Regional background and glacial history
Windermere is a glacial ribbon lake in the south-east of the English Lake District (Fig. 1). Tertiary uplift of the Cumbrian Mountains initiated a regional radial drainage pattern with the Windermere River Valley forming the south-eastern spoke (e.g. Mitchell, 1956). Successive glaciations throughout the Pleistocene have overdeepened the pre-glacial river valley into a steep-sided (>20°), deep (basal surface up to 110 m below present lake-level) glacial lake, dammed at its southern end by a moraine-capped bedrock bar that forces water to drain westwards into the river Leven (Fig. 1c; Wilson, 1987). The lake is cut into the Windermere Supergroup (largely Silurian mudstones and siltstones) and is divided into a South and North Basin by a shallow bedrock plateau that supports numerous small and one larger island called Belle Isle (Fig. 2).
Figure 1. Maximum extent and major ice flow directions of the Late Devensian British and Irish Ice Sheet (a), after: Chiverrell and Thomas (2010) and Clark et al. (2012). Younger Dryas ice limits in the Lake District (b), and glaciogenic landforms in the south-eastern Lake District (c), after: Gresswell (1952), Sissons (1979, 1980), Wilson and Clark (1998, 1999), McDougall (2001), Wilson (2004), Brown (2009) and Hughes et al. (2010). In (a), the thick black line indicates BIIS maximum extent, and thin black lines indicate ice flow directions. In (c), pale grey landforms are associated with YD readvance, while black landforms are believed to be Late Devensian structures. Panel (a) coordinates shown as latitude/longitude (WGS84), panels (b) and (c) as Easting/Northing in UTM zone 30 N (WGS84).
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Figure 2. Summary of data presented. Black lines indicate seismic reflection profiles, circles 6-m cores acquired by the Fresh Water Biological Association (Holmes, 1964), and squares 16-m shell and auger cores acquired in 1971 by Dredging Investigations Ltd.
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During the Devensian glaciation (c. 75–14.7 ka BP; Chiverrell and Thomas, 2010; Clark et al., 2012) snow-blown wet-based glaciers (>800 m thick; Ballantyne et al., 2009) nucleated in the Lake District, Pennines and Snowdonia, forming the central and southern parts of the British portion of the BIIS (Fig. 1a). In north-west England, ice flow was initially dominated by Scottish ice flowing southward from the Southern Uplands, forcing English ice (nucleating in the northern Lake District) eastward across the Pennines at Stainmore (Evans et al., 2009). However, the superposition of subglacial flowsets in the Vale of Eden and Solway Lowlands indicate this ice divide subsequently moved northward as the Lake District ice drained first eastward through the Tyne Gap, and then westward into the Irish Sea through the Solway Firth, before retreating southward again during retreat from the Last Glacial Maximum (LGM) (Evans et al., 2009; Livingstone et al., 2010a, b, 2012). While the Kendal–Lancaster drumlin field (east of Windermere) and lineations near Mosedale suggest that the eastern and north-eastern Lake District contributed to the northward flow of ice through the Vale of Eden (Livingstone et al., 2010b, 2012; Hughes et al., 2010), drumlin fields near Barrow-in-Furness and Grange-over-Sands show that at least part of southern Lake District ice fed directly into the Irish Ice Stream through Morecambe Bay (Hughes et al., 2010).
In Windermere, valley glaciers entered the North Basin from Great Langdale, Grassmere/Stock Ghyll and Troutbeck in the north (Figs 1 and 2). While the western slope of the North Basin steeply rises to a significant topographic high (c. 300 m), the eastern side is low-lying and drumlinized east of Bowness-on-Windermere (Fig. 1c; Hughes et al., 2010), suggesting that these glaciers divided at the southern end of the North Basin. Only part moved south over the remnant bedrock high around Belle Isle into the South Basin, whereupon it confluenced with ice from Esthwaite Water entering near Cunsey Beck. A drumlin field (Hughes et al., 2010) and recessional moraines (Gresswell, 1952; Clark et al., 2004; Evans et al., 2005) in the lower Cartmel Valley show that ice from the Windermere valley formed part of the southerly drainage through Morecambe Bay.
