The ice still blocked the Mohawk and St. Lawrence valleys on the east, while on the west it had retreated far to the north.  The lakes become confluent in wide expanses of water, whose depths and margins, as shown by their old lake beaches, varied at different times with the position of the confining ice and with warpings of the land.  These vast water bodies, which at one or more periods were greater than all the Great Lakes combined, discharged at various times across the divide at Chicago, near Syracuse, New York, down the Mohawk valley, and by a channel from Georgian Bay into the Ottawa River.  Last of all the present outlet by the St. Lawrence was established.

The beaches of the glacial lakes just mentioned are now far from horizontal.  That of the lake which occupied the Ontario basin has an elevation of three hundred and sixty-two feet above tide at the west and of six hundred and seventy-five feet at the northeast, proving here a differential movement of the land since glacial times amounting to more than three hundred feet.  The beaches which mark the successive heights of these glacial lakes are not parallel; hence the warping began before the Glacial epoch closed.  We have already seen that the canting of the region is still in progress.

The Champlain subsidence.  As the Glacial epoch approached its end, and the Labrador ice field melted back for the last time to near its source, the land on which the ice had lain in eastern North America was so depressed that the sea now spread far and wide up the St. Lawrence valley.  It joined with Lake Ontario, and extending down the Champlain and Hudson valleys, made an island of New England and the maritime provinces of Canada.

The proofs of this subsidence are found in old sea beaches and sea-laid clays resting on Wisconsin till.  At Montreal such terraces are found six hundred and twenty feet above sea level, and along Lake Champlain—­where the skeleton of a whale was once found among them—­at from five hundred to four hundred feet.  The heavy delta which the Mohawk River built at its mouth in this arm of the sea now stands something more than three hundred feet above sea level.  The clays of the Champlain subsidence pass under water near the mouth of the Hudson, and in northern New Jersey they occur two hundred feet below tide.  In these elevations we have measures of the warping of the region since glacial times.

The western united states in glacial times.  The western United States was not covered during the Pleistocene by any general ice sheet, but all the high ranges were capped with permanent snow and nourished valley glaciers, often many times the size of the existing glaciers of the Alps.  In almost every valley of the Sierras and the Rockies the records of these vanished ice streams may be found in cirques, glacial troughs, roches moutonnecs, and morainic deposits.