318 PLEISTOCENE OF INDIANA AND MICHIGAN. 



largely an original rock basin, or perhaps a syncline out of which soft rocks, probably in the 

 main those below the limestone, have been eroded. 



Thus, the shape and size and arrangement of the lake valleys were primarily dependent 

 on the geologic structure — on the relative position and thickness of the soft beds and the dis- 

 tribution of their exposed parts. Where the soft beds were exposed to effective stream erosion 

 they were removed more rapidly than the harder rocks and thus became the main valleys of the 

 region. 



In the present attitude of the land the Paleozoic strata dip distinctly but gently south in 

 the basin of Lake Ontario; south in the eastern part and southwest and west in the western 

 part of the basin of Lake Erie; southwest in the main part and south in the northwestern part 

 of the basin of Lake Huron; south in the northeastern part and east in the southern part of the 

 basin of Lake Michigan, and south in the peninsula east of Marquette; steeply north in the west- 

 ern part and variously in other parts of the basin of Lake Superior. 



That these valleys were going through the process of development by erosion during practi- 

 cally all the. time from the close of the Paleozoic to the beginning of the glacial epoch seems not 

 improbable. Indeed, the time must have been very long to have permitted the making of such 

 extensive valleys by so slow a process. It might be thought that some movement of elevation or 

 tilting had turned these old valleys into lake basins long before the time of the ice epoch, but no 

 certain evidence indicating such a change has been found. Up to or nearly to the beginning of the 

 ice epoch the valleys appear to have had complete drainage by rivers and to have held no lakes. 

 Although the whole region was probably reduced to a peneplain more than once in the long time 

 of preglacial erosion, it appears to have stood at a relatively high altitude above sea level toward 

 the close of that time. This is indicated by the deeper lake basins, by the submarine valleys of 

 the St. Lawrence, the Hudson, and other rivers. 



DIFFERENTIAL UPLIFT. 



Some time before the beginning or possibly in the earlier part of the ice epoch, the northern 

 lands were uplifted in such a way as to warp or tilt the region of these great valleys. The lands 

 in the north were uplifted more than those in the south, and as the outflow of most of these 

 valleys was northerly, their lower courses were elevated more than their headward parts, and 

 they became water-filled basins or lakes. It is not known with certainty when the first impor- 

 tant tilting took place. If the great period of diastrophism, which began in late Cretaceous 

 time and reached a climax in the middle or later part of the Tertiary, was characterized 

 by a southwestward creeping of the entire crustal sheet of the continent, as certain features seem 

 to suggest, it may well be that the principal part of the warping which turned the open valleys 

 into lakes occurred at that time and was the result of that movement. At present, however, 

 the facts bearing upon this question are too few to warrant more than the briefest mention. 



GLACIAL HISTORY OF THE GREAT LAKES BASINS. 



EARLY STAGES. 



The glacial epoch as a whole has been found to be made up of at least four distinct 

 stages of glaciation separated by intervening warm periods when the ice sheet either shrank to 

 relatively small proportions or disappeared altogether. The last ice sheet deposited what is 

 known as the Wisconsin drift. There is abundant evidence, however, that the lake basins existed 

 substantially as they are to-day during at least one of the earlier stages of glaciation. In the 

 southeastern and central parts of the southern peninsula of Michigan there is a great body of 

 older drift underlying the drift of the Wisconsin stage, and in the northern part of the same penin- 

 sula there are thick deposits of lake clays, which were deposited hi the basin of Lake Michigan 

 before the last advance of the last ice sheet, for they are overlain by drumlins of Wisconsin age. 

 (See pp. 314-315.) Possibly they were deposited before the Wisconsin glaciation. The position 

 and relation of these beds indicate that this part of the Lake Michigan basin was then in all respects 

 the same as it is to-day, except that the water stood relatively somewhat higher upon the land; 



