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 (<em>c</em>. 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 <em>c</em>. 50 million m³; and (3) a proposed ‘zone of crush’ model that allows a large failed mass, <em>c</em>. 100–200 m thick, to descend incrementally by <em>c</em>. 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 <em>c</em>. 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成支谷上方、形态格外醒目的冰斗状凹坑，经评估为大规模（约1 km²）岩坡失稳（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，但野外实地证据包括干涸冲沟与无冰斗湖洼地；清晰的侧翼陡坡与高陡岸坎趾部表明该失稳体曾发生复活。间接支撑该假说的证据来自英国首创的整个山谷区域岩坡失稳前地貌示意图重建工作，该研究采用‘克隆式填绘法’（clone-stamping）流程，成功解释了区域内主要地貌异常。该岩坡失稳假说得到了以下三方面的有效验证：（1）匹配的替代凹坑体积与滑体体积分别为3500万与2500万立方米，二者的体积差值归因于末次冰盛期山谷冰川的削蚀改造；（2）与其他岩坡失稳事件在面积、凹坑参数以及由推断平衡长剖面得出的现存总体积（约5000万立方米）开展对比分析；（3）提出的‘挤压带’（zone of crush）模型可解释厚约100–200米的大型失稳体以约300米的距离渐进式滑移而未发生解体。研究发现一处地貌悖论：现今该山谷呈现不对称的准河成剖面，冰斗状凹坑下方的斜坡被冰碛物覆盖，但推断其在岩坡失稳前曾为冰川地貌形态，且存在末次冰盛期冰川活动。该悖论可通过以下推演得到合理解释：（a）末次冰盛期前的岩坡失稳形成了凹坑，此后冰斗冰川至少两次利用该凹坑发育；（b）末次冰盛期的山谷冰川对该失稳体进行了剥蚀改造；（c）末次冰盛期后，失稳体再次下滑至谷底。岩坡失稳事件起源于末次冰盛期前这一推论，既可解释冰碛物覆盖的滑体，也为现今冰斗后壁下切50–100米、以及受持续迁移的河流与山谷冰川驱动的对岸谷坡后退约50米提供了充足的时间。若该推论成立，则多夫峭壁（Dove Crags）将成为英国与爱尔兰冰川山区中首批被识别出的大规模末次冰盛期前岩坡失稳事件之一，这也意味着该类地貌事件在区域内应有更多未被发现的案例。