e map on page 451, the easiest and most natural approach to
the subject of the structure and history of the earth is by studying the
effects of the continental glacier which formerly moved down over this
region.

Tracks of the Glacier

When we see the foot-prints of an animal in the mud or in the snow, we
are sure that an animal has passed that way at some previous time. Those
who live in Canada or northern United States (See map page 451) can be
just as sure that a great glacier or ice-sheet formerly moved down over
northern North America, by the tracks it has left. Although it is
estimated by geologists that between 10,000 and 40,000 years have
elapsed since the Great Ice Age, these tracks or evidences can still be
seen by any one who lives in this region or who can visit it. The
principal ones are: (1) Boulders or Lost Rocks which were brought down
by this glacier; (2) The Glacial Drift or Boulder Clay which covers
nearly all of the glaciated region; (3) Scratches on the bed-rock which
show the direction the glacier moved.

Notice in the field the size and shape of the glacial boulders, where
they are found, evidence of the place where the glacier melted off
(terminal moraine). Do these boulders increase or decrease in size as we
go south over the glaciated area? Can you discover any place where they
can be traced back in their native ledge? Present-day glaciers, like the
Muir Glacier in Alaska, can be seen transporting boulders and drift just
as this great prehistoric ice-sheet must have done.

The drift which consists of clay mixed with pebbles, cobblestones, and
boulders, varies greatly in depth. In some places there is none, while
at St. Paris, Ohio, it is 550 feet deep. It probably averages 100 feet
thick or less.

In your locality note the depth of the drifts in cuts made naturally by
creeks and rivers or those made artificially for railroads. Oil-wells
furnish evidence on this point. Collect a few good examples of scratched
or glaciated pebbles or cobblestones which are abundant in the drift.
These were scratched while frozen in the bottom of the glacier and
pushed along on the bed-rock under the weight of the ice above.

Collect ten different kinds of rock from the glacial boulders and
drift,--there are more than one hundred kinds to be found,--and with the
aid of some such book as "Rocks and Rock Minerals," by Louis V. Pirsson
(John Wiley & Sons) or "Common Minerals and Rocks," by Wm. O. Crosby (D.