting that the effects were relatively minor in character and thus limited to the edges of the basin.
Finally, there is no evidence that a rising lake level caused peat-forming environments to expand over previously dry ground (contra Cloutman 1988a, p 14); aquatic material was not recorded in the deposits in trenches H and I, and the sedimentary sequences and plant macrofossils are indicative of a terrestrial fen. Comparable environments were also recorded in Cloutman’s pollen analysis at Seamer Carr (Profile K5), which described a herb and fern-rich fen forming above the former lake shore (Cloutman 1988b, pp 28–30). Rather, the expansion of peat-forming environments over areas of dry ground was probably caused by the accumulation of peat at the lake edge, which would have inhibited drainage from the adjacent mineral sediments above the shore causing them to become waterlogged (a form of edaphic paludification) (Rydin and Jeglum 2006, pp 125–126).
The timing and nature of wetland succession in the Lake Flixton basin
Organic sediments began to form at the Flixton SHF lake edge in the centuries around 11,000 cal bp with marl accumulating in the deeper water at the site of core A (and potentially B and C). Species diversity was high within the lake. Phragmites was growing at trench F, and other emergent species, notably Menyanthes, Typha spp. and Carex (including C. paniculata and C. rostrata), were growing in the surrounding area. A similar range of taxa was recorded from contemporary sediments at Star Carr (Dark 1998a; Taylor and Alison 2018) and No Name Hill (Taylor 2011), indicating the rich and diverse nature of the marginal vegetation. Aquatic plants Nymphaea, Myriophyllum and Potamogeton spp. were also present, probably in deeper water further from the shore, along with beds of the aquatic algae Characeae. Trees (Populus tremula, Betula pubescens and B. pendula, and Salix spp.) were growing at the water’s edge. Beyond the lake, pollen analysis has recorded a landscape of grassland and scrub with Juniperus, Salix and Betula that was replaced by Betula woodland from ca 11,100 cal bp (Fig. 4 in Blockley et al. 2018).
The lake-edge environments changed rapidly over the following centuries. Emergent and aquatic species had colonised core A by ca 10,750 cal bp. At trench F the deposits were only seasonally submerged by ca 10,500 cal bp, encouraging the local growth of Cladium, with the deposits forming beyond the reach of the lake by ca 10,400 cal bp. This allowed trees (Salix/Populus) to spread onto the peat. The same sequence has been recorded at Star Carr where Cladium increases following the shift to a shallower environment (Taylor and Alison 2018, p 130), before the lake edge deposits became terrestrialised and resulted in fen and carr forming by 10,450–10,165 cal bp (Supp Info 20–21 in Blockley et al. 2018,). Assuming a similar rate of sedimentation in other parts of the basin the embayments at Seamer Carr, Lingholme, and Cayton (Fig. 2) were probably also becoming terrestrialised at this time (see Taylor et al. 2018b, p 49). However, within the main body of the lake the extent of these environments was still limited to the area close to the shore with vegetation characteristic of deeper water persisting at core A, and open water also persisting in the vicinity of cores B and C. On the shore, Viburnum spp. and Corylus were growing at Flixton SHF from ca 10,500 cal bp, the presence of the latter in the wider landscape also being reflected in the low but consistent presence of its pollen (e.g. Cloutman 1988b; Dark 1988a, b; Day 1996). The dating undertaken by Cloutman (1988a) shows that peat-forming environments were already established on areas of dry ground at Seamer Carr Site K (Fig. 2).
The period from ca 10,000 to ca 9,000 cal bp was characterised by changes to both the wetland and terrestrial environments. At ca 9,900 cal bp the lake level fell causing the area around core A to become terrestrialised. Emergent and aquatic plants probably expanded further into the basin creating an extensive area of wetland vegetation (though not as far as core B where open water conditions persisted). A subsequent rise in the level of the lake, up to 370 years later, resulted in flooding in the area around core A and reversion to an aquatic swamp. Deposits at the lake edge were now too wet to support growth of trees. Fen communities consisting of herbaceous plants (including Thalictrum and Eupatorium) replaced carr at trench F and other locations around the former shore. At ca 9,500 cal bp the local water-table fell causing the deposits around the edge of the lake to dry out and humify, before wetter conditions returned up to 350 years later. Peat was starting to encroach over previously dry ground at VP Site E and parts of the Star Carr peninsula (see Fig. 2) by this time (see Cloutman 1988a), and was probably also developing at Flixton SHF. On drier ground Corylus became the principal component of the woodland, an event dated at Star Carr to 10,250–9,730 cal bp (8,940 ± 90 bp. OxA-4377) (Dark 1988a, p 133), and Ulmus, and to a lesser extent Quercus, began to expand into the area, shading out much of the understory vegetation (Day 1996, pp 16–17; Dark 1998b, p 170).
