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 

 debris 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 cruat, 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 



