The next part of the RSW climbs to the top of Tolsford Hill, the highest named hill in East Kent, at an elevation of 181 m AOD (above sea level). The route then passes onto the Chalk of the North Downs.
The British Geological Survey (Aldiss et al., 2012) has published a document explaining how to recognise the boundaries of the Chalk Formations by noting the nature of the change in slope at their boundaries and the type of “Brash” (stones brought to the surface by weathering and agriculture). That document advises that, to identify a change in slope, it may be necessary to lie prone and look along the slope at ground level, which would presumably require several changes of position before succeeding. This is probably beyond the enthusiasm of the average walker following the RSW, so Formation boundaries will only be noted if the changes in slope are easily seen.
As described in the previous section, the Chalk begins at the point where the slope gradient increases, extending up to Etchinghill. The lowest part of the Chalk Group is the West Melbury Marly Chalk Formation, one of two formations comprising the Grey Chalk Subgroup. It underlies the village of Etchinghill, with a finger extending northwards along the East Brook stream towards North Lyminge.
The point where the route crosses the road on the south side of Etchinghill is also the boundary between the West Melbury Marly Chalk Formation and the overlying Zig Zag Chalk Formation (named after a former exposure in a road called Zig Zag Hill near Shaftesbury). This is the top part of the Grey Chalk Subgroup. The Zig Zag Chalk forms the bedrock beneath most of Tolsford Hill, and the outcrop extends northwards along the bottom of the Elham Valley beyond Lyminge. It is mostly firm, pale grey to off-white blocky chalk, with a lower part containing alternate layers of marls (clayey chalk) and firm white chalk. It can be regarded as a transition between the underlying grey clayey, sandy chalk and the overlying typical white chalk.
At the end of the steep climb near the small metal gate, the reduction in gradient coincides with the mapped boundary between the Zig Zag Chalk and the overlying Holywell Nodular Chalk Formation, which underlies the top of Tolsford Hill. The latter is the lowest formation in the White Chalk Subgroup, which forms the majority of the Chalk Group and comprises seven formations, although the two highest are missing in east Kent. The complete White Chalk sequence spans the period from about 94 to 66 million years ago.
Beyond the metal gate, the route follows the northern edge of a typical chalk coombe (a steep-sided, dry valley with a steep headwall). Many of the chalk valleys follow fault lines, and the abrupt change in direction to the south-east in this coombe coincides with the mapped position of a fault running north-west to south-east (Photo 1).
A south-west view from the top of Tolsford Hill allows you to see the easternmost part of the eroded anticline’s core, mentioned in the previous section. This is the sea cliff at Fairlight, visible beyond the Dungeness peninsula.
Tolsford Hill and many of the tops of the North Downs are capped with the Clay-with-flints Formation. The clay promotes the development of topsoil and more substantial vegetation than the grass found on those parts of the downs directly underlain by chalk. This distinguishes the more tree-covered North Downs from the grasslands of the South Downs, where the Clay-with-flints deposit is much less abundant.
The British Geological Survey classifies the Clay-with-flints within the “Superficial deposits” category, yet it is much older than the other superficial deposits, which date from the Ice Age. The Survey places its formation between 23 million and 2.6 million years ago in its Lexicon of Named Rock Units, and on its digital maps between 23.03 million and 11.8 thousand years ago (the end of the last glaciation). The warmer climate at that time induced chemical weathering of the rocks. The Clay-with-flints is a residual weathering product of that process.
The normal Clay-with-flints Formation is a stiff, red-brown to chocolate-brown sandy clay with abundant flint nodules. At the base of the deposit, the clay’s sand content disappears, leaving a dark brown to black clay with black-stained flint nodules.
On Tolsford Hill and elsewhere along the escarpment crest between Folkestone and Wye, there are isolated deposits of Sand In Clay With Flints. This comprises reddish brown clay or sandy clay with abundant flint pebbles, iron-rich sandstone, and rare sarsen sandstone. The sarsen sandstone (silcrete) is another residual deposit formed by chemical weathering. It is a silica-cemented sandstone, rather than the more usual calcite-cemented variety, and is therefore unusually durable. It may be more familiar as Sarsen Stone found on the Wiltshire Downs (see the entry for Clay-with-flints Formation in ‘Rock types encountered along RSW’ and ‘Local Sources of Ironstone’ for more information on this sandy deposit). An example can be seen on the surface beyond the large wooden gate near the top of the hill (Photo 2) and thrown up in mole hills near the top of the coombe (Photo 3).
The descent from Tolsford Hill marks the start of the next section of the RSW, The Elham Valley. As you descend, note the distinctive broad fan-shaped top of the valley.
Continued in the section called “The Elham Valley”.
Andrew Coleman
Rev. 14/010/2026
Reference:
Aldiss, D. T., Farrant, A. R., & Hopson, P. M. (2012). Geological mapping of the Late Cretaceous Chalk Group of southern England: A specialised application of landform interpretation. Proceedings of the Geologists’ Association, 123(5), 728–741. https://doi.org/10.1016/j.pgeola.2012.06.005
