266 GEOLOGY. 



tility, it would adapt itself to tension brought to bear upon it so slowly 

 as are many of the tensions which produce crevassing. In its behavior 

 under tension therefore a glacier is notably unlike a river. 



SURFACE FEATURES. 



Topography. Many of the minor irregularities of the surface of a 

 glacier are the result of crevassing. After the ice is crevassed, the sun's 

 rays and the air which has been warmed by them penetrate the openings 

 ancl melt the ice. The melting is most rapid at the top, and decreases 

 downward. The result is that the sections of ice between adjacent 

 crevasses are narrowed into wedges. If there be cross-crevassing, as is 

 common, points instead of wedges result. As the sort of surface shown 

 in Fig. 239 develops, any debris which was on the ice slides into the 

 crdvices, and the upper surface becomes clean. 



Where ice is crevassed transversely, and where melting is not rapid, 

 the crevasses may close as the ice moves forward, and the regelation 

 of adjoining faces heals the rents in the surface. Even in this case, 

 however, the surface is hkely to be more or less undulating because of 

 the waste on the sides of the crevices before they are closed. After 

 regelation, surface ablation tends to obliterate the protuberances. 



The topography of the surface of the ice is affected by other con- 

 ditions. All parts of the surface of the ice are not equally compact, 

 and the least compact portions melt most rapidly, giving rise to depres- 

 sions, while the more sohd parts occasion protuberances. Both depres- 

 sions and protuberances may be regular or irregular in form (Figs. 240 

 and 241). Undulations of the bed often show themselves in the surface of 

 the ice as suggested by Fig. 242. In such cases, ponds or lakelets some- 

 times accumulate on the surface of the ice. The topography of the 

 ice in such cases seems to show that the ice is forced up slope. 



Surface moraines. — The surface of a glacier is often affected by 

 debris of one sort or another, and this also influences its topography. The 

 debris is sometimes disposed in the form of belts or moraines (Figs. 237, 

 243). The surface moraines may be lateral, medial, 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 not necessarily in or near the 



