XXII ABSTRACT OF VOLUME. 



Chapter IV: The Glacial period and its drift deposits. — Tho continental ice-sheet attained an 

 area of aVioiit 4,000,000 square miles, and had a maximum thickness, in its central portion, of 

 probably 1 to 2 miles. It extended from the Atlantic to the Pacific and from the northern United 

 States to the Arctic Sea, probably enveloping the Rocky Mountains in the region of the Peace 

 River and northward. The closing stage of this glaciation was the time of existence of Lake 

 Agassiz. On the greater part of the lacustrine area the drift is from 100 to 300 feet thick, con- 

 sisting chiefly of till or bowlder-clay. A series of twelve terminal moraines is found in proceeding 

 from south to north and northeast in Minnesota and North Dakota. The last six of those, 

 named the Dovre, Fergus Falls, Leaf Hills, Itasca, Mesabi, and Vermilion moraines, were contem- 

 poraneous with Lake Agassiz, besides probably others to be traced farther north. Birds Hill, 

 near Winnipeg, a remarkable esker, indicates that much drift was contained in the lower part 

 of the ice-sheet. The deltas of Lake Agassiz were formed chiefly of modified drift, brought by 

 streams from the receding ice. Very little transportation of bowlders and other drift was 

 eff'ected by icebergs or floes on this lake. 



Chapter V: History op Lake Agassiz. — The records of glacial lakes are their outlets .across present 

 lines of watershed ; eroded cliffs, beach ridges, and deltas at the levels of the former outlets ; and 

 lacustrine sediments in the basin inclosed by the old shores. Lake Agassiz grew from south to 

 north as fast as the ice-sheet receded, forming its series of moraines. The outlet by the River 

 Warren was eroded to a depth of about 90 feet. Afterwards lower outlets were opened toward 

 tho northeast. Probably the early northeastward outflow passed along the ice border and 

 through the upper Laurentian lakes to the Mississippi, then to tho Hudson River, and later to 

 the much enlargeil Gulf of St. Lawrence. Finally the outflow was tributary to Hudson Bay 

 when the ice had melted so far as to admit the sea to that basin. With the uncovering of the 

 course of the Nelson River, Lake Agassiz ceased to be held by the ice barrier, and became Lake 

 WinniiJeg. Epeirogeuic uplifting of the area of Lake Agassiz, increasing in vertical extent from 

 south to north, gave to its beaches a northward ascent, and caused the several shores of its south- 

 ern part to become double or multiple as they are traced northward. The molluscau fauna of 

 Lake Agassiz, so far as it has been discovered, consists of five fresh- water species. The amount 

 of tho shore erosion of Lake Agassiz and the volume of its beaches, compared with tho post- 

 glacial erosion and beach deposits of the present Great Lakes, have a ratio approximately as 

 one to ten. The duration of postglacial time is believed to have been from seven to ten thousand 

 years; of Lake Agassiz, probably not more than one thousand years. 



Chapter VI: Beaches and deltas of the Herman stages. — These shore deposits are described 

 in detail. The earliest and highest beach, named from Herman, Minn., has been mapped, with 

 determination of its height by leveling, through an extent of about 175 miles in Minnesota, from 

 Lake Traverse east to Herman, and thence north to Maple Lake. In 140 miles, from south to 

 north, this shore-line ascends from 1,050 feet to 1,170 feet, approximately, above the sea. Near 

 Maplo Lake four lower beaches, successively about 8, 15, 30, and 45 feet below the highest, were 

 also formed during the time of accumulation of the single Herman beach at the south ; and on the 

 west side of the lake in Manitoba the Herman series of beaches is increased to seven. In North 

 Dakota the uppermost Herman shore has a northward ascent of about 180 feet in the distance 

 of 224 miles from Lake Traverse to the international boundary, where its height is 1,230 feet 

 above the sea. At the latitude of Gladstone, in Manitoba, 84 miles farther north, the altitude of 

 1,315 feet is attained by the second of the Herman shores, which is the highest one extending so 

 far. Six noteworthy deltas were brought Into Lake Agassiz, contemporaneously with the for- 

 mation of the Herman beaches, by streams which were exceptionally supplied with uuich modi- 

 fied drift by the molting ice-she(^t. Those are tho Bufl",-ilo River and Sand Hill River deltas in 



