22 GLACIAL GRAVELS OF MAINE. 



the readily warmed shore, and thus become detached. They then drift 

 hither and thither under the action of winds or tides, and finally drop their 

 biTrden of drift upon the floor of the sea or lake, or upon the shore where 

 they may have been stranded. 



River ice. — Thls dififcrs from the floe of shore ice only in situation. The 

 ice of rivers freezes fast to stones and bowlders, either on the shores or in 

 shallow channels. When the ice breaks up in the spring, these stones and 

 bowlders are often transported long- distances. Frequently as the ice goes 

 out it forms jams or gorges in its channel. When. the dam at last yields to 

 the pressure of the water behind it, the ice often pushes along with it large 

 quantities of bowlders and other drift. The moving ice dam acts as a sort 

 of glacier, the units of ice motion being the blocks of ice, and not indeter- 

 minate masses, as in the glacier. Similar dams must frequently form in 

 the channels of superficial streams on the ice, as well as in those of the 

 subglacial streams. 



SHAPES OF DRIFT FRAGMENTS. 



Crystalline forms, or those due to crystalline cleavage. lu MaiuC, nOt UufreqUeUtly, CryS- 



tals of garnet, quartz, and other hard minerals can be found in sand and 

 other forms of drift. Easily cleavable minerals, such as feldspar, are 

 usually found in their cleavage forms, more or less modified by attrition. 



Fracture forms. — Tficse arc the augular, prismatic, or more or less irregular 

 forms into which r(-)cks and minerals fracture under the influence of heat 

 and cold, joints, etc. The forms vary according to the composition and 

 structure of the rocks, each kind of rock having a prevailing form peculiar 

 to itself. These forms are so characteristic that one can often know the 

 nature of a bowlder from its shape alone. 



Weather-rounded forms. — Whcu rocks nvo of rather unifomi composition and 

 structure, their fracture forms naturally weather faster at the exposed angles, 

 and thus tend toward the spherical form. For instance, the surface of a 

 weathered granite bowlder is somewhat rough, being composed of a great 

 number of small crystalline, fracture, and cleavage surfaces, but its general 

 shape is rounded. The most of the granitic and syenitic bowlders owe their 

 rounded shapes, not to the attrition of the glacier, but to weathering. They 

 are no rounder than similar bowlders under the tropics in Egypt. A good 

 example of the progressive changes from angular blocks of fracture to 



