62 THE SUCCESSIVE CULTURES AT ANAU. 
that its progress must be very slow, and that the time during which these 2,200 feet 
of sediments were laid down can be expressed only in terms of thousands of centu- 
ries. Since the uppermost of the sediments are of recent formation, it is evident 
that the column, as represented in the well, must extend into the glacial period, 
and probably include at least the last of the glacial epochs. We should therefore 
expect to find, in the character of the sediments, variations recording phases of the 
glacial period. The record of the boring shows uniformly brown loam with occa- 
sional layers of sand or of gravel, more rarely of cobbles, all of which are distinctly 
delta-oasis formations. An examination of the record on fig. 21 shows at once a 
great difference in the distribution of the coarser layers. Below 750 feet the 
layers of gravel range from 150 to 200 feet apart, and the lower 500 feet of the 
well has only a solitary and thin layer of any kind in the loam. On the other 
hand, between 500 and 750 feet, the coarse layers, and here largely cobble, recur 
with short intervals. And the percentage of coarse material, including sand, 
between 500 and goo feet, is several times as great as in the rest of the column. 
This would seem to indicate a period when the stream had exceptionally great 
transporting power. Since there can be no doubt that at least the last ice- 
epoch of the glacial period was contemporaneous with some part of this column, 
it would seem more probable that the conditions that caused the rapid recurrence 
of coarse beds are assignable to climatic rather than to orogenic causes. Looking 
at the column from this point of view, we might see the climax of the ice-epoch in 
the part between 600 and 800 feet. Without attempting to analyze closely the 
significance of all the details of the column, it may be worth while to carry this 
tentative speculation farther, and determine approximately the age of this part 
of the record. Since the beds coarser than loam clearly represent alluviation, 
they are evidence that the column has grown pari passu with the sinking of the 
zone of depression. The maximum rate of growth would be that of the maximum 
of alluviation on the sinking zone, and this rate is very nearly represented by that 
of the growth of irrigation sediments where practically all of the silts are held up 
artificially. For this we may take the rate of 1 foot per century determined on 
the neighboring Anau delta. Applying this rate, the space between 500 and 900 
feet would cover the period between 50,000 and 90,000 years ago; and the mean 
point at 700 feet would be 70,000 years ago. 
