Geology . Fig. 520.—Lake Erie and Lake Iroquois; a stage in the history of the eastern GreatLakes, after the ice had retreated so as to open the Mohawk outlet. (Gilbert,U. S. Geol. Surv.). Fio. 521.—The Great Lakes at the Algonquin-Iroquois stage. (After Taylor.) 402 GEOLOGY With further stages of Ice retreat, the outlet was lei down to the Cham-plaio arm of the sea presently to be noted. By this time Lake Algon-quin had given place to the greal Nipissing Lakes (Fig. 522), whichhad their outlet via Lake Nipissing to the Ottawa, and thence to theChamplain arm of the sea. Subsequently the outlet
Geology . Fig. 520.—Lake Erie and Lake Iroquois; a stage in the history of the eastern GreatLakes, after the ice had retreated so as to open the Mohawk outlet. (Gilbert,U. S. Geol. Surv.). Fio. 521.—The Great Lakes at the Algonquin-Iroquois stage. (After Taylor.) 402 GEOLOGY With further stages of Ice retreat, the outlet was lei down to the Cham-plaio arm of the sea presently to be noted. By this time Lake Algon-quin had given place to the greal Nipissing Lakes (Fig. 522), whichhad their outlet via Lake Nipissing to the Ottawa, and thence to theChamplain arm of the sea. Subsequently the outlet was shifted to]its present position, probably by gentle warpings of the Without doubt similar complicated lake histories attended thiretreat of the ice in the Mackenzie and Hudson Bay basins, but littleis yet known regarding them. A very important lake was also formed in the Red River valleyof the north (Lake Agassiz), discharging in its earlier history, into theMinnesota river at Lake Traverse. As Lake Agassiz was not con-nected with the complex system of basins of the St. Lawrence valley,it had a comparatively simple history. It grew to the northward withthe retreat
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