SCOUTING FOR GIRLS 
453 
of years. During one age, the coal was formed of plants 
that grew luxuriantly on the earth’s surface. At one 
period in the development of the earth there were many 
kinds of invertebrate animals, but no animals with back- 
bones. Later, the vertebrates appeared. At one time the 
whole Mississippi Valley was under the water of the sea. 
(“The Story of Our Continent,” by N. S. Shaler. Ginn 
& Co.). These statements suggest just a few of the 
things that have been going on in the history of the earth. 
By the study of Geology we can learn much more about 
it, and we should supplement our study of books with the 
more important actual observation of conditions out-of- 
doors. To those living in that part of North America, 
which is shaded in the accompanying map, 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 Can- 
ada or northern United States (See map above) 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 
