300 CASA GRANDE RUIN. [eth.akn.13 



incessant change, new bands or minor divisions of the tribe appearing on 

 the scene, other divisions leaving the parent village for other sites, and 

 the ebb and flow continuing until at some period in its history the 

 population of a village sometimes became so reduced that the remainder, 

 as a Tuatter of precaution, or for some trifling reason, abandoned it en 

 masse. This phase of pueblo life, more prominent in the olden days 

 than at present, but still extant, has not received the prominence it 

 deserves in the study of southwestern remains. Its effects can be seen 

 in almost every ruin ; not all the villages of a group, nor even all the 

 parts of a village, were inhabited at the same time, and estimates of 

 population based on the number of ruins within a given region, and 

 even those based on the size of a given ruin, must be materially 

 revised. As this subject has been elsewhere' discussed, it can be dis- 

 missed here with the statement that the Casa Grande gi'oup seems to 

 have formed no exception to the general rule, but that its population 

 changed from time to time, and that the extent of the remains is uo 

 criterion (jf the former population. 



It will be noticed that in some of the mounds, noticeably those in the 

 immediate vicinity of the Casa Grande ruin, the surface is very irregu- 

 lar. In this instance the irregularity indicates a recent formation of 

 surface ; for at this point many walls now marked only by mounds were 

 standing within the historical period. External contour is of course a 

 product of erosion, yet similarity of contour does not necessarily indicate 

 either equal erosion or equal antiqiiity. Surface erosion does not become 

 a prominent factor until after the walls have fallen, and one wall may 

 easily last for a century or two centuries longer than another similarly 

 situated. The surface erosion of a standing wall of grout, such as these 

 under discussion, is very slight; photographs of the Casa Grande ruin, 

 extending over a period of sixteen years, and made from practically 

 the same point of view, show that the skyline or silhouette remained 

 essentially unchanged during that period, every little knob and projec- 

 tion remaining the same. It is through sapping or undermining at the 

 gi-ound surface that walls are destroyed. An inspection of the illustra- 

 tions accompanying this ijaper will show what is meant by sapping: 

 the external walls are cut away at the ground surface to a depth vary- 

 ing from a few inches to nearly 2 feet. After a 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 mois- 

 ture rises by capillary attraction a foot or two above the ground surface, 

 rendering the walls at this level softer than elsewhere, and as this por 

 tion is more exposed to the flying saud which the wind sweeps over the 

 ground it is here that erosion attains its maximum. The wall is gradu- 

 ally cut away at and just above the ground surface until finally tlie 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 



' See pp. 179-261 of this Report, "Aboriginal Remains in Verde Valley." 



