near Ubly, Mich., whence they discharged down the Cass River, with a descent of 35 to 40 feet, to glacial Lake Saginaw, thence westward to glacial Lake Chicago in the Lake Michigan Basin, and thence to the Illinois River by an outlet near Chicago. (See pl. 12.) This stage of the lake is known as glacial Lake Whittlesey, and its shore line, the Belmore or Whittlesey beach, is marked in the Cleveland district by the Middle Ridge of the Berea quadrangle and associated littoral deposits at a level about 735 feet above the sea. The bases of the wave-cut cliffs of this stage are at about 720 feet.

Glacial Lake Wayne.-When the recession of the ice front again lowered the waters there was a fall of 75 to 80 feet from the Lake Whittlesey level. To this low stage the name Lake Wayne has been given. Lake Wayne was the first of the glacial lakes in the Erie Basin to have an eastward discharge. Its waters extended along the front of the Ontario ice lobe eastward past the Finger Lakes of western New York to a channel near Syracuse that discharged eastward down the Mohawk and Hudson Valleys to the Atlantic Ocean. To this lake is referred the cutting of the lake cliff under the North Ridge of the Berea quadrangle. It was largely obliterated in the Cleveland and Euclid quadrangles by the rise to the Lake Warren stage.

Glacial Lake Warren.-By an advance of the ice in New York the outlet near Syracuse was closed, and the water level was raised so that it again reached the Grand River outlet. Glacial Lake Warren occupied the Saginaw Basin, the Erie Basin, the intervening low areas on the border of Lake St. Clair, and the southern part of Lake Huron. (See pl. 13.) Its extent was nearly the same as that of Lake Arkona, and it had the same outlet, through the Grand River Valley to Lake Chicago, in the Michigan Basin. It was about 15 feet lower than Lake Arkona, because the discharging waters of Lake Saginaw had deepened the Grand River outlet that much during Lake Whittlesey time. The reversal in the direction of discharge from an eastward course to the Atlantic to a westward course to waters that drained to the Gulf of Mexico was caused by the advance of the Ontario ice lobe in western New York across the line of eastward discharge. At this stage was formed the Warren or Forest beach, which on the map of the Berea quadrangle is designated North Ridge. West of the Cuyahoga River little more of the Cleveland district was submerged than at the preceding Lake Wayne stage. East of the river the lake extended to the Woodland Avenue beach, about 680 feet above sea level.

Glacial Lake Lurdy (Elkton, Dana).—When the waters were again lowered below the level of Lake Warren they halted first at the Grassmere beach, at about 640 feet above sea level, and later at the Elkton beach, at about 620 feet, each marking short transitional stages before the waters of the Erie Basin became finally separated from those

in the Huron and Ontario Basins. (See pl. 14.) There has been little study of the beaches formed at these stages east of Michigan, partly because they are weak and difficult to identify. The outlet at both stages appears to have been eastward near Syracuse, N. Y., and thence down the Mohawk and Hudson Valleys to the Atlantic. The sandy strip along St. Clair Avenue in the Euclid quadrangle about 640 feet above sea level may represent the Grassmere beach. Only faint traces of the shore were found west of Cleveland. The Elkton beach is faintly developed in the northwestern part of the Berea quadrangle, but most of it has been cut away by the encroachment of the present Lake Erie, whose bluffs now rise above the 620-foot contour throughout much of the Cleveland area. The opening of the Mohawk outlet lowered the lake to a level below the Niagara escarpment and restricted the waters in the Erie Basin to about their present (See pl. 15.)

area.

POSTGLACIAL LAKE

Lake Erie.-When the ice had receded in western New York sufficiently to allow water to escape along its southern edge through the low plain between the Niagara escarpment and the shore of Lake Ontario into the Mohawk Valley, the level of the lake waters east of the Erie Basin was drawn down below the level of the Niagara escarpment, and the remaining body of water had about the extent of the present Lake Erie. The Niagara River connected the two lakes, and Niagara Falls became operative and began their recession. With the institution of these features the direct influence of the oscillating ice dam on the lake in the Erie Basin ceased, and its postglacial history began.

