that if the ice remained long at work in a
region it must do a vast deal of erosion. They were right in a part of
their premises, but, as we shall see, probably in another part wrong.
Looking carefully over the field where the ice has operated, we note
that, though at first sight the area appears to have lost all trace of
its preglacial river topography, this aspect is due mainly to the
irregular way in which the glacial waste is laid down. Close study
shows us that we may generally trace the old stream valleys down to
those which were no larger than brooks. It is true that these channels
are generally and in many places almost altogether filled in with
rubbish, but a close study of the question has convinced the writer,
and this against a previous view, that the amount of erosion in New
England and Canada, where the work was probably as great as anywhere,
has not on the average exceeded a hundred feet, and probably was much
less than that amount.

Even in the region north of Lake Ontario, over which the ice was deep
and remained for a long time, the amount of erosion is singularly
small. Thus north of Kingston the little valleys in the limestone
rocks which were cut by the preglacial streams, though somewhat
encumbered with drift, remain almost as distinct as they are on
similar strata in central Kentucky, well south of the field which the
ice occupied. In fact, the ice sheet appears to have done the greatest
part of its work and to have affected the surface most in the belt of
country a few hundred miles in width around the edges of the sheet. It
was to be expected that in a continental glacier, as in those of
mountain valleys, the most of the _debris_ should be accumulated about
the margin where the materials dropped from the ice. But why the
cutting action should be greatest in that marginal field is not at
first sight clear. To explain this and other features as best we may,
we shall now consider the probable history of the great ice march in
advance and retreat, and then take up the conditions which brought
about its development and its disappearance.

Ice is in many ways the most remarkable substance with which the
physicist has to deal, and among its eminent peculiarities is that it
expands in freezing, while the rule is that substances contract in
passing from the fluid to the solid state. On this account frozen
water acts in a unique manner when subjected to pressure. For each
additional atmosphere of press