Late Quaternary evolution of a large rock slope deformation (Robinson, English Lake District) - a first such exploration by cosmogenic exposure dating in the British mountains

The temporal dynamics of the Robinson rock slope failure (RSF), the largest such feature in the Lake District of northwest England, have been assessed through cosmogenic (10Be) surface-exposure dating of quartz veins on the headscarp/rupture surface and an upper-mid-slope antiscarp. Movement along the headscarp/rupture surface began during the Lateglacial ~ 15.4–14.3 ka, and movement could have continued, incrementally or episodically, until the mid-Holocene ~ 4.6 ka. Displacement rates for the headscarp/rupture surface averaged over that time are in the range 3–4.5 mm/a. These are the first rates to be reported for an RSF in Britain and Ireland. Ages from the upper-mid-slope antiscarp (~ 8–6.5 ka) indicate early-mid-Holocene movement, although 10Be inheritance may have aged these samples somewhat. Reasons for RSF activity at Robinson at those times are complex, but probably involved paraglacial stress release augmented by the concentrated erosion of bedrock in a trough below a glacial breach, along with the structural characteristics of the local bedrock. The cosmogenic ages suggest that other RSFs in this area may have a longer history of instability than hitherto thought.