There is an oddly aligned dry valley north of Postling. It runs down eastwards parallel to the crest of the escarpment into the Postling Gap where it makes a right-angled turn to the south (See photo 1). The oddity here is that the stream did not take the more direct route from its origin, down the adjacent scarp slope rather than running parallel to it for a considerable distance (about a kilometre).
As well as its odd alignment, this valley is shallower with more gentle slopes than is usual for the upper ends of those valleys which originate close to the Chalk escarpment. The form of those valleys is attributed to the effects of melting permafrost, but the shape of this valley is more typical of a conventional river valley.
British Geological Survey digital maps Geoindex and BGS Geology Viewer (viewed 2025) show Alluvium along the base of this valley which suggests that at one time it contained flowing water. The printed sheet map, published 1966 re-print 1990, also shows Alluvium, although maps from Digimap Geology show Head along the bottom of this valley. But whichever, Head can accumulate in valley bottoms. (The Survey seems to use the term interchangeably, apparently naming old Alluvium as Head – see the abrupt boundary between the two deposits shown in the East Brook valley on the southern side of Lyminge – TR 16357 40755).
One explanation of the odd alignment may be found in the scenario outlined in “Some origins of Chalk Downland landscape” section 2.2, “The effects of uplift on drainage in south east Kent.” That describes how early rivers flowed down the north side of the upward flexing Weald-Artois anticline, before the current east-west escarpment had fully developed. These rivers were joined by tributaries flowing east and west down the sides of the valleys. Perhaps this oddly aligned valley was formed by one of those early eastward flowing tributaries. It did not flow southwards down the scarp slope because it wasn’t there.
The Geological Survey Memoir for Canterbury and Folkestone “The Memoir” (Smart et al., 1966) includes an alternative explanation for this right-angled turn. It explains it in terms of stream capture changing the course of what was originally a head water of the ’Little Stour’ (The Nailbourne). Follow this link to Examples of stream capture?
There is a second valley west of the head of the shallow valley referred to above which has the form of a typical coombe. The top part of that valley aligns with the shallow valley to the east but then turns abruptly south, cutting through the adjacent scarp slope (Photos 2 – 4). The change of course shows that the adjacent scarp slope was present and influenced the direction of its stream. It also implies that this coombe is much younger than the shallow valley to the east.
N.B. The marked difference in the colour of the grass between the base the sides of this valley is only apparent in early spring. This reflects the difference between the underlying chalk and solifluction deposit (“Head”).
Given the proximity of this coombe to the well-researched Devil’s Kneading Trough (Some origins of Chalk Downland, Section 4.3) it is likely to share a similar history, i.e., it is a recent feature, much of it formed during the end of the last glaciation and ice melt.
The British Geological Survey digital maps (Geoindex and BGS Geology Viewer) show the base of this valley to be underlain by “Head”. In these previously glacial tundra environments, Head usually refers to material which is derived from mass flow deposits produced by thawing of permafrosted chalk. Photo 5 shows brecciated chalk and weathered, probably in-situ, chalk exposed in a slump scar in the side of the upper part of the coombe.
There is a similarly aligned coombe on the south slope of Tolsford Hill (Photo 1, Royal Saxon Way, Tolsford Hill). It would seem likely that both coombes were initiated during one of the interglacial periods by streams following the line of joints, faults, or both. The Tolsford Hill coombe follows the alignment of two faults shown on the BGS Geoindex map.
In summary, the coombe and the shallow valley are likely to have been formed by different processes at different times. Confirmation of the nature and age of the sediments in the base of both valleys would go a long way to resolving uncertainty surrounding their histories. Perhaps, it is pertinent to note that in their review Whiteman & Haggart (2018) remark that given the length of time involved (from the late Cretaceous through to the Quaternary), it would be surprising if the origin of chalk landscape features were not the result of more than one process. These two valleys appear to demonstrate this diversity.
Andrew Coleman
15/12/2025
Geomusings 