Rocks and Their Origins

by Grenville A. J. Cole

On the whole, however, areas of siliceous conglomerate and sandstone are given over, even in temperate climates, to forest and heather. Where the sandstone is still in the sand-rock state, bare patches are likely to appear even in the heath that has grown across it, and from these the wind carries away shifting sands.

Everyone familiar with the Carboniferous areas of the English midlands will realise the influence of hard grit and sandstone in forming "edges" across the country. The contrast between these escarpments and the slopes of crumbling shale that often underlie them gives diversity to the scenery of Yoredale and the Peak. The more yielding sandstones of Cretaceous age round about the Weald, or at the foot of the Chiltern Hills near Woburn, form rounded hills, mostly clad with woods of coniferous trees. In Surrey, unpaved cart-tracks, used for centuries, have cut gullies in the unconsolidated Folkestone Sands.

The underlying Hythe Beds, however, stand out between Reigate and Guildford as a bold escarpment, and it is interesting to reflect that this fine feature of south-eastern England is probably due to the chert which the beds contain (see [p. 62]). The local growth of siliceous sponges in a Lower Cretaceous sea enables Leith Hill in our days to dominate even the arch of Ashdown Forest, where another unfilled sandstone area rises in the centre of the Weald.

The sands of Bagshot Heath, and numerous similar areas in the Paris Basin, show how impossible it is to cultivate such strata, even near the best of markets. The flint gravels that cover much of the upland in the New Forest may also be borne in mind, as presenting the worst features of highly siliceous lands.

In a semi-arid climate, or one with only seasonal rains, the processes by which sandstone begets sandstone tend to develop desert wastes. The soils produced by weathering do not cake together, and are carried away by wind during the drier months. The bare rock appears over broad surfaces, just as it does in storm-swept limestone areas, and any hollow where shelter is afforded tends to become filled with sand (see [Fig. 5]).

The hummocky and extremely irregular surface of some of our Silurian areas, such as parts of the Southern Uplands of Scotland and the hard-won farmlands of Down and eastern Monaghan, is due to the presence of resisting sandstones among the shales. These sandstones, passing into true grits, are repeatedly folded, and their upturned edges have resisted even the passage of glacier-ice. They jut out along the crests of ridges, and even the smaller beds furnish angular fragments to the soils.

Fig. 6. Siliceous Conglomerate. Characteristic weathering; moraine-blocks at Coumshingaun, Co. Waterford.

Far wilder scenery is formed by the more continuous sandstone masses of the Harlech Beds in western Wales, which are grits so firmly cemented that the rock breaks across the quartz-grains. Much of the Old Red Sandstone is of equally hard quality ([Fig. 6]). Its purple or grey conglomerates, the pebbles of which are quartzite in a quartz cement, form bare and rugged masses in the Great Glen south-west of Inverness, and are responsible in Kerry for some of the wildest rock-scenery in the British Isles. Variations in coarseness allow of the development of a marked stratification on the weathered mountain sides, and differential erosion of the beds has taken place where ice has pressed against them. Even on precipices, grassy ledges may occur, marking bands of sandstone or shale in the conglomeratic mass.

The red sandstones and conglomerates that form huge outstanding bluffs from Applecross to the north of Sutherland represent the denudation of a pre-Cambrian mountain region. These Torridon Sandstones cover a very irregular surface of old gneiss, with which their almost level strata are in striking contrast. P. Lake[39] has compared them with the deposits styled dasht in Baluchistan and Afghanistan, which similarly fill up valleys and cover hills, as products of extensive and rapid denudation. There is much, indeed, to suggest that the Torridon Sandstone, some 10,000 feet in thickness, was accumulated in a dry country on a continental surface, with the aid of floods during occasional rainy seasons.