There has been a slight differential uplift since that time at the east end of the lake, which raised the outlet and caused the waters to extend a little farther west, but, in general, the first stage of Lake Erie was nearly of the same area and extent as the present lake. When Lake Erie became an independent body of water the ice still covered the greater part of the Lake Huron Basin and discharged its waters southward to Lake Erie along the line of the present drainage from Lake Huron to Lake Erie (pl. 15), but as the ice melted back in the Huron Basin and in the district on the east an outlet for the waters was opened, leading from a place near Kirkfield, Ontario, down the Trent River Valley into a lake in the Ontario Basin known as Lake Iroquois. By this change the Erie Basin had its drainage area much reduced, and the level of the water in it became correspondingly lower. It is estimated that its level may have been at least as low as 560 feet above the sea, or 13 feet or more lower than the present level of the lake. After a long period of eastward discharge through the Kirkfield or Trent River outlet the region through which that outlet passed was uplifted so that it could no longer carry the water from the Huron Basin. (See figs. 10 and 11.) This uplift caused the water to rise again to a sufficient height at the south end of Lake

Huron to discharge southward along the present line of drainage to Lake Erie. By this time the ice had been melted away from the Michigan and Superior Basins also, and the water from these three great lakes was turned into Lake Erie. This accession of water appears to have brought the lake up practically to its present height, 573 feet above sea level. The Erie Basin drained into the Atlantic by way of the Mohawk and Hudson Rivers. For a time after the St. Lawrence region was free of ice its altitude was below sea level,

FIGURE 10.-Hinge lines of Lakes Whittlesey, Wayne, and Warren

and an arm of the ocean extended westward into the Ontario Basin. (See pl. 16.) At that time Lake Erie was again slightly lower than now, for the waters of the upper lake basins, comprising the Nipissing Great Lakes, flowed eastward from Georgian Bay by way of the Ottawa River into the St. Lawrence arm of the sea.

The Sandusky River at that time ran across what is now Sandusky Bay. The old channel is traceable through the bay, and its banks

about 560 feet above sea level. This low stage of Lake Erie was maintained until an uplift in the region traversed by the Ottawa River raised the head of the outlet to a level high enough to cause the withdrawal of the sea from the Ontario Basin and the discharge southward

50

100

ICE

200 Miles

are less than 10 feet below the surface of the bay. From this evidence it appears that the level of the river at ordinary stage was probably but little more than 10 feet below the present level of Lake Erie, or

FIGURE 11.-Isobases and hinge line of Lake Algonquin. (After Taylor.) The figures above the isobases indicate altitude above the horizontal or unaffected part of the beach south of the hinge line; the figures in parentheses below the isobases and the scattered figures elsewhere indicate altitude above sea level

into the Erie Basin from the basins of Lakes Superior, Michigan, and Huron to be resumed.

Studies of the Niagara Gorge and the rate of recession of the falls indicate that the lake has been at this higher level for less than 4,000 years.12 The present stage of Lake Erie is thus of relatively recent date. Yet in this brief period the lake appears to have cut back its coast line in the Cleveland district more than the amount of cutting accomplished in this area by any of the glacial lakes.

The level of Lake Erie is determined by the rock bed at the head of the Niagara River, its present outlet. Since the separation of Lake Erie from the waters on the northeast the falls have cut back about 7 miles, with a much smaller flow than at present for a considerable part of the time. There remains about 18 miles for the falls to cut back to the edge of Lake Erie. It is estimated that the present rate of recession of the main or Horseshoe Fall is fully 4 feet a year. At this rate it will not be many thousand years until the cataract reaches the head of the outlet. The lake will then be partly drained and become smaller.

POSTGLACIAL STREAMS

Since the Wisconsin ice sheet disappeared from the Cleveland district the streams have been subject to several distinct base-levels, owing to the changes in the level of the lake waters described above. At each succeeding lower stage the streams were extended to the new position of the shore line. At each succeeding rise in the lake level the lower courses of the streams directly tributary to the lake were flooded and probably partly refilled with sediment, and the opening of valleys was by so much retarded. Owing to the complexity of the relations between the streams and the shifting base-level it is difficult to differentiate clearly the history of each stream at each stage of the lake. The valleys of to-day represent the net result of the process of development.

The Cuyahoga River has cut a crooked flat-bottomed valley 100 to 180 feet deep, bordered by steep bluffs and ranging in width from about a mile down to scarcely one-fifth of a mile. Most of this cutting has been done in unconsolidated material, a large part of which is laminated clay. Only at three places-near Boston, Brecksville Station, and Tinkers Creek-is the preglacial rock bluff exposed. North of Brecksville Station the valley cuts across a projecting rock point, and there its trough is very narrow.

The Rocky River has worked mostly in shale. Except where the river follows or crosses its preglacial valley, it occupies a narrow, steep-walled channel. (See pl. 17, B.) Down to relatively recent

12 U. S. Geol. Survey Geol. Atlas, Niagara folio (No. 190), 1913.