THE GLACIER'S SURFACE FEATURES 391 



be picked out of the partially melted ice as articulating balls the 

 size of the fist or larger. Glacier ice has therefore a structure 

 quite different from that of lake ice, since the latter is developed 

 in parallel needles perpendicular to the freezing surface. 



Crevasses and seracs. Prominent surface indications of gla- 

 cier movement are found in the open cracks or crevasses, which 

 are the marks of its yielding to tensional stresses. Crevasses 

 are apt to run either directly across the glacier, wherever there is 

 a steep descent upon its bed, or diagonally, running in from the 

 margin and directed up-glacier (r, r, r, of Fig. 416), though they 

 occasionally run longitudinally with the glacier when there is 

 a rock terrace at the side of the valley beneath the ice. The 

 diagonal crevasses at the glacier margin are due to the more 

 sluggish movement where the ice is held back by friction upon the 

 walls of the valley, as will be clear from Fig. 416. The square a 

 has by this movement been distorted into the lozenge a', so that 

 the line xy has been extended into x'y', with the obvious tendency 

 to open cracks in the direction ss. 



Every glacier surface below its neve is marked by steps or 

 terraces, which are well understood to overlie corresponding steps 

 of the cascade stairway to be seen in all vacated glacier valleys 

 (plate 19). The steep risers of these steps are usually marked 

 by parallel crevasses which cross the glacier. Under the rays 

 of the sun, which strike them more from one side than from 

 the other, the slices into which the ice 

 is divided are transformed into sharp- 

 ened blades and needles which are 

 known as seracs (Fig. 401, p. 376, and 

 Fig. 417). ^ 



The numerous Crevasses tell US that FIG. 417. Transverse crevasses 



the ice is many times wrenched apart at the fall below a glacier step 



during its journey down the glacier. transformed by unsymmetri- 

 J cal melting into sSracs. 



This has been illustrated by some- 

 what grewsome incidents connected with accidents to Alpinists, 

 but as they illustrate in some measure both the mode and the rate 

 of motion of Swiss glaciers, they are worthy of our consideration. 



Bodies given up by the Glacier des Bossons. In the year 

 1820, during one of the earlier ascents of Mont Blanc, three guides 

 were buried beneath an avalanche near the Rochers Rouges in 



