clay formed underneath the great glacier, containing abundant
examples of stone showing by their scratched surface that they
have been ground along underneath the glacier. The rocks on the
sides of the mountains are scratched exactly as are those in the
Alps. By observing how high up on the mountains the striae are,
we know the thickness of the ice-sheet; and the direction in
which it moved is shown in several ways.<22>

Briefly, then, the geologist assures us that when the cold of
the Glacial Age was at its maximum glaciers streamed down from
all the mountains of Scotland, Wales, and Northern England;
that the ice was thick enough to overtop all the smaller hills,
and on the plains it united in one great sea of ice some
thousands of feet in thickness, that it stretched as far south
as the latitude of London, England. But that to the west the ice
streamed out across, the Irish Sea, the islands to the west of
Scotland, and ended far out into what is now the Atlantic.<23>
But these glaciers, vast as they were, were very small compared
with the glaciers that streamed out from the mountains of Norway
and Sweden. These great glaciers invaded England to the south-
west, beat back the glacier ice of Scotland from the floor of
the North Sea, overran Denmark, and spread their mantle of
bowlder clay far south into Germany.<24>

While such was the condition of things to the north, the
glaciers of the Alps were many times greater than at present.
All the valleys were filled with glacier ice, and they spread
far out on the plains of Southern Germany and westward into
France. The mountains of Southern France and the Pyrenees also
supported their separate system of glaciers. Ice also descended