The Geology of Chaco Canyon, New Mexico In Relation To The Life And Remains Of The Prehistoric Peoples Of Pueblo Bonito Smithsonian Miscelleanous Collections Volume 122, Number 7

The various relics of man's activity found exposed in the walls of the modern arroyo indicate that man inhabited Chaco Canyon during the latter part of the period of alluviation. The greatest depth at which potsherds were seen during the course of our investigation was in a bed ranging from 17 feet to 20 feet 6 inches below the surface of the valley floor (section No. 3, fig. 1). At another site, section 8, charcoal was found at a depth of 21 feet. Man may have been present during the earlier phases of sedimentation but his presence during deposition of the last 21 feet of fill is definitely established. It seems a fair inference that in the prehistoric period most streams in this region were building up their flood plains and primitive man witnessed the process.

Chaco Canyon was by no means unique in this respect. Ashes, pottery, artifacts, and like evidences of man have been recovered at various depths in the alluvium of other Southwestern valleys. Published records are not so numerous as might be expected but those available, together with some of the present writer's observations, are tabulated herewith:⁴

Relics of man in the recent alluvium

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⁴It is to be emphasized that this paper was begun in 1925 and was added to from time to time until 1940, when it was left incomplete. Hence the absence of later references—N. M. J.

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In Chaco Canyon, potsherds and other artifacts were collected from noteworthy depths at many different places. Generally, sherds from the upper 4 to 6 feet of the valley fill are of relatively late types of pottery (Pueblo III). Earlier types are found at greater depths. From our data it seems clear that the upper 4 to 6 feet of the main valley fill is not only younger than the lower horizons but that the division between the prehistoric periods known as Pueblo II and III lies at the base of this layer.

Two Pueblo I pit houses in Chaco Canyon were excavated and described by Judd (1924). One of them, so far as critical evidence was preserved, may have been built after alluviation had filled the valley to its present level. It was situated at the base of the talus on the south side of the canyon, opposite Pueblo Bonito. The other and more perfect example, at the locality marked "Pit House" on figure 1, was evidently occupied when the surface of the alluvial plain was 5 feet 11 inches or, in round numbers, 6 feet below the present surface (pl. 6, lower). Because potsherds of precisely the same type as those recovered from these two P. I pit houses have been found repeatedly in the valley fill, one can scarcely avoid the conclusion that during the time when strata at depths of 4 to 21 feet were being deposited, Chaco Canyon was occupied predominantly by people of this cultural stage.

The exact relationship between the Pit House people and later inhabitants of the valley is not wholly clear at this writing. Pottery similar to that from Judd's two pit houses has been found in and below rubbish associated with the older parts of Pueblo Bonito and portions of a slab-lined pit house were encountered 12 feet beneath the west court of the great ruin. I leave solution of this puzzle to the archeologists but, at the moment, it would seem as though the inhabitants of Chaco Canyon had passed rather abruptly from a P. I to a P. III society. So direct a transition in human culture appears physically possible for the valley fill records no break in sedimentation; hence the environment must have been relatively uniform during the change.

As previously stated, numerous relics of the Pueblo III people have been found in the upper 4 feet of the main body of the valley alluvium. Most distinctive of these remains are house walls which, in a number of instances, rest on undisturbed material 3 feet or more below the general surface. There is no better example than the small house on the south bank of the arroyo, opposite Pueblo del Arroyo. Its relationship to the underlying strata is clearly shown in plate 6, upper. One corner had recently been undermined when Jackson first noted its precarious situation in 1877. Since then the little house has paid annual tribute to Chaco floods and its complete destruction is only a matter of time.

Near section No. 2 (p. 53) the foundations of another small building reach a depth below the surface of 8 feet, but this is exceptional. Apparently at this site the alluvial fan of the adjacent tributary has been built up several feet above the normal level.