Bowman — Geologic Relations of the Guzco Remains. 319 



tributary to the unglaciated part of the valley. In time the 

 entire flat-floored portion will be dissected to the point where 

 no flat floor remains ; the steep marginal walls will ultimately 

 be reduced in gradient, and the striae obliterated. The form of 

 the upper valley will then harmonize with the form of the 

 unglaciated portion of the valley farther down. If the gravels 

 had been deposited in post-glacial time, glacial forms in the 

 valley heads would be destroyed by. that great headwater ero- 

 sion which the valley alluvium demands. 



The bluff in whose face the Cuzco man and associated ver- 

 tebrate remains were found leads up by steep slopes to a broad, 

 smooth, and almost flat-topped gravel spur, one of a group of 

 spurs whose upper surfaces fall into a common plane as in fig. 

 1. In all cases the borders of the spurs are marked by bluffs 

 of steep descent, indeed in many cases they are unscalable 

 and the infrequent paths run by selected routes. The highest 

 bluffs rise several hundred feet above the valley floors. Their 

 height and steepness are clearly dependent on the climate, 

 which is semi-arid and marked by light, infrequent rains. 

 Seepage lines are rare and occur only at low levels. So coarse 

 and steep are the bluffs, so well drained, and so scantily 

 watered that many of them exhibit not the slightest sign of 

 seepage though all bear on their surfaces and margins signs of 

 water action. Later alluvium derived from older rock waste is 

 in evidence everywhere either as a fringe about the bases of 

 the bluffs or as long trailing masses of gravel along the dry 

 channels of the tributary streams. 



The relation of the bones to the surface of the bluff leads 

 to some important considerations. The finding of material on 

 the immediate face of the bluff does not merely by virtue of 

 that position indicate with certainty natural burial during the 

 upbuilding of the formation and reexposure as a result of 

 present erosion. Though the bluff is very steep, a number of 

 plant forms cling to it. These catch particles of falling or 

 sliding material and even pieces of pottery. In a number of 

 cases it was noted that the vegetation responsible for such 

 obstruction in time dies and may be entirely or almost entirely 

 removed. Surflcial objects are then left attached to the 

 face of the bluff, from which they may be easily removed. 

 The steeper the bluff the more difficult the retention on a 

 sloping surface becomes. The patchy mantle of foreign mate- 

 rial is always loose, unstratified, fine-textured, and in strong 

 contrast to the undisturbed material directly beneath it. As 

 contrasted to such surface drift, it is noteworthy that the verte- 

 brate remains were not on the face of the bluff but eight 

 inches back from the face measuring to the median line of 

 the deposits : also that they were stratified with the gravels, 



