Page:Early Man in Britain and His Place in the Tertiary Period.djvu/144

116 rocks. In some cases, as near Liverpool, these grooves are found near the present sea-level, and in others they pass far below it. It is very probable that the ice may have arrived at the Atlantic shore at a considerable distance from the present coast-line, and that it may have been continuous with that of Scandinavia, where similar traces have been met with.

The ice at this time was sufficiently thick to override Schihallion in Perthshire at a height of 3500 feet, and the hills of Galway and Mayo at 2000 feet. Its southern limit in Britain is uncertain. According to Professor Ramsay and Dr. James Geikie it extended as far south as the latitude of London: but the hypothesis upon which this southern extension is founded—that the boulder clays have been formed by ice melting on the land—is open to the objection that no similar clays have been proved to have been so formed, either in the Arctic regions, where the ice-sheet has retreated, or in the districts forsaken by the glaciers in the Alps or Pyrenees, or in any other mountain chain. Similar deposits, however, have been met with in Davis Straits and in the North Atlantic, which have been formed by melting icebergs, and we may therefore "conclude that the boulder clays have had a like origin.

To this ice-sheet may be referred the groovings in the rocks underlying the lowest boulder clays of Britain and Ireland, as well as the lines of erratics which sometimes can be traced in directions not coinciding with the present valleys, as, for example, those at Norber, near