ctised eye can in a way judge how long a valley
has been subjected to glacial action by the extent to which it has
been widened by this process.

In the valleys of Switzerland and other mountain districts which have
been attentively studied it is evident that glacial action has played
a considerable part in determining their forms. But the work has been
limited to that part of the basin in which the ice is abundantly
provided with cutting tools in the stone which have found their way to
the base of the stream. In the region of the _neve_, where the
contributions of rocky matter to the surface of the deposit made from
the few bare cliffs which rise above the sheet of snow is small, the
snow-ice does no cutting of any consequence. Where it passes over the
steep at the head of the deep valley into which it drains, and is
riven into the _seracs_, such stony matter as it may have gathered is
allowed to fall to the bottom, and so comes into a position where it
may do effective work. From this _serac_ section downward the now
distinct ice river, being in general below the snow line, has
everywhere cliffs, on either side from which the contributions of rock
material are abundant. Hence this part of the glacier, though it is
the wasting portion of its length, does all the cutting work of any
consequence which is performed. It is there that the underrunning
streams become charged with sediment, which, as we have noted, they
bear in surprising quantities, and it is therefore in this section of
the valley that the impress of the ice work is the strongest. Its
effect is not only to widen the valley and deepen it, but also to
advance the deep section farther up the stream and its tributaries.
The step in the stream beds which we find at the _seracs_ appears to
mark the point in the course of the glacier where, owing to the
falling of stones to its base, as well as to its swifter movements and
the firmer state of the ice, it does effective wearing.

There are many other features connected with glaciers which richly
repay the study of those who have a mind to explore in the manner of
the physicist interested in ice actions the difficult problems which
they afford; but as these matters are not important from the point of
view of this work, no mention of them will here be made. We will now
turn our attention to that other group of glaciers commonly termed
continental, which now exist about either pole, and which at various
times in the