192 Frank Carney 



worn away more on the stoss side, the side approached by the ice. 

 Mountain slopes would be broken through localized ice action, 

 and the mountain tops would be rounded. There would be very 

 little residual soil left. Rock areas, where not covered by drift, 

 may show the efTects of glacial scouring, either in smoothed, stria- 

 ted, or grooved surfaces. 



Tools of glacier ice. Clear ice never abrades rock surfaces, 

 any more than water, without tools, erodes the beds of rivers. 

 But glacier ice acquires tools readily. As it first moves into a 

 country, it finds a great mass of residual and loose rock. This 

 material is gradually worked into the ice and held as tools, to 

 rasp all surfaces against which the ice moves. After the residual 

 soil has been removed, the acquirement of a further load is an 

 easy task. Man has never yet gone so deep in the earth that he 

 has not found the rocks broken by joints and faults. These divi- 

 sions make it easier for rivers and ice to remove blocks. This is 

 especially true when ice moves through a valley or around hills 

 and mountains. The great bowlders, sometimes weighing many 

 tons, scattered over the states, were brought from areas north, 

 probably from the slopes of valleys or the sides of hills. 



Furthermore, effectiveness of glacial erosion depends directly 

 on the hardness of its tools. Some rocks are so soft that they can 

 accomplish very little abrasion, even against the same kind of 

 rock. The size of the tool is of slight importance. A grain of 

 sand, will probably wear rock beneath the glacier more than would 

 a slab of shale weighing many pounds, because quartz is ver}- 

 hard. The erosion accomplished by a glacier is not entirely on its 

 bed. The rock which is taken into the basal parts of a glacier 

 is raised from one level to another, through the buckling and shear- 

 ing of the ice ; the material may ascend even to the surface of the 

 glacier. Consequently much mutual attrition of block on block 

 tends to wear the tools in transit. Streams issuing from glaciers 

 bear heavy loads of exceedingly fine material, partly produced by 

 this mutual attrition. 



Evidences of erosion. The most universally convincing proof 

 of the power of glacier ice to wear rock is seen in a surface that 

 bears striae, grooves, or gouges. Such a surface merely shows a 

 stage in glacial eroson. It is by the continuous removal of the 

 rock, in registering even delicate scratches upon it, that hundreds 

 of feet of solid rock have been gradually rasped awa}' from the 