Despite these hydrological changes the lake environments continued to develop in the manner of a classic hydrosere. By ca 9,200 cal bp the deposits at core A were forming beyond the reach of the lake water. Emergent species were probably already present around core B, and by ca 8,800 cal bp had been replaced by Cladium fen, creating a zone of terrestrialised wetland that extended lakewards at least 150 m from the former shore. Aquatic and deep water emergent communities may also have been present around core C from this time, and were colonising the deeper parts of the basin by 8,600–8,300 cal bp (94.6% probability) (7,640 ± 83 bp. OxA-4042) (Day 1996, p 17; Dark 1998b, p 170). On dry ground, the pollen evidence indicates that Quercus became more common, and, along with Ulmus and Corylus, formed a significant component of the woodland whilst Alnus spread to the area (Day 1996, p 17; Dark 1998b, p 170), and probably colonised drier areas within the wetland.
From ca 8,000 cal bp the hydrosere entered its final phase as the water around core C became increasingly shallow and then terrestrialised at ca 7,750 cal bp. Standing water may have persisted in some places, and may have resulted in small pools within a wider mosaic of swamp, fen and carr. The deposits around trench F dried out for a second time at ca 7,500 cal bp, though the effects appear to have been limited to the edge of the basin. Shortly after organic sedimentation resumed Alnus colonised the site (ca 7,250 cal bp) and probably the area around core A. Fen environments persisted around cores B and C, though the occurrence of Alnus and Betula macrofossils suggests at least some tree cover locally.
Implications for Mesolithic activity
The first Mesolithic groups arrived in the area at ca 11,300 cal bp (Milner et al. 2018), with evidence for activity at multiple locations around the lake within the following centuries (Conneller et al. 2016; Taylor 2018). This phase of settlement is strongly associated with the lake environments and the exploitation of wetland resources. Activity areas were located on or close to the shore and have yielded evidence for fishing, fowling, hunting of aquatic mammals, and the collection and processing of wetland plants (Clark 1954; Taylor 2011, 2012, 2018; Taylor et al. 2018a; Robson et al. 2018).
Initial changes to the lake environments had little effect on this pattern of activity with sites continuing to be occupied as the lake edge became shallower and ultimately terrestrialised, and emergent vegetation extended further from the shore (see Taylor 2011, 2012, 2018; Milner et al. 2018). Indeed, the early stages of wetland succession were probably beneficial to early Mesolithic groups as the expansion of swamp and carr within the shallower embayments created a mosaic of habitats suitable for fish, waterfowl, and some of the larger mammals.
This pattern of settlement began to change from the latter stages of the early Mesolithic as sites on low-lying ground close to the shore were abandoned. The recent excavations at Star Carr, which focused on the area between 24 m and 25 m a.s.l., show occupation ending ca 10,500 cal bp (Milner et al. 2018; Blockley et al. 2018), whilst activity at Seamer Carr site C (which lay at a similar elevation) had ceased by ca 10,000 cal bp (Conneller et al. 2016) (site locations in Fig. 2). In addition, there is no evidence for late Mesolithic settlement on early Mesolithic occupation sites at VP-D, VP-E, Seamer Carr site D, and Flixton Island (Table 1 in Conneller and Schadla-Hall 2003), all of which lie on or below 25 m a.s.l. (Fig. 2), and only minimal activity at Seamer Carr site K, which lies at a similar elevation. In contrast sites on higher ground, such as Flixton SHF, Flixton School Field, Barry’s Island and No Name Hill continued to be occupied into the late Mesolithic along with new sites (such as Rabbit Hill) on similarly elevated positions (Fig. 2).
The changes described above were probably a response to the expansion of peat-forming environments over areas of dry ground, a process that would have rendered low-lying areas adjacent to the shore unsuitable for settlement. In turn, the shift to higher ground changed the environmental context of the later sites, which now lay either at the junction between fen and the terrestrial woodland or, in the case of the former islands or peninsulas, on small wooded hills surrounded by wetlands. Indeed, from ca 9,000 cal bp it is more appropriate to describe these sites as occupying a wetland edge, rather than a lake-edge environment, as areas of open water receded into the basin.
As well as the shift in settlement, patterns of economic activity would have changed as both terrestrial woodland and wetland succession altered the availability of plant resources and the habitats of prey animals. However, these changes need not have been as unfavourable to the human population as has been suggested (contra Mellars 1998, p 230). Though the lake was infilling with fen, this would have been bordered by an extensive woodland-edge environment made up of younger saplings and shade intolerant species such as Salix and Populus tremula. This would have resulted in habitats suitable for browsers such as red and roe deer. Given that the development of closed canopy woodland is generally thought to have reduced the size of large mammal populations (e.g. Mellars 1976), the persistence of large areas of fen and woodland-edge resulted in the Lake Flixton landscape remaining an important area for both settlement and hunting throughout the much of the Mesolithic.