Abstract

The southwest slope of Ben Attow (NH 010180) features numerous obsequent (uphill-facing) scarplets which are up to 10 m high and 800 m long (Figs 1 and 2). de Freitas and Watters (1973) discussed similar features at Glen Pean, Scotland, invoking obsequent-scarplet formation by toppling. The Ben Attow slope exhibits the largest extent of obsequent-scarplets in Scotland known to the authors. This note examines its mode of instability and the formation of obsequent-scarps.

The slope is composed of coarse Moinian psammites of locally varying metamorphic grade. Joint surveys revealed the presence of four joint sets (Fig. 3): one parallel to the foliation (F), one striking parallel to and dipping into the slope (J1) and two sets that dip gently west (J2 and J3). The scarplets strike subparallel to J1 and terminate westwards at a 1 to 2 m high step-like feature.

Watters (1972) undertook a reconnaisance investigation of the slope and hypothesized that the scarplets were formed by sequential upslope separation and translation of blocks on a failure plane at depth. This was followed by erosion upslope of the tension cracks between displaced blocks and their later infilling with slope debris. However, no slide toe exists at the base of the slope, nor is it likely that there ever was one, since any erosion of a toe would have removed the essentially delicate obsequent-scarplets as well. Further, in situ rock crops out across the base of the slope. The scarplets are less than 30 m apart in places and translation

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