Page:VCH Cornwall 1.djvu/55

 GEOLOGY these terraces in their present position has often materially changed the geographical features ; for instance, the present peninsula of St. Anthony in Roseland, above two miles in length, was before the uprise an island as is demonstrated by the deposits of sand charged with marine shells on the neck of the peninsula ; while the peninsula of Pendennis, forming the opposite front of the entrance to the Falmouth estuary, has similarly emerged from the sea which completely encircled it. The antiquity of these old beaches is still further borne out by their frequently being overlain by the material which has long been known m Cornwall, and to geologists generally, as head.' It is sometimes stratified, and may contain beds of sand and fairly rounded gravel ; more often however the tendency to stratification is but slight, and it presents an irregular accumulation of stones, mostly angular, occasion- ally subangular. Such an accumulation is made up of material similar to the subsoil of the district. If this subsoil were transported from higher to lower levels it would probably form a deposit similar to the so-called head,' which often merges so gradually into the subsoil as not to be separable from it. That the surface burden which forms the subsoil is ever creeping to lower levels may be seen in the sections afforded by the Cornish lanes, along the steeper valley slopes, where their banks have been cut through soil and subsoil into the solid rock. In these banks, no matter in what direction the lower strata are dipping, the upper surface invariably bends down the hill, the downward drag of the superficial accumulations involving the solid rock in its creep. The action of rain- wash in the passage of debris to lower levels has already been alluded to but it is evident that modern processes are not evolving the tumultuous and stratified accumulations of 'head' such as are common features along the sea front, and have acted as a protective covering to the raised beaches. An explanation must be sought elsewhere. In Pleistocene times the whole of Britain, except its highest peaks, as far south as the Bristol Channel, was probably buried beneath a mantle of ice, and during these arctic conditions the face of the country was profoundly modified by the grinding effects of the glaciers which over-rode it. While nowhere in Cornwall is there evidence of the county having been invaded by that ice cap, its proximity to the edge of the ice field must necessarily have entailed the rigours of an arctic climate, under which the land was incapable of supporting any but the sparsest vegetation. In winter, not only would the higher ground be swathed beneath a covering of snow, but the crumbling debris which so deeply covers our slopes would be frozen for many feet below the surface. The melting of the winter snows and the ice which bound the frozen soil, acting on a surface unchecked by vegetation, would involve a sweeping of material down the slopes that would amply account for the abnormal character of that deposit. In its downward course it has filled the hollows on the coast line and covered the shelf of the ancient beach ) which it has afforded a protection, so that the thickest deposits of