Fan deltas at Buttermere and Crummock Water, Cumbria

[NY 14 18]–[NY 14 19], [NY 15 17]–NY 15 19], [NY 16 16]–[NY 16 19], [NY 17 16]–[NY 1717], [NY 18 14]–[NY 18 15], [NY 19 15]

Potential GCR site

Highlights

The site includes excellent examples of postglacial fan deltas (Figure 4.6) and (Figure 4.7).

Introduction

Where steep mountain catchments issue into Buttermere and Crummock Water, there are excellent examples of postglacial fan deltas. Each one has two parts: a submerged delta and an exposed alluvial fan. The area proposed includes a range of fan sizes, from small steep forms such as that at Comb Beck, to the large, low-angle, fan delta at Buttermere Village. Surprisingly, since Hay's (1926) study there has been no detailed work published on their sedimentology, stratigraphy and morphometry, but they offer scope for further study of the geomorphology of British Holocene alluvial fans and fan deltas, and contain in their sediment an as yet unexplored record of postglacial geomorphic events in the Lake District.

Description

The Lake District was intensively glaciated during the Devensian glaciation by ice-cap and then by valley glaciers, creating the classic mountain glaciated mountain topography of the Buttermere area. The lakes Buttermere and Crummock Water were formed as one lake during the decay of the Late Devensian valley glacier. Small mountain streams feeding directly into the lakes have deposited fan deltas during the Late Pleistocene and Holocene, and Mill and Salt Becks deposited the large fan delta at Buttermere Village, dividing the former large lake into the two modern lakes. The individual fan deltas proposed exclude the main headwater of the Buttermere valley because of human modification to the channel, but include others ranging in size from the large Buttermere Village fan delta fed by Mill and Salt Beck catchments, through intermediate-sized features at Gatesgarth, Rannerdale, Scale Beck and Crag Wood to the small steep fan of Comb Beck, fed by a steep bedrock-dominated mountain stream. Also included are steep debris cones and recently active debris flows on the flanks of Mellbreak Mountain above Crummock Water.

Interpretation

The Devensian glaciers had probably melted by c. 13 000 BP, and Buttermere Water formed as one large lake in the Buttermere trough. Small glaciers formed at the head of the Buttermere valley during the Loch Lomond Stadial (Sissons, 1980), but did not advance as far as the lake basin. The lakes therefore preserve a sedimentation record from the earlier decay of the main Devensian glaciers. The regional Late Pleistocene and Holocene vegetation sequence is fairly well-understood (Oldfield, 1963; Pennington, 1970; Gale, 1985), but little is known of the geomorphic sequence. The regional vegetation record and the regional lake diatom sequence (e.g. Haworth and Allen, 1982) suggest a late glacial period of major sediment input, followed by a stable Early Holocene and an increase in erosion rates during the Late Holocene. The fan deltas are important as they offer the potential for linking the lake sediment record with landform development within their mountain catchment areas, in addition to their inherent value as the best developed suite of fan deltas in England. Little is understood of sediment transport by Lake District mountain streams. Only one recent study (Newson and Leeks, 1985) deals with sediment transport in the Buttermere area, but the fan deltas offer the potential for relating sediment transport to fan delta sedimentation.

The Buttermere and Crummock Water fan deltas are the best developed suite of such features in England. Despite the lack of recent work on their geomorphology they provide the potential for further understanding of Holocene alluvial fans and fan deltas, as well as containing a record of sedimentation since the Late Pleistocene, that might allow a link to be made between lake sediment sequences and erosional and depositional landform development in mountain catchments.

Conclusions

Buttermere and Crummock Water exhibit an excellent suite of fan deltas, formed over the period since the Late Pleistocene. They include a wide variety of sizes and morphologies, as well as providing potential sites for further investigation of the Holocene landform sequence in the Lake District.

References