Behind the terminal moraines lie wide till plains, in places studded thickly with drumlins, or ridged with an occasional esker.  Great outwash plains of sand and gravel lie in front of the moraine belts, and long valley trains of coarse gravels tell of the swift and powerful rivers of the time.

The loess of the Mississippi valley.  A yellow earth, quite like the loess of China, is laid broadly as a surface deposit over the Mississippi valley from eastern Nebraska to Ohio outside the boundaries of the Iowan and the Wisconsin drift.  Much of the loess was deposited in Iowan times.  It is younger than the earlier drift sheets, for it overlies their weathered and eroded surfaces.  It thickens to the Iowan drift border, but is not found upon that drift.  It is older than the Wisconsin, for in many places it passes underneath the Wisconsin terminal moraines.  In part the loess seems to have been washed from glacial waste and spread in sluggish glacial waters, and in part to have been distributed by the wind from plains of aggrading glacial streams.

The effects of the ice invasions on rivers.  The repeated ice invasions of the Pleistocene profoundly disarranged the drainage systems of our northern states.  In some regions the ancient valleys were completely filled with drift.  On the withdrawal of the ice the streams were compelled to find their way, as best they could, over a fresh land surface, where we now find them flowing on the drift in young, narrow channels.  But hundreds of feet below the ground the well driller and the prospector for coal and oil discover deep, wide, buried valleys cut in rock,—­the channels of preglacial and interglacial streams.  In places the ancient valleys were filled with drift to a depth of a hundred feet, and sometimes even to a depth of four hundred and five hundred feet.  In such valleys, rivers now flow high above their ancient beds of rock on floors of valley drift.  Many of the valleys of our present rivers are but patchworks of preglacial, interglacial, and postglacial courses (Fig. 366).  Here the river winds along an ancient valley with gently sloping sides and a wide alluvial floor perhaps a mile or so in width, and there it enters a young, rock-walled gorge, whose rocky bed may be crossed by ledges over which the river plunges in waterfalls and rapids.

In such cases it is possible that the river was pushed to one side of its former valley by a lobe of ice, and compelled to cut a new channel in the adjacent uplands.  A section of the valley may have been blockaded with morainic waste, and the lake formed behind the barrier may have found outlet over the country to one side of the ancient drift-filled valley.  In some instances it would seem that during the waning of the ice sheets, glacial streams, while confined within walls of stagnant ice, cut down through the ice and incised their channels on the underlying country, in some cases being let down on old river courses, and in other cases excavating gorges in adjacent uplands.