The deposits of the Post-Glacial period are represented throughout Russia, Poland and Finland, as also throughout Siberia and Central Asia, by very thick lacustrine deposits, which show that, after the melting of the ice-sheet, the country was covered with immense lakes, connected by broad channels (the fjarden of the Swedes), which later on gave rise to the actual rivers.
This lake drained southward into the Gulf of Mexico via the Minnesota and Mississippi rivers, until the ice sheet which had prevented its natural drainage to the north had melted sufficiently to allow it to be drained off into Hudson Bay by way of the Nelson River.
North America is bathed in frigid waters around its broad northern shores; its mountains bear huge glaciers in the north-west; the outlying area of Greenland in the north-east is shrouded with ice; and in geologically recent times a vast ice-sheet has spread over its north-eastern third; while warm waters bring corals to its southern shores.
After the continental ice sheet entirely disappeared from the state, local valley glaciers lingered in the Adirondacks and the Catskills.
This region presents no striking topographic features except the numerous small lakes which occupy the hollows created by the continental ice-sheet.