242 



GEOLOGY 



at the margin sometimes appear to be the result of the tension 

 developed on a curve. Oblique crevasses on the surface near the 

 sides are commonly ascribed to the tension between the faster- 

 moving center and the slower-moving margins, and in like manner 

 cracks that rise obliquely from the bottoms are attributed to the 

 tension between the faster-moving parts above and the slower- 

 moving parts below. All these crevasses indicate strains to which 

 a liquid, whose pressures are equal in all directions, does not offer 



Fig. 198. Crevassed glacier, the cracking due to change in grade of bnl. 

 North Greenland. 



a close analogy. Longitudinal crevasses may affect both the nar- 

 row part of a glacier and its deploying end, and are the result <>f 

 tension developed by movement within the ice itself, to which, 

 again, rivers offer no analogy. All cracks show that the glacier is 

 a very brittle body, incapable of resisting even very moderate 

 strains brought to bear upon it very slowly. In its behavior un< In- 

 tension, therefore, a glacier is notably unlike a river. 



Surface moraines. The surface of a glacier is often affecte< 1 1 y 

 rock debris, which is sometimes disposed in the form of belts or 

 moraines (Fig. 199). The surface moraines may be lateral, wnlinl. 

 or terminal. A lateral moraine is any considerable accumulation 

 of debris in a belt on the side of a glacier. A medial moraine is a 

 similar accumulation at some distance from the margins, but nol 

 necessarily in or even near the middle. There may be several medial 



