Glacial Meltwater Channels of the Stiperstones Area

ISSN 1750-855X (Print) ISSN 1750-8568 (Online)

David Pannett1

PANNETT, D. (2010). Glacial Meltwater Channels of the Stiperstones Area. Proceedings of the Shropshire Geological Society, 15, 17–19. Consideration of recent contour maps reveals that glacial meltwater channels in the Stiperstones and Long Mynd areas have elevations compatible with an ice margin sloping from west to east cutting across SW-NE ridges where they would have dammed water in the heads of the valleys, forcing some flow across cols in the main watershed. Such channels suggest that ice once lapped around the Stiperstones to a height of at least 330 m.

1Bicton, Shropshire, UK. E-mail: [email protected]

BACKGROUND THE SHROPSHIRE HILLS One thing the author soon discovered about It is well known that the Shropshire Hills upland Ordnance Survey maps when first teaching in the area encompassing the Stiperstones and Long Shropshire Hills, was the unreliability of some Mynd stood above the glacial ice which filled the contour lines. Each 100 foot line (ca. 30 m) had surrounding lowlands during the last “Ice Age” been accurately surveyed up to 1,000 feet (ca. 330 (the Devensian). m), but those between and above had only been Tors and blockfields still crown the highest estimated from spot heights (Anon., 1953). All ridges, while the Long Mynd Plateau is covered credit must go to the original surveyors with their with locally derived “head” (Goudie & Piggott, basic equipment, but some details were missed out. 1981) (Figure 1). At lower levels, the distribution Contouring can now be achieved by of glacial till (formerly known as “boulder clay”) photogrammetry. A contour interval of 5 m is should offer some clue to the actual ice levels. achievable, revealing fantastic detail, initially on However, the BGS surveyors admit that some of the 1:50,000 Pathfinder Series (Anon., 1994a, b, c, this on the recently published maps could in fact d) and, even more clearly, on the 1:25,000 be solifluction debris or even lake clay deposits Explorer Series (Anon., 1999a, b, c). Features now laid down in valley systems dammed by the revealed more clearly include several small lowland ice (Cave, 2008). meltwater channels cut into the hills around the Stiperstones and Long Mynd.

Figure 1. Sketch of river and valley profiles drawn from map contours of the Northern Longmynd upland area. Channel patterns and gravel deposits give clues to former glacial ice levels.

ISSN 1750-855X (Print) ISSN 1750-8568 (Online)

Laminated lake clays have been observed around channels crossing from west to east and usually Bridges in the Upper Onny Valley (Pocock et al., just below steps in the ridge profile. 1938), but later surveyors have played down their Such channels at the Pontesbury end of the significance because they appeared to contain Stiperstones are to be described in the some pebbles (Greig, 1968). They did not realise forthcoming Geotrail for Poles Coppice (Anon., that dirty “mini” icebergs can float around such 2009), at Lords Hill [SJ 394 051], Granhams lakes and drop debris. Regardless of their origin, Moor [SJ 385 036] and Nills Hill [SJ 394 051]. such clays were much appreciated by local Their waters would have entered the Habberley builders for brick making (Pocock et al., 1938), Valley where gravel deltas and lake clays suggest in spite of such pebbles. As a result of these lakes at various levels governed by outlets in the interpretations, the most recently published ridge beyond [SJ 415 032]. The lowest of these memoir covering the Long Mynd (Greig, 1968) has left the Habberley Brook permanently invokes almost impossible tongues of ice diverted around Earls Hill and, at one stage, it fed penetrating the uplands in order to explain the water via another channel [SJ 426 060] to mapped distribution. Nevertheless, the mapped Longdon. All these channels may have brought distribution of glacial moraine in the head of the “Late Glacial Flood Gravels” to the Lyth Hill Kinnerton Valley [SO 369 964] raises the (Dorrington) area. possibility of a local east-facing “lee side snow On the south side of the hills, a similar group patch” mini glacier. with successively lower elevations cut the ridges of volcanic rock outcropping around Hyssington. Those at the foot of Corndon Hill cross an GLACIAL MELTWATER CHANNELS extension of its dolerite and, perhaps for this The network of glacial meltwater channels around reason, they appear to have developed waterfalls the Long Mynd has since been mapped and [SO 325 656]. One such chanel cuts the volcanic considered by the BGS during their mapping of rocks at Hyssington Castle and may easily be the Shelve Special Sheet (BGS, 1991) and the mistaken for a massive man-made defensive ditch Montgomery Sheet (BGS, 1994; Cave & Hains, [SO 324 946], while another may have formed a 2001). waterfall at SO 323 951. Whilst covering similar areas to Pocock, the A similar set of features occurs around BGS surveyors were able to recorded many more Norbury [SO 370 934] and Wentnor [SO 303 channels, but missed some that are detectable on 923]. At Myndtown water has cut quite dramatic the latest generation of OS contour maps. For channels at the base of the Long Mynd fault scarp instance, Marrington Dingle (below Churchstoke) [SO 391 895]. and the Hope Valley show up on maps because of their size but it is the smaller ones, about the size of a railway cutting, that appear to have been CONCLUDING REMARKS overlooked. Taking the evidence as a whole, these glacial First of all, one needs to realise that ideas meltwater channels have elevations compatible about glacial meltwater channels have changed with an ice margin sloping from west to east from during the 20th Century. Once they were thought Hyssington to the Plowden area. All these to indicate actual ice margins or the spillage of channels suggest that ice once lapped around the lake water across cols. While this may still be the Stiperstones to a height of at least 330 m (ca. case with some, it is also realised that they can be l,000 feet), where it would have dammed water in the products of water within the ice encountering the heads of the valleys, forcing some flow across the uneven rock surface; some can have humped cols to the main watershed. For instance, at profiles (Sissons, 1960; 1961). Water trapped by Leazous Bank [SO 401 996], such impounded ice can also escape during brief catastrophic water has cut a notch at an elevation of 325 m, but events. Either way, the major ridges projecting the flow direction is not obvious. into the lowlands, due to the “Caledonian trend” Furthermore, did northward-spilling water cut of the rock outcrops, have become scarred by the channel below Gatten [SJ 386 001] without cutting a notch in the 355 m col? Perhaps clean

