THE GLACIAL GEOLOGY OF GRAND MESA, COLOILiDO 677 



reading, near the eastern end, is only 11,300 feet, though that was 

 along the trail and therefore not quite the highest point of the mesa. 

 The top is a slightly rolling plateau, sloping gently to the southwest. 

 At the base of the escarpment is a shelf two or three miles wide, 

 extending all along the north and east sides, covered by an intricate 

 network of moraines, with scores of lakes and ponds, mostly 

 morainal. Near the escarpment, which provided an abundant 

 supply of detrital material, the moraines are mostly high, narrow, 

 steep sided, and strike one another at various angles, in many 

 places forming basins of small radius. The more distant moraines, 

 representing the greatest extension of the ice, perhaps two or three 

 miles from the cHff, are lower and not so strongly defined. 



Probably the less pronounced character of the more distant 

 moraines is partly due to the fact that they have been longer 

 uncovered and subjected to greater erosion, and partly to their 

 greater distance from the source of supply. It is likely also that 

 at the time of the greatest extension of the ice the cliff was more 

 completely covered with ice and snow, so that less material fell 

 upon the surface of the glacier to be carried down to the moraine. 



The number, size, character, and relation to one another of 

 the moraines near the cliff record frequent and rapid changes 

 in the shape of the ice-front, with numerous small ice tongues 

 extending beyond the general line of the front. This would be 

 expected from the fact that the glacier (or glaciers) was doubtless 

 formed and maintained chiefly by drifted snow, driven over the 

 top by prevaiHng westerly winds and lodging along the rim of 

 the cliff. Even now huge drifts remain along the rim throughout 

 the summer. The drifts must have varied more or less from season 

 to season, from cycle to cycle, and as a result the points of maximum 

 and minimum accumulation, and consequently of greatest extension 

 and contraction, must have shifted from time to time. 



It seems likely, from our rather hurried investigation, that 

 at the time of greatest expansion the ice along the north and east 

 sides constituted one continuous glacier from ten to fifteen miles, 

 perhaps more, in width and not more than three or four miles 

 long. Such a condition would be unfavorable to the formation 

 of well-defined cirques, though in its inception probably a number 



