Page:Cornwall; Cambridge county geographies.djvu/69

 THE COAST 53 land of something like 30 ft. at least since the period when man in the late stone or early bronze age inhabited Cornwall. The submarine forests grew on the top of the tin ground. Of these many have been noted and recorded, not only on the south coast, but on the north as well. The trees were oak and hazel, alder and elm, but they never reached a large size. . Above this bed lie the raised beaches, some 40 or 50 ft. above high water mark. The tin-beds in the Cornish valleys towards the sea do not exhibit such an upheaval. Generally the raised beach rests on the original rock, and consists of rolled stones, frequently of large size, mixed with smaller gravel and sand. The " Head of Rubble," with some intervening perplexing beds of sand, may be noticed on the coast. This Head is from 40 to 50 ft. in depth. It is composed of angular fragments of rock, often large, many of quartz, with no signs of stratification. The Rubble bed in Cornwall has yielded no organic remains, but elsewhere in it have been found the bones of the mammoth, elephant, woolly rhinoceros, reindeer, etc. It was not formed by the disintegration of the subjacent rocks, but by aqueous transport. It owes its origin to a powerful force of water, acting violently and rapidly. It caps the heights, and is not in the valleys where the tin ground has been deposited. It has not therefore been found to overlie it. It was due to a sudden and brief overrush of water, and the fragments of stone carried before the flood did not travel sufficiently far to have their angles rubbed down.