MA LA SPINA GLACIER. 231 



bottom, so that the basins in vertical cross section have some- 

 thing of an hour-glass form. The walls are frequentl}^ from 

 50 to 100 feet high, with a slope of 40° to 50°, and some- 

 times are nearly perpendicular. Near the water's edge the 

 banks are undercut so as to leave a ridge projecting over the 

 water. The upper edge of the walls is formed of the sheet of 

 debris which covers the glacier, and the melting of the ice 

 beneath causes this material to roll and slide down the ice slopes 

 and plunge into the waters below. The lakes are usually less 

 than 100 feet in diameter, but larger ones are by no means 

 uncommon, several being observed which were 150 or 200 yards 

 across. Their waters are always turbid owing to the mud which is 

 carried into them by small avalanches and by the rills that trickle 

 from their sides. The rattle of stones falling into them is fre- 

 quently heard while traveling over the glacier, and is especially 

 noticeable on warm days, when the ice is melting rapidly, but is 

 even more marked during heavy rains. The crater-like walls 

 inclosing the lakes are seldom of uniform height, but frequently 

 rise into pinnacles. Between the pinnacles there are occasion- 

 ally low saddles, through which in some instances the lakes over- 

 flow. Frequently there are two low saddles nearly opposite to 

 each other, which suggests that the lakes were formed by the 

 widening of crevasses. The stones and dirt which fall into 

 them, owing to the melting of the walls, gradually fill their bot- 

 toms. Instances are numerous where the waters have escaped 

 through crevasses or openings in the bottom of the basin, leav- 

 ing an exceedingly rough depression, with a heavy deposit of 

 debris at the bottom. 



As the general surface of the glacier is lowered by melting, the 

 partially filled holes gradually disappear and their floors, owing 

 to the deep accumulation of debris on them, which protects the 

 ice from melting, become elevated above the surrounding sur- 

 face, in the same manner that glacial tables are formed. The 

 debris covering these elevations slides down their sides as 

 melting progresses, and finally a rugged pyramid of ice, 

 covered with a thin coating of debris, occupies the place of the 



