e presence of a large part of the
superficial deposits lately ascribed to the agency of the sea, during
temporary subsidences of the land, will be found in the melting of the
ice-fields. To this cause I would refer all those deposits which I have
designated in former publications as remodelled drift. When the sheet of
ice, extending from the Arctic regions over a great part of North
America and coming down to the sea, slowly melted away, the waters were
not distributed over the face of the country as they now are. They
rested upon the bottom deposits of the ice-fields, upon the glacial
paste, consisting of clay, sand, pebbles, boulders, etc., underlying the
ice. This bottom deposit did not, of course, present an even surface,
but must have had extensive undulations and depressions. After the
waters had been drained off from the more elevated ridges, these
depressions would still remain full. In the lakes and pools thus formed,
stratified deposits would be accumulated, consisting of the most
minutely comminuted clay, deposited in thin laminated layers, or
sometimes in considerable masses, without any sign of stratification;
such differences in the formation being determined by the state of the
water, whether perfectly stagnant or more or less agitated. Of such pool
deposits overlying the drift there are many instances in the Northern
United States. By the overflowing of some of these lakes, and by the
emptying of the higher ones into those on a lower level, channels would
gradually be formed between the depressions. So began to be marked out
our independent river-systems,--the waters always seeking their natural
level, gradually widening and deepening the channels in which they
flowed, as they worked their way down to the sea. When they reached the
shore, there followed that antagonism between the rush of the rivers and
the action of the tides,--between continental outflows and oceanic
encroachments,--which still goes on, and has led to the formation of our
eastern rivers, with their wide, open estuaries, such as the James, the
Potomac, and the Delaware. All these estuaries are embanked by drift, as
are also, in their lower course, the rivers connected with them. Where
the country was low and flat, and the drift extended far into the ocean,
the encroachment of the sea gave rise, not only to our large estuaries,
but also to the sounds and deep bays forming the most prominent
indentations of the continental coast, such as th