Page:Popular Science Monthly Volume 31.djvu/349

Rh flow each through a bead-roll of small lake-basins, walled around by solid rock. Through their margins the rivers for untold ages have been deepening their channels until the lake-bottoms have become dry land, and the homes of men. The Great Lakes themselves have, from a like cause, been much reduced from their former dimensions. The evidences are abundant that Lakes Michigan, Erie, and Huron are but the relics of what was once a large body of water, covering all the intervening and much of the adjacent lands. The work of depletion is still going on. Not only is Niagara deepening its channel and sinking thereby the surface of Lake Erie, but by the gradual recession of the falls a much greater work is prophesied. It is only a question of time when Erie will be robbed of its waters, and the other Great Lakes reduced to insignificant parts of their present dimensions. Lake Pepin, now but an expansion of the Mississippi, was once a much larger body; and Peoria, a similar widening of the Illinois, once spread over the adjacent level lands equaling in area that of Lake Champlain.

Doubtless, in many of these lake-outlets, natural fractures and marginal depressions have not only given direction to the effluent streams, but greatly aided in the work of abrasion. The evidence, however, of the former greater extent of these lakes is abundant and apparent. The railroad from Lafayette, Indiana, northward cuts through several low lake-margins, marking the gradual retreat of the waters; and runs within sight of several sand-hills similar to those on the lake-shore near Michigan City. There are evidences that the Illinois was once the outlet of Lake Michigan and the Wabash that of Erie, carrying the waters of these lakes to the Gulf of Mexico.

It is now generally conceded that the whole northern part of the continent, reaching southward in some places to the thirty-eighth parallel, once wore an ice-cap of immense thickness, through which only the mountain-peaks projected. I have already alluded to the work of the ice-plow in the excavation of lake-basins. I am now about to give to the glacier the credit of their preservation when formed.

Although it has been found that the glacier flows like the water of a river, only more slowly, the ice, except when wedged in between two walls, as the Mer de Glace, could not have been confined in narrow channels, and can not, therefore, have grooved out long, tortuous river-beds. The abrasion and drainage were indeed going on, but by a slow process, as compared to the work of the released and active waters.

When the ice-field began to disappear it gradually receded northward, first uncovering that part of the drift-region in which the lakes have been wholly drained. The southern half of the continent has had even larger rivers than now fed by the ice and snow of the gradually disappearing glacier. The length of time during which these rivers were doing their work of excavation, while the North was still