User:Aliveness Cascade/Holmesdale

Britain's Structure and Scenery, L. Dudley Stamp.
Between the greensand cuesta and the scarp-face of the Chalk of the North Downs there is a usually narrow valley known in parts of Kent and Surrey somewhat redundantly as the Vale of Homesdale (or Holmesdale) or, more simply, as Homesdale. It coincides broadly with the outcrop of that dark clay, the Gault, though the southern part may be partly on the soft Folkestone Sands and the northern part is obscured by hill wash from the Chalk. Along the northern face of the South Downs the Lower Chalk tends to form a step or terrace, so that the change from the Downs to the Gault vale is less abrupt and the latter appears to be less marked.

Not only is the Upper Greensand thin; in the eastern part of the Weald it is rarely seen and makes no separate feature. At the western end of the Weald, the Malmstone gives rise to a cuesta of its own and it was here that Gilbert White noted and described a feature typical also of the Lower Greensand—the sunken roads which are the result of centuries of wheeled traffic powdering the rock and permitting it to be removed by wind and rain. These sunken roads are typical of loess covered chalk lands across the Channel in northern France: they occur locally and less fully developed in the chalk lands of south-eastern England.

Geology of London and South-East England, G. M. Davies.
The Gault stretches inland from Folkestone, forming a vale between the Chalk escarpment and dip-slope of the Lower Greensand. This is really a succession of valleys cut by subsequent streams, except where broken by obsequent streams as at Oxted. In the neighbourhood of Reigate it as known as the Vale of Holmesdale. The switchback form of the vale, distinguishing it from a valley, may be noted on a railway journey from Reigate to Guildford: the line goes up to Betchworth, down to the Mole, up to Wotton, and down to the Wey. The Gault outcrop carries many streams and ponds, in contrast to the dry, permeable beds to the north and south. It is mainly pasture land, with fine trees, the marly Upper Gault giving a more fertile soil than a pure clay. The thickness increases westward from Folkestone to about 200 feet; at Caterham it reaches 343 feet.

Many small pits have been opened in the Gault for the manufacture of bricks, tiles, drainpipes, etc. A few are still in work. The Cheriton brickfields near Folkestone are very fossiliferous. Upper Gault is worked at Burham and Aylesford, in the Medway Valley. The best exposure of the Gault near London is close to Dunton Green Station, where some 30 feet of Upper Gault is separated by a layer of phosphatic nodules from 15 feet of Lower Gault. Fossils are abundant here. The phosphatic nodules of the mammillatum zone are seen at the top of some sand pits in the Folkestone Sand in Surrey. There is a small brickyard in Gault at Albury and a pottery at Wrecclesham.

=== Geology of the Country Round Sevenoaks and Tonbridge, H. G. Dines, S. Buchan, S. C. A. Holmes, and C. R. Bristow ===

Pages 2 to 4
Between the Chalk and Lower Greensand escarpments lies the Vale of Holmsdale, about four miles wide, its northern slopes being the scarp face of the North Downs and its southern the dip-slope of the Lower Greensand, Gault lying on the floor.

The greater part of the map area (Figure 2) drains into the Medway catchment with that of its tributary the River Eden. These rivers drain the whole of the area south of the Lower Greensand escarpment, as well as the eastern and western parts of the Vale of Holmsdale. The central part of the the Vale of Holmsdale, however, constitutes the head of the River Darent catchment, which includes the downs above Kemsing. This river flows northward into the Thames Valley through the Darent gap at Otford. The Chalk downs of the north-west corner of the district are traversed by a number of dry coombes within catchments of the rivers Wandle, Ravensbourne and Cray. The distribution of the watersheds is shown in Fig. 2

Although the Darent probably originated as a normal consequent stream, the headwaters of which, within the Weald, were captured by the Medway, the encroachment of the Medway system into Holmsdale, both on the west (Gibb's brook, near Limpsfield) and on the east (River Shode, near Ightham) was probably influenced by Head deposits that for a period may have completely blocked parts of the Vale of Holmsdale and interfered with its normal drainage. Trains of later Head on the Weald Clay, particularly southward of Limpsfield, show that the Medway had already lowered its valley well towards present base-level at the time of their formation (Dines, Hollingworth and others 1940, p221). Further erosion finally dissected the original fans.

on Angular Chert Drift
Like the Clay-with-flints, the Angular Chert Drift extends in general down to about 450 ft O.D. and is dissected by the dry valleys and coombes of the dip-slope.. Being one of the older drifts of the area (Dines, Hollingworth and others 1940, p. 220) it has contributed to a large extent to the later head deposits on the Weald Clay and in Holmsdale (p. 121).

on Petrography of Pebbles in the Limpsfield Gravel
[Selected specimens examined, in thin section and by heavy mineral concentrations, came from east of Limpsfield, on the northern side of Limpsfield Common.]

It seems likely that the pebbles examined came from a general westerly direction along the Vale of Holmsdale, thought this westerly source for the Limpsfield gravel is at variance with the views of Gossling (1963, 1937, 1941) upon drainage in the Vale of Holmsdale. He claimed that pebbles of siltstone and ferruginous mudstone found at Limpsfield, Westwood and Westerham are of Wealden derivation, a view which he recognized as conflicting with the observations of Topley (1875, pp. 188, 295), who had stated emphatically that the Darent deposits contain no pebbles of Wealden sandstone and had recoded none from Limpsfield.

Page 121
on Head Deposits on Weald Clay

[Head deposits on the Weald Clay are divisible into three groups on the twin basis of composition and geographical distribuiton (Dines and other, 1940 p. 121).]

The third division [of head deposits] includes patches of subangular battered and pitted flints, cherts, sandstones and occasional well-rounded flint pebbles. Similar flints are to be found in the Vale of Holmsdale where it is also possible to match the pebbles.

on Scarp Drift and Downwash
Prestwich suggested that torrential streams in late Glacial time may have accounted for much of the debris from the scarp in the 'Shode drift-gravels', the rest being derived from the remnants of an old unstratified gravel on the Gault in the Vale of Holmsdale.