great changes came about. But geologists have connected
them with the alternating rise and fall of the surface
of the northern continent and its altitude at various
times above the level of the sea. Thus it seems probable
that the glacial period with the ice sheet of which we
have spoken was brought about by a great elevation of
the land, accompanied by a change to intense cold. This
led to the formation of enormous masses of ice heaped up
so high that they presently collapsed and moved of their
own weight from the elevated land of the north where they
had been formed. Later on, the northern continent subsided
again and the ice sheet disappeared, but left behind it
an entirely different level and a different climate from
those of the earlier ages. The evidence of the later
movements of the land surface, and its rise and fall
after the close of the glacial epoch, may still easily
be traced. At a certain time after the Ice Age, the
surface sank so low that land which has since been lifted
up again to a considerable height was once the beach of
the ancient ocean. These beaches are readily distinguished
by the great quantities of sea shells that lie about,
often far distant from the present sea. Thus at Nachvak
in Labrador there is a beach fifteen hundred feet above
the ocean. Probably in this period after the Ice Age the
shores of Eastern Canada had sunk so low that the St
Lawrence was not a river at all, but a great gulf or arm
of the sea. The ancient shore can still be traced beside
the mountain at Montreal and on the hillsides round Lake
Ontario. Later on again the land rose, the ocean retreated,
and the rushing waters from the shrunken lakes made their