Anomalous terrain at Dove Crags ‘cirqueform’ and Gasgale Gill asymmetric valley, English Lake District, attributed to large-scale rock slope failure of pre-LGM origins

An unusually bold, cirque-like cavity, above a small, apparently fluvial side valley rich in landscape anomalies, is evaluated as a large-scale (c. 1 km2) rock slope failure (RSF) with possible pre-Last Glacial Maximum (LGM) origins. Although the failed mass is much subdued, field evidence includes dry gullies and a tarnless hollow; a sharp flank scarp and a high cutbank toe suggest reactivation. Indirect support is provided by a pioneering (in Britain) schematic reconstruction of a whole-valley pre-RSF landscape, employing a ‘clone-stamping’ procedure, which resolves the main terrain anomalies. The RSF interpretation is satisfactorily tested by: (1) compatible proxy cavity and slipmass volumes of 35 and 25 million m3 respectively, with the deficit attributed to trimming by an LGM valley glacier; (2) comparisons with other RSFs by area, cavity parameters, and a total extant volume (derived from an inferred balanced long section) of c. 50 million m3; and (3) a proposed ‘zone of crush’ model that allows a large failed mass, c. 100–200 m thick, to descend incrementally by c. 300 m without disintegrating. A landscape paradox is identified whereby the valley now has an asymmetric, quasi-fluvial profile, with till covering the slope below the cirqueform cavity, yet is inferred to have had a pre-RSF glacial form and an LGM glacier. This is resolved by envisaging: (a) that a pre-LGM RSF created a cavity that was exploited by cirque glaciers on at least two occasions; (b) that the RSF was pared back by an LGM valley glacier; and (c) that it subsequently re-descended to the valley floor after the LGM. A pre-LGM origin for the RSF would account for the till-covered slipmass. It also allows time for excavation of the present cirque headwall by 50–100 m, and for the opposite valleyside to erode back by c. 50 m in response to a progressively displacing river and valley glacier. If this proposition is correct, Dove Crags would be one of the first substantial pre-LGM RSFs to be recognized in the glaciated mountains of Britain and Ireland, implying that others ought to be present.一处位于富含地貌异常特征的小型河流侧谷上方、形态罕见独特的近似冰斗状大型凹坑，被评估为一处规模约1平方千米的岩质边坡失稳（rock slope failure, RSF）事件，其起源可能早于末次冰盛期（Last Glacial Maximum, LGM）。尽管失稳体已大幅夷平，但野外证据包括干涸冲沟与无冰斗湖洼地；清晰的侧翼陡崖与高切坡趾部表明其曾发生复活。间接支撑证据来自一项英国范围内首创的、采用"克隆冲压法（clone-stamping）"流程重建全谷史前RSF地貌的示意性重建工作，该方法厘清了主要地貌异常特征。该RSF成因解释通过以下三点得到了充分验证：(1) 匹配的代理凹坑与滑体体积分别为3500万与2500万立方米，体积缺口归因于末次冰盛期山谷冰川的侵蚀削切；(2) 与其他RSF在面积、凹坑参数及由推断平衡长剖面得出的约5000万立方米现存总容积进行对比；(3) 提出的"挤压带（zone of crush）"模型，该模型允许厚度约100–200米的大型失稳体以约300米的幅度渐进式滑移而未发生解体。研究发现一处地貌悖论：当前该山谷呈现不对称的准河流地貌形态，冰碛物覆盖于冰斗状凹坑下方的坡体之上，但推断其在RSF发生前曾为冰川地貌，且末次冰盛期存在山谷冰川。该悖论可通过以下设想得到解释：(a) 末次冰盛期前的RSF形成了凹坑，该凹坑至少两次被冰斗冰川利用；(b) 末次冰盛期的山谷冰川对该RSF进行了削蚀改造；(c) 末次冰盛期后，失稳体再次滑移至谷底。该RSF的末次冰盛期前起源可解释覆盖于滑体之上的冰碛物，同时也为冰斗后壁下切50–100米、以及因河流与山谷冰川持续推移导致对岸谷坡后退约50米提供了时间条件。若该推论成立，多夫崖（Dove Crags）将成为英国与爱尔兰冰川化山区中首批被识别出的大型末次冰盛期前RSF之一，这意味着该类事件在当地应广泛存在。