24 GLACIAL GEAVELS OF MAINE. 



grinding themselves and wearing away the underlying rock. Concussion 

 and attrition usually accompany each other, and it is sometimes difficult to 

 distino-uish between them. Concussion alone would leave the surfaces with 

 small granular projections. It is the office of attrition to rub these off The 

 attrition scratches of water-transported fragments are necessarily short, 

 since friction against the sides of the stones causes them to rotate, thus 

 giving them a tumbling motion, with consequent concussion. The distance 

 traveled by water-rolled pebbles in becoming rounded must depend on 

 many circumstances, including the velocity of the current, the abundance 

 of the di-ift, the condition of the bed of the stream (whether a uniform 

 declivity or a series of waterfalls and rapids), and the size, specific gravity, 

 hardness, brittleness, etc., of the fragments. 



Forms carved by water-borne sand. FrictiOU IS rliythmical, aud wlieUeVCr tlie 



solid rock or fragments, which for any cause are stationary for a consider- 

 able time, are swept by rapid currents bearing sand and gravel, they are 

 carved into conchoidal depressions or furrows separated by rather angular 

 ridges usually tranverse to the motion. Sand carving shows what sort of 

 work is constantly being done by the finer detritus — if not too fine — trans- 

 ported by a stream. Stones which from time to time are moved into new 

 positions owe their shapes to concussion and attrition of large stones as well 

 as to sand carving, and do not show the peculiar depressions due to the 

 rhythmical movement of the water over a stationary surface. Instances of 

 sand carving can be seen at most of the rapids and waterfalls of Maine 

 where the rock is hard and resists weathering sufficiently well. Quartz 

 veins in granite afford the finest examples of this process, as, for instance, 

 those at Rumford Falls. Sometimes the peculiar markings of sand carving 

 are very distinct on small stones which have become wedged into a cavity 

 of the solid rock. I found some such near the head of Rumford Falls 

 which might have remained fixed in position for several years. The upper 

 extremity was faceted to a plane surface, except that it showed the con- 

 choidal grooves characteristic of sand carving as distinctly as any of the 

 rock in situ. The pebbles of sea and lake beaches are perhaps rounded 

 more by concussion than by attrition. According to Sorby and Daubrc^e, 

 very fine sand grains remain angular after motion in water. 



Forms carved by wrind-biown sand. — Saud aud fiue gravcl, wlicu impelled by the 

 wind against bowlders and other stationary objects, rapidly wear them 



