rain the ground, and that portion of the walls at present below its
surface, retains moisture much longer than the part of the walls which
stands clear; the moisture rises by capillary attraction a foot or two
above the ground surface, rendering the walls at this level softer than
elsewhere, and as this portion is more exposed to the flying sand which
the wind sweeps over the ground it is here that erosion attains its
maximum. The wall is gradually cut away at and just above the ground
surface until finally the base becomes too small to support it and it
falls en masse. Then and not till then surface erosion becomes an
important factor and the profile of the mass becomes finally rounded.
But it will be readily seen that a slight difference of texture, or
thickness, or exposure, or some trifling difference too minute for
observation, might easily add many decades to the apparent age of a
mound. The walls once fallen, however, the rounding or smoothing of the
mounds would probably proceed at an equal rate throughout the group, and
study of the profile gives a fairly good estimate as to the comparative
age of the mounds. On this basis the most ancient mounds are those
specified above, while the most recent are those in the immediate
vicinity of the Casa Grande ruin. This estimate accords well with the
limited historical data and with the Pima traditions, which recount that
the Casa Grande ruin was the last inhabited village in this vicinity.

[Illustration: Fig. 328.--Map of large mound.]

Probably intermediate in time between the Casa Grande ruin and the
rounded mounds described above should be placed the large structure
occupying the northern-central part of the map. This mound is deserving
of more than a passing notice. It consists of two mounds, each four or
five times the size of the Casa Grande ruin, resting on a flat-topped
pedestal or terrace about 5 feet above the general level. The summits of