MOUNTAINS AND HILLS 
217 
tops are flat, without peaks. Many of the round 
mounds that are to be seen on the plains and 
watersheds of the world were probably formed 
in still another way. They are composed of 
débris of clay and gravel, and were perhaps 
pushed to their places by some glacier of the 
Ice Age. They were not caused by breaks in 
the crust, and have no splintered rock strata 
about them. Then, again, there are so-called 
hills that are not hills at all, but exposed por- 
tions of the earth’s crust caused by the erosion 
of rivers. The bluffs that fringe the banks of 
the Upper Mississippi are not mountain-heights; 
they indicate merely the level of the prairie. 
The river passing over a sand-stone crust has 
cut through it and sunk its bed five hundred 
feet or more below the prairie surface. Many a 
hill or mountain in the valley of the Hudson or 
the Connecticut has been formed by the water 
passing around it and wearing through the 
softer portion of the rock, leaving the harder 
portion standing. It is even said that parts of 
the Catskills, and many of the mountains in 
Colorado, were formed, not by folds in the crust, 
but by erosion—the cutting out of the valleys 
about them by water. 
It is seldom that a mountain-ridge or chain 
Exposed 
crust, 
Mountains 
by erosion, 
