THE QUATERNARY OF THE LAKE DISTRICT

Field Guide

Edited byDerek A. McDougall & David J.A. Evans

2015

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Cover Photograph: Stony Cove Pike, looking towards Brothers Water and Ullswater (D. McDougall).

Produced to accompany the QRA Annual Field Meeting based at Blencathra Field Studies Centre, 21-24 May 2015.

© Quaternary Research Association, London, 2015.

All rights reserved. No part of this book may be reprinted or utilized in any form or by any electronic, mechanical or other means, now known or hereafter invented, including photocopying and recording or in any information storage or retrieval system, without permission in writing from the publisher.

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Adlard Print & Reprographics Ltd., The Old School, The Green, Ruddington, Nottinghamshire, NG11 6HH.

Recommended reference:

McDougall, D.A. and Evans, D.J.A. (eds) (2015) The Quaternary of the Lake District: Field Guide. Quaternary Research Association, London.

ISSN: 0261 3611

ISBN: 0 907 780 164

QRA contribution to The Geological Society’s Year of Mud.

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8Gray Crag and The Knott rock slope failuresP. Wilson

IntroductionGray Crag (698 m OD, NY 427119) and The Knott (739 m OD, NY 437127) are hills within the southeastern sector of the Ullswater catchment, eastern Lake District (Figure 8.1). More specifically Gray Crag is part of the watershed ridge between the valleys of Hayeswater Gill and Pasture Beck (aka Threshthwaite Glen), while The Knott is entirely within the catchment of Hayeswater Gill. Both valleys are in rocks of the Borrowdale Volcanic Group and each held a glacier, of ~3-4 km and ~2-3 km length respectively, during the Loch Lomond Stadial (LLS: 12.9-11.7 ka; Manley, 1959; Pennington, 1978; Sissons, 1980; McDougall, 2013). Each summit can be attained by a reasonably fit hill-walker within ~1–1.5 hours from the car park at the eastern end of Hartsop (180 m OD, NY 410130), although it is not necessary to go as far as the summit of Gray Crag

Figure 8.1. Topography and rock slope failures in the vicinity of Gray Crag and The Knott. For details of backdrop mapping, see Figure 7.2.

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Figure 8.2. A: View south along the ‘split’ ridge of Gray Crag. B: Closer view of the shattered crag, depression and bounding ridge in the middle section of the ‘split’ ridge. C: The hillside embayment (upper centre) on the Pasture Beck flank of Gray Crag with oblique ridges on the lower slope.

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in order to see the evidence for rock slope failure (RSF). The Gray Crag RSF was listed in the database compiled by Wilson et al. (2004), whereas The Knott RSF was identified by the author later that same year.

Gray CragThis medium-sized RSF (0.11 km2) can be reached by leaving the Hartsop-Hayeswater track at NY 422129 and ascending the steep grass slopes that constitute the blunt northern end of the Gray Crag ridge. At 550 m OD (NY 423125) the gradient eases and for the next ~300 m the ground consists of a series of ~crest-parallel depressions and flanking ridges, followed by a pair of wedge cavities invading the ridge axis by 10 m, all with scarps and trenches of 1–4 m amplitude. Most depressions / ridges are grass-covered with little visible rock but one small and shattered crag, now consisting almost entirely of boulders, forms a distinctive marker about half way along the section (Figures 8.2A and B). The depressions are interpreted to result from tensional spreading, that caused slicing of the rock body, with an associated outwards and downslope movement towards the Pasture Beck valley.

This length of ‘split’ ridge forms the crest of a shallow embayment at the top of the slope falling to Pasture Beck. The embayment is bounded to north and south by projecting bedrock outcrops interpreted as flank scarps of a failure scar, both 5 m high but with that to the south being the considerably more prominent. The embayment represents the source scar / failure surface for a large mass of bedrock evacuated from the site (Figure 8.2C). Close to the embayment head and near to its southern limit the smooth grass-covered slope is diversified by three benches each extending ~20 m along slope and up to ~10 m in width. These are mainly extensional deformation features (including a hole), but on the south a distinct slice has slipped 10–20 m downslope.

The slope below the embayment head is generally smooth and steep, with a lack of open water courses. Two typical RSF basal springs 4 m apart at 370 m OD (NY 421124) merge 10 m downslope and form the only stream on this part of the hillside. There are no apparent accumulations of RSF run-out debris or other materials with marked surface expression until at ~300 m OD several broad ridges materialize and trend obliquely downvalley (Figure 8.2C). These have been mapped as LLS moraines (Sissons, 1980; McDougall, 2013) and this is a reasonable view given their morphology, alignment, and the presence of other undoubted moraine ridges and mounds immediately across the valley floor.

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A question to be posed is: to what extent, if any, do the oblique ridges at the foot of the embayment consist of morainised RSF debris? There are no exposures to assist in resolution of this issue but it is important with respect to understanding:

1. the contribution that RSF debris might have made to LLS moraines (not only at this site but at other locations occupied by LLS ice).