Deglaciation of the BIIS from its maximum extent (c. 26–21 ka BP) proceeded rapidly (e.g. Scourse et al., 2009). Locally, 14C dates of organic sedimentation in Windermere (14 623 ± 360 14C BP; Coope and Pennington, 1977: calibrated by Vincent et al., 2010 to 17.7 ± 1.0 cal ka BP) and 36Cl exposure dating from Lingmell Coll (750 m OD; 17.3 ± 1.1 ka BP; Ballantyne et al., 2009) suggest the southern Lake District could have been ice free before 17.0 ka BP, and pollen records from lake cores (Pennington, 1943, 1977; Coope and Pennington, 1977) prove the Lake District was ice free prior to the YD (12.9–11.7 ka BP; Golledge and Phillips, 2008). The YD readvance caused a brief return of cirque and valley glaciation in the high fells (Sissons, 1979, 1980; Wilson and Clark, 1998, 1999; McDougall, 2001; Brown, 2009; Fig. 1b), but in England the readvance was less extensive than in Scotland (Chiverrell and Thomas, 2010).
Terrestrial evidence of the YD glaciation is abundant in the Lake District. Frontal/hummocky moraines, meltwater channels and peri-/para-glacial features afford a robust reconstruction of YD ice extent (e.g. Sissons, 1980; Wilson and Clark, 1998, 1999; McDougall, 2001; Clarke and Wilson, 2004; Wilson, 2005; Wilson and Smith, 2006; Brown, 2009; Fig. 1b). In contrast, there is a paucity of identifiable and/or dateable landforms that can be clearly correlated with local BIIS retreat, especially in the central and southern Lake District (e.g. Clark et al., 2004; Hughes et al., 2010). This lack of recessional moraines has led authors to propose that rapid climate amelioration produced extensive in-situ downwasting (e.g. Hollingworth, 1951; Pennington, 1978). However, recent studies have identified recessional moraines beyond the agreed YD limit in upper Eskdale and south of Windermere (Fig. 1c; Wilson, 2004; Clarke et al., 2004). If these were formed by still-stands/small readvances of decaying valley glaciers during a late stage of the BIIS, then they indicate active glacial retreat and the nature of regional BIIS ice-loss requires re-evaluating.
Over 150 km of 2D MCS reflection profiles provided a grid (50- to 200-m line spacing) of very high-resolution (c. 0.5-m vertical; metre-scale horizontal) intersecting lines, suitable for pseudo-3D structural interpretation (Fig. 2). A catamaran-mounted Boomer source and 60-m multi-channel streamer were used, augmented by single-channel Boomer/Chirp profiles and a 100 × 400-m decimetre-resolution 3D seismic volume (Vardy et al., 2010).
Multi-offset data acquired with the 60-channel streamer provided two advantages. First, data could be migrated into geometrically correct depth sections, improving the horizontal and vertical resolutions and providing accurate package thicknesses. Secondly, analysis of variations with offset allowed the construction of seismic velocity models that provided an additional layer of information for discriminating between ice-contact structures (a proxy for the variations in consolidation of the seismic facies; see Supplementary Information).
Previous work on Windermere
Windermere, nearby Esthwaite Water, and the surrounding tarns have a long history of scientific study, including: coring (Pennington, 1943, 1975, 1977, 1978; Smith, 1959; Holmes, 1964; Coope and Pennington, 1977); terrestrial seismic profiles (Wilson, 1987); and pinger profiles (Howell, 1971). The core archive is extensive and describes a generic sedimentary succession for Windermere (Pennington, 1943, 1977; Coope and Pennington, 1977). The base consists of boulder-clay, sand and gravel-rich deposits fining upwards into varved clays during the Lateglacial, overlain by organic-rich lacustrine sedimentation from the Windermere Interstadial (Allerød). During the YD, sedimentation reverted to being inorganic and clay-rich (extensively reworked), followed by organic, lacustrine Gyttja during the Holocene. These transitions between inorganic/organic sedimentation can be identified in pollen records (Pennington, 1943, 1977; Coope and Pennington, 1977) and dated using early 14C dates (Coope and Pennington, 1977: calibrated by Vincent et al., 2010).