Sandbeds Fan

[NY 234 291]

J. Boardman

Introduction

Fans are sites of sediment storage. Alluvial fans are an important geomorphological feature in the Lake District landscape. They represent deposition by fluvial and mass-movement processes from hill-slope sources. They generally are located at breaks of slope, or valley widening sites where channel gradient decreases, or width increases. It is reasonable to assume that alluvial fans in the Lake District post-date the Late Devensian glaciation and potentially contain sediments accumulated over a period of ± 16 000 years. However, sediment transport is greatly enhanced under bare ground conditions and it is assumed that major growth of fans will occur in periods of periglacial climate.

The Sandbeds Fan [NY 234 291] is situated at the point where a small stream, Sandbeds Gill, debouches on to the valley bottom. The fan is at the break of slope between a steep Skiddaw Group slope and till-covered lowland, adjacent to Bassenthwaite Lake. A small quarry provides excellent exposures in the fan. The site is described by Boardman (1985b).

Description

The morphometry of the Sandbeds Fan and its catchment is shown on (Figure 7.24), and exposure in the west-facing wall of the quarry is shown in (Figure 7.25). The surface of the fan is totally vegetated with grass, heather and coniferous trees, and the present-day stream is confined to a deep gorge in the northern part and does not contribute sediment to the fan surface. At the base of the quarry, diamicton is exposed and macrofabric analysis shows a strong east–west orientation, suggesting post-depositional solifluction in an area where Late Devensian ice was moving to the north. At one site in the quarry, diamicton also appears to overlie fan gravels, implying solifluction (debris flow activity) during early stages of fan formation.

The sediments exposed in the quarry are predominantly gravels, which vary from fine- to very coarse-grained and discrete beds of sand are rare. The degree of sorting is variable and the amount of interstitial material in the gravels, principally sand, also is variable. A feature of many beds are the silt caps on gravel clasts. The fact that some beds lack silt caps, and underlying beds possess them, implies that the caps result from the washing in of silt during fan aggradation. MI gravel units in the fan contain high proportions of edge-rounded clasts in contrast to scree deposits in the same lithology, which are predominantly angular.

In the main face of the quarry, gravels can be seen to infill a channel of about 11 m width and 7 m depth (Figure 7.25). Units of coarse blocky gravels are seen at the base and top of the channel. The coarse gravels contain occasional large slate clasts up to 0.5 m in length and also glaciated and non-slate lithology clasts. They are poorly bedded and sorted. The margins of the channel are marked by a prominent iron-stained sandy gravel horizon and the high angle of the northern channel wall (35°) suggests rapid channel cutting and filling before collapse. At the north end of the quarry, well-bedded, fine-grained, sorted gravels are exposed, which probably represent flood overbank discharges from the main channel. Directions of flow in the fan gravels are indicated by the rose diagrams in (Figure 7.26) and show flow divergence from the apex of the fan.

At the southern end of the main quarry sec tion a prominent iron-stained sandy gravel horizon occurs, which may represent the margins of a shallow channel. Below this horizon are loose, openwork gravels. The boundary between the two units is involuted, the size of the involutions being almost 1 m. These features represent disturbance within an active layer under periglacial conditions and as such imply the existence of a temporarily stable land surface on part of the fan. The marked contrast in texture between the two units, and the availability of water on the fan surface, would no doubt encourage the formation of involutions (Johnson, 1975). Burial of the involuted horizon occurred owing to flooding from the main channel, or a change in the location of channels on the fan. Involutions have not previously been reported from gravels or scree sediments in the north-eastern Lake District, although similar forms are described in the southern Lake District (Johnson, 1975).

Interpretation

The character of the fan gravels suggests that they originated largely as a result of frost shattering but were subsequently transported by fluvial processes. Present-day fluvial activity within the catchment is confined to the channel of Sandbeds Gill. Under periglacial conditions, however, drainage densities may have been high in response to seasonal snow-melt discharges.

Overland flow on slopes underlain by permafrost also may have been an effective means by which weathered debris was moved to channels. Rapid rates of weathering and transport would have been favoured by the absence of vegetation.

The Quaternary history of the area implies that the fan has formed since the Late Devensian glaciation and became inactive after the establishment of forest cover on the slopes at the beginning of the Holocene Epoch. The volume of the fan (150 000 m³) represents a mean surface lowering within the catchment of 0.5 m, based on the assumption that the fan formed during the c. 1000 years of the Loch Lomond Stadial. This is roughly an order of magnitude higher than present-day yields from steep mountain streams in the Lake District (Newson and Leeks, 1985). The evidence for a single period of rapid aggradation is equivocal, however. The steep sides of the infilled channel imply rapid cut and fill, whereas the involuted horizon suggests a significant intervening ground-surface stability during which permafrost existed. Moreover, a well-developed soil profile would be expected beneath the fan surface if the fan had been inactive since the Loch Lomond Stadial. Soil development is, in fact, very limited, suggesting that the upper part of the fan, above the iron-stained, involuted horizon may well be of late Holocene age.

Conclusions

The Sandbeds Fan site is an excellent example of an alluvial fan. Such landforms are areas of sediment storage and as such offer the opportunity for reconstruction of environmental history during the period of accumulation. The fan is relatively inactive at the present time but was formed largely under Late-glacial periglacial conditions when rates of weathering and sediment transport were greatly enhanced as a result of a colder climate and the existence of bare ground.

References