GLACIAL MELTWATER CHANNELS OF THE STIPERSTONES AREA water spilt from a lake could not have eroded here Anon. (2009). GeoTrail for Poles Coppice. as effectively as sediment-charged glacial Shropshire Geological Society. [in press] meltwater? If not, where did all the water come BGS (1991). Map for 1:25,000 Geological from to explain this strange feature? Did water Sheet SO29, 39, SJ20, 30 Shelve. British also enter this East Onny lake system via a Geological Survey, Keyworth. channel at 340 m by Cold Hill Farm [SO 362 BGS (1994). Map for 1:50,000 Geological 963]? At about the same altitude water may have Sheet 165 Montgomery. British Geological crossed the West Onny watershed near Shelve Survey, Keyworth. [SO 343 991]. Cave, R. & Hains, B.A. (2001). Geology of the Significantly, it is only above the general level country around Montgomery and the of these channels that tors and blockfields suggest Ordovician rocks of the Shelve area. an ice-free periglacial environment. Actual tors Memoir for 1:50,000 Geological Sheet 165 only survive above 400 m, but even near this with part of Sheet 151 (Welshpool). HMSO, altitude there is still a hint of a channel crossing London, 160 pp. the col between the Bog and the Knolls [SO 352 Cave, R. (2008). Geology of the Welshpool 975]. In the field its two branches are almost at a District. Brief explanation of 1:50,000 scale of a man-made feature. Contours suggest a Geological Sheet 151 (Welshpool). British similar narrow channel across a lower col near Geological Survey, Keyworth. Bromlow Callow [SJ 321 009]. Goudie, A. & Piggott, N. (1981). Quartzite Clearly the contour patterns are helping an tors, stone stripes and slopes at Stiperstones, understanding of the landscape while also raising Shropshire, England. Biuletyn many questions. This would seem a good time to Peryglacjalny. 28, 47-56. take a closer look in the field. Greig, D.C., Wright, J.E., Hains, B.A. & Mitchell, G.H. (1968). Memoir for one-inch Geological Sheet 166 (Church Stretton, REFERENCES Craven Arms, Wenlock Edge and Brown Anon. (1953). First Series 1:25,000 Clee). HMSO, London, 379 pp. topographic map Sheets SO39 and SJ30. Pocock, R.W., Whitehead, T.H., Wedd, C.B. & Ordnance Survey, Southampton. Robertson, T. (1938). Shrewsbury District Anon. (1994a). 1:50,000 Pathfinder Series including the Hanwood Coalfield. Memoir Sheet 888. Ordnance Survey, Southampton. of the Geological Survey of England & Anon. (1994b). 1:50,000 Pathfinder Series Wales Sheet 152. HMSO, London, 297 pp. Sheet 889. Ordnance Survey, Southampton. Sissons, J.B. (1960). Some aspects of glacial Anon. (1994c). 1:50,000 Pathfinder Series drainage channels in Britain. Part 1. Scottish Sheet 909. Ordnance Survey, Southampton. Geographical Magazine, 76, 131-46. Anon. (1994d). 1:50,000 Pathfinder Series Sissons, J.B. (1961). Some aspects of glacial Sheet 910. Ordnance Survey, Southampton. drainage channels in Britain. Part 2. Scottish Anon. (1999a). 1:25,000 Explorer Series Sheet Geographical Magazine, 77, 15-36. 216 Welshpool & Montgomery. Ordnance Survey, Southampton. Anon. (1999b). 1:25,000 Explorer Series Sheet Copyright Shropshire Geological Society © 2010. 217 The Long Mynd & Wenlock Edge. Ordnance Survey, Southampton. ISSN 1750-855x Anon. (1999c). 1:25,000 Explorer Series Sheet 241 Shrewsbury. Ordnance Survey, Southampton.