Page:Handbook for Boys.djvu/137

116 the breaking-up process may be seen; the outer portion is softer, more easily broken, and of different color from the fresh rock, as shown by breaking of a large piece. The wearing away of the land surface is well shown in rain gullies, and the carrying along and depositing of sand and  gravel may be seen in almost any stream. In the Northern states and Canada, which at one time were covered by  a great sheet of ice, moving southward and grinding of the surface over which it passed, most of the rock outcrop are smoothly rounded and many show scratches made by pebbles dragged by the ice. The hills too have