The application of the hypothesis to the map brings out some interesting results. In the first place, it may be seen that in the lowest mounds, such as those in the northwestern corner of the sheet, on the southern margin, and southwest of the well-marked mound on the eastern margin, the contours are more flowing and the slopes more gentle than in others. This suggests that these smoothed mounds are older than the others, and, further, that their present height is not so great as their former height; and again, under this hypothesis, it suggests that the remains do not belong to one period, but that the interval which elapsed between the abandonment of the structures whose sites are marked by the low mounds and the most recent abandonment was long. In other words, this group, under the hypothesis, affords another illustration of a fact constantly impressed on the student of southwestern village remains, that each village site marks but an epoch in the history of the tribe occupying it—a period during which there was constant, 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 matter 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[9] discussed, it can be dismissed here with the statement that the Casa Grande group 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 no criterion of 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 irregular. 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 antiquity. 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 projection remaining the same. It is through sapping or undermining at the ground surface that walls are destroyed. An inspection of the illustrations accompanying this paper will show what is meant by sapping: the external walls are cut away at the ground surface to a depth varying 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 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.
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 these mounds, which are nearly flat, are some 13 feet above this level. The sides of the mounds slope very sharply, and have suffered somewhat from erosion, being cut by deep gullies, as shown in figure 328, which is an enlargement from the map. It has been stated that these structures were mounds, pure and simple, used for sacrifice or worship, resembling somewhat the well-known pyramid of Cholula; but there is no doubt that they are the remains of house-structures, for a careful examination of the surface on the slopes, reveals the ends of regular walls. The height is not exceptional, the mound on the east being less than 3 feet lower, while the one on the southeast lacks less than 4 feet of its height. The characteristic feature, however, and one difficult to explain, except on the hypothesis stated, is the sharp slope of the sides. It will be noticed that the raised base or terrace on which the mounds are located is not perfectly flat, but on the contrary has a raised rim. This rim seems quite inconsistent with the theory which has been advanced that the terrace was built up solidly as a terrace or base, as in that case it would seem natural that the slope from the base of the mounds to the edge of the terrace would be continuous.
There is an abundance of room between the crest of the rim and the base of the terrace for a row of single rooms, inclosing a court within which the main structures stood, or such a court may have been covered, wholly or partly with clusters of rooms, single storied outside, but rising in the center, in two main clusters, three or more stories high. Such an agglomeration of rooms might under certain conditions produce the result seen here, although a circumscribing heavy wall, occupying the position of the crest of the rim and inclosing two main clusters each rising three or more stories, might also produce this result. The difficulty with the latter hypothesis is, however, that under it we should expect to find a greater depression between the base of the mounds and the edge of the terrace. The most reasonable hypothesis, therefore, is that the space between the base of the mounds and the edge of the terrace was occupied by rooms of one story. This would also help to explain the steepness of the slopes of the mounds themselves. The walls of the structures they represent, being protected by the adjacent low walls of the one-story rooms, would not suffer appreciably by undermining at the ground level, and if the central room or rooms of each cluster were higher than the surrounding rooms, as is the case in the Casa Grande ruin, the exterior walls, being usually heavier than the inner walls, would be the last to succumb, the clusters would be filled up by the disintegration of the inner walls, and not until the spaces between the low one-story walls surrounding the central cluster were nearly filled up would the pronounced disintegration of the outer walls of the structures commence. At that period the walls were probably covered and protected by debris dropping from above, and possibly the profile of the mounds was already established, being only slightly modified by surface erosion since.
| [PLATE LII] |
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GROUND PLAN OF CASA GRANDE RUIN |
About the center of the eastern side of the terrace, and also on the western side, the water which falls on the surface of the structure is discharged through rather pronounced depressions at these points. These depressions are not the work of running water, though doubtless emphasized by that agency, but represent low or open spaces in the original structure, probably passageways or gateways. Furthermore, before or inside each gateway there is a slightly depressed area, just where we would expect to find it under our hypothesis, and showing that the process of filling in is not yet completed. If the structure were to remain undisturbed for some decades longer these spaces would doubtless be filled up from material washed from the mounds, giving eventually a continuous slope from the base of the mounds to the edge of the terrace.
On the eastern margin of the map and in the southeastern corner two small and sharply defined mounds, differing in character from any others of the group, are represented. That shown on the eastern margin rises about 6 feet and the other about 10 feet above the surrounding level, and both stand out alone, no other remains occurring within a hundred yards in any direction. These mounds seem a thing apart from the other remains in the group; and it is probable that they represent the latest period in the occupancy of this site, or possibly a period subsequent to its final abandonment as a place of residence. Analogous remains occur in conjunction with some large ruins in the north, and there they represent single rooms, parts of the original structure kept in a fair state of preservation by occasional repairs while the remainder of the village was going to ruin, and used as farming outlooks long after the site was abandoned as a place of residence. As these farming outlooks have been discussed at some length in another paper[10] it is not necessary here to enlarge upon their function and the important part they play in Pueblo architecture. If the high mounds in question mark, as supposed, the sites of farming outlooks such as those which are found in the north, they indicate that the occupancy of the region in which they occur was continued after the abandonment of the Casa Grande structure by the people who built it or by people of similar habits and customs.
An inspection of the map will show a number of depressions, some of quite large area, indicated by dotted contour lines. The principal one occurs a little west of the center of the area, and is worth more than a passing notice since similar structures are widely distributed throughout this region. It may be roughly characterized as a mound with excavated center. The ground for some distance about the structure (except for two depressions discussed later) is quite flat. From this flat surface as a base the structure rises to a height of 5 feet. From the exterior it has the appearance of an ordinary mound, but on reaching the top the interior is found to be hollowed out to a depth which even at the present day is below the surrounding surface, although not below the depressions adjoining. The main structure or mound is shown in figure 329 (an enlargement from the map). It measures on top of the crest 150 feet from north to south and about 80 feet from east to west, but covers a ground area of 200 feet by 120 feet or over half an acre. The crest is of the same height throughout, except for slight elevations on the eastern and western sides and a little knoll or swell in the southwestern corner. There is no indication of any break in the continuity of the crest such as would be found were there openings or gateways to the interior. The bottom of the depression in the main structure is at present about a foot below the surrounding ground surface, but it must have been originally considerably more than this, as the profile indicates long exposure to atmospheric erosion and consequent filling of the interior. No excavation was made and the character of the construction can not be determined, but the mound is apparently a simple earth structure—not laid up in blocks, like the Casa Grande ruin.