166 Olaciation of Mountains^ Etc. — Upham. 
its greatest depth. That this is true of Mt. Washington is 
shown by rare transported bowlders found by professor C. H. 
Hitchcock on this highest summit of the White mountains, 
6,293 feet above the sea. 
In the Ice Age, the latest completed period of geologic his- 
tory, the climate of the northern half of North America be- 
came very cold, with so much precipitation of snow that the 
summer's heat was not sufficient to melt it. The depth of the 
snow therefore slowly increased from year to year, until its 
lower portion became changed to ice by the pressure of its 
own weight, as glaciers are formed in the Alps, and as an ice- 
sheet now covers the interior of Greenland and another sur- 
rounds the south pole. In our country the southern limit 
of the ice-sheet in its maximum extent reached from Nan- 
tucket, Martha's Vineyard, and Block Island westward along 
the terminal moraine, which is commonly called the backbone 
of Long Island, across northern New Jersey and Pennsylva- 
nia, southern Ohio, Indiana and Illinois, central Missouri, 
and northeastern Kansas ; thence it extended northwestward 
through Nebraska and Dakota; and from the vicinity of Bis- 
marck trended again westward through northern Montana, 
Idaho and Washington. 
North of this boundary the land was deeply covered by ice, 
as is known by its transported boulders and drift, and by its 
striae, which are furrows and scratches engraved on the bed- 
rock over which the ice moved, as fragments of stone frozen 
in the bottom of alpine glaciers wear the underlying rock 
surfaces. Toward the northeast from Nantucket and cape 
Cod this ice-sheet probably terminated on the remarkable sub- 
marine plateaus known as the Fishing banks ; and on the 
west it pushed into the edge of the Pacific, across the islands 
that border the coast of British Columbia and Alaska. About 
a quarter part of the United States, the entire Dominion of 
Canada, the area of Hudson bay, and probably much of 
Alaska and of the large islands in the Arctic sea between the 
mouth of the Mackenzie and Baffin bay, were wrapped in a 
sheet of ice, which was replenished by the yearly snow-fall and 
was caused, by the pressure of the vast weight of its central 
portion, to flow slowly outward on all sides. Its greatest 
depth, estimated by professor Dana to have been about two 
miles, was on the highlands between the St. Lawrence and 