2. the age of the Gray Crag RSF.

With regard to the first point, the disproportionate size of some LLS moraines relative to the size of their glacier and the timescale involved has been noted in the past and has sometimes been explained by LLS glaciers reworking and incorporating tills deposited by earlier retreating valley glaciers. This is likely to have happened. Equally, RSF could have made a significant contribution (Ballantyne, 2002). In the Highlands of Scotland, apparent RSF scars with no run-out debris below could indicate pre-LLS events with debris removed by LLS glaciers (Ballantyne, 2013b). Similarly, RSF deposits segueing into lateral moraines can be observed, e.g. Glen Ey (Deeside), with RSF source areas above likely LLS limits, while valley-floor ‘pods’ of coarse angular debris are often found short distances down-ice from RSFs, e.g. Glen Kingie (D. Jarman, pers. comm.).

With regard to the second point, the absence of RSF run-out debris at the foot of the Gray Crag embayment indicates that the failure that created the embayment must pre-date the Holocene (although the ‘split’ ridge and benches near the top of the embayment may represent Holocene reactivation). It is possible that the embayment is a product of pre-Last Glacial Maximum (LGM: ~26-21 ka) RSF with the debris having been removed during the LGM and the ice having protected the southern (north-facing) flank scarp and eroded (subdued) the northern (south-facing) scarp. Under this scenario the ‘split’ ridge and embayment benches could have occurred at any time post-LGM.

The KnottThe Knott is most easily reached by following the track from Hartsop to Hayeswater and then crossing the outflow and ascending the clear path ahead. The path passes north of the summit then turns south to cross the neck that separates The Knott from Rampsgill Head and the High Street upland. This neck is notched by a conspicuous ravine descending SW to Hayeswater. The summit of The Knott is about 100 m west of the neck alongside a ruined wall.

Distant indication of RSF on The Knott can be seen from the summit of Gray Crag across the valley to the southwest. Just below the summit of The Knott on

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Figure 8.3. A: The Knott as seen from Gray Crag. The arrow indicates the upper surface of the main slipped mass. B: Shallow depression (0.5 m deep) and bounding ridge on southwest slope of The Knott. C. Boulder-filled depression and sinuous grass-covered ridge on southwest slope of The Knott.

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its west side at 710-720 m OD (NY 436127) is a broad (~30 m) gently-sloping vegetation-covered bench marking the upper surface of a mass of hillside that has descended ~10-20 m (Figure 3A). The base of the outer slope of the failed mass is ill-defined on a hillside continuously steep for ~250 m and characterised by low shattered outcrops, scree patches and debris-slide scars and tracks.

At the south end of the bench and passing around the southwest arc of the hill, and rising to slightly higher levels, are a few narrow sinuous ridges and depressions (Figures 8.3B and C). The depressions are shallow and ridge heights

Figure 8.4. The ravine, colluvial fan-delta and some of the LLS moraines, Hayeswater.

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low (<1.5 m) and are, in part, littered with boulders derived from outcrops on their upslope sides. The ridges and depressions follow the curvature of the hillside as it trends east towards the head of the ravine at NY 438126. Low ridges / shallow depressions occur on the south side of the ravine at a lower level.

The ravine descends steeply from the neck at 720 m OD to ~500 m OD where the bed gradient slackens and the enclosing flanks diminish as the lower valley side is reached. From the foot of the ravine a colluvial fan-delta extends to the shore of Hayeswater and below the waterline (Figure 8.4). With a radius of 300 m, at least 80 m in height, and extending over 0.07 km2, this is an unusually large fan-delta for the Lake District. It bears comparison with that debouching from Gasgale Gill, with its large RSF (Jarman and Wilson, 2015), which is 40 m high but 500 m in radius.

The fan is contained on its northern side by moraines of the LLS glacier; while in its centre and along its southern edge, debris has accumulated around moraines, and has possibly buried some, all indicating a post-LLS age for its present state.

The question posed by the fan-delta is: to what extent has RSF around the head of the ravine acted as a source of debris? The ridge / depression topography around the ravine head suggests cross-slope continuity of slope weakness; significant rock-slope collapse may have occurred with subsequent fluvial reworking and debris flow as likely causes of onward debris transport.

Although the relationship between the present fan-delta and the moraines indicates a Holocene age for the former, ravine erosion is likely to have been intense during valley glacier decay following the LGM and again during the Windermere Interstadial. As with Gray Crag, RSF debris from The Knott ravine and its precursor fan-delta is likely to have contributed to the LLS moraines downvalley of the fan-delta.

AcknowledgementsThe comments and suggestions made by David Jarman on the initial version of this account are much appreciated.

ISSN: 0261 3611ISBN: 0 907 780 164