Langstrathdale, Cumbria

[NY 26 09]–[NY 26 13], [NY 28 12]–[NY 28 13]

Potential GCR site

Highlights

The site comprises a large river with alternating bedrock and alluvial reaches, typical of mountain environments (Figure 4.2) and (Figure 4.3).

Introduction

The valley exhibits typical features of a 'young' Holocene fluvial system adjusting to Late Pleistocene glacial erosional topography. The area was intensively glaciated during the main Devensian glaciation, first by ice-cap glaciation, then by valley glaciers moving down Langstrathdale and the Greenup Valley. Loch Lomond stage glaciers occupied the head of the Greenup Valley and Langstrathdale as far down valley as Blackmoss Pot (Sissons, 1980). Upstream of this point the valley floor is wholly Holocene, whereas lower downstream and on the lower part of the Greenup Gill the postglacial features relate both to the Late Pleistocene and the Holocene.

There has been no recent published work on this valley, but it includes a wide range of fluvial features typical of a 'youthful' mountain river.

Description

Three steep valley-floor segments alternate with gentler, former basin reaches. In the steep segments the modern channel is cut in bedrock and the detailed channel morphology exhibits rock-controlled small gorges and waterfalls. The intervening basin reaches have been characterized by aggradation during the Holocene. Within the uppermost of these aggrading reaches, the modern channel exhibits a well-developed braided channel pattern. Lower downstream the alluvial reaches demonstrate a rapid downstream decrease in sediment size with distance away from each bedrock reach. Also included are Late Pleistocene to Holocene slope forms, incised gullies with alluvial fans/debris cones at their bases.

On the upper part of the Langstrath Beck, above Tray Dub, the channel is cut in bedrock, and exhibits immature erosional features: small gorges, waterfalls and an irregular profile. Between there and Blackmoss Pot, Holocene alluviation has filled a glacially scoured rock basin, initially presumably by Late Pleistocene proglacial outwash during recession of the Loch Lomond glacier. The modern channel is a well-developed braided channel with multiple bars and islands. At Blackmoss Pot the channel crosses a rock bar, which Sissons (1980) interpreted as marking the Loch Lomond stage glacial limit. Below the steep bedrock reach is another alluvial reach, presumably an earlier out-wash-filled rock basin, before the channel enters a steep gorge at Johnny's House. From here to beyond the Greenup Gill confluence the channel is confined by the gorge, and includes several waterfalls before entering the bouldery alluvial reach of Stonethwaite Beck beyond Galleny Force.

Each of the alluvial reaches demonstrates a marked decrease in sediment size away from the bedrock outcrops. In addition to the channel features, these valleys exhibit postglacial hillslope gullies with debris cones at their base. These are of uncertain age.

Interpretation

Langstrathdale Beck is a typical 'youthful' mountain river exhibiting an excellent suite of fluvial landforms characteristic of an immature river system adjusting to a recently deglaciated landscape. Although there has been no research on either the Holocene geomorphic sequence or on the contemporary channel, the valley is an_, excellent example of its type.

Conclusions

The valleys of Lanstrathdale Beck and its tributary Greenup Gill contain a fine suite of mountain fluvial landforms, representative of steep, immature mountain river systems. These channels alternate between steep bedrock-controlled and alluvial reaches. The hillslopes exhibit a series of, as yet undated, postglacial gully and debris cone forms.

References