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

One of the notable discoveries of W. H. Jackson during this keen observer's visit in 1877 was a buried channel just south of Pueblo del Arroyo, 14 feet deep and containing numerous potsherds, flint chips, and other human relics (Jackson, 1878, pp. 443-444). Unquestionably this exposure was among those later seen by Prof. Richard E. Dodge and reported in published excerpts from his field notes of 1899 and 1901 (in Pepper, 1920, pp. 23-25). It is perhaps only natural, therefore, that the present writer, although these earlier discoveries were unknown to him at the time, should have happened upon the same ancient channel mentioned by Jackson and Dodge.

As shown in plate 7, lower, this buried channel is round-bottomed and crescentic in cross section. Its boundaries are sharp and clearly cut across the horizontal bedding of the older valley fill. The materials within the channel are such as might be expected: gravel and cross bedded sand 3 to 4 feet thick form the base of the deposit and are succeeded by clay which, in turn, is overlain by successive beds of sand and sandy clay. The first clay bed seems to indicate a ponding of flood water in this channel while the main floods were diverted into another. A series of sandstone blocks in the clay bed resembles stepping stones placed to provide a passage over the mud but such an explanation of the blocks, though plausible, can scarcely be considered as established.

Buried potsherds, broken bones, and beads were revealed also in a reentrant of the modern arroyo which existed in 1924 and 1925 a few yards from the south wall of Pueblo del Arroyo and a hundred feet, more or less, west of the exposure described above. The potsherds included some of the latest varieties of pottery made in Pueblo Bonito and were, according to Mr. Judd, produced somewhere around A.D. 1100. Although this deposit was visible for about 75 feet its lateral contacts were, at the time of my examination, unfortunately obscured by a small gulley on the east and by unusually severe weathering on the west.

With two exposures in line, extension of this buried channel eastward under the plain before Pueblo Bonito was obvious. To provide a third contact, a test pit (pit No. 4) was dug 60 feet back from the edge of the bank where the channel was first observed. At a depth of 18 feet sherds of the latest Pueblo Bonito pottery were found, proving this pit to be located in the buried channel.

Nearer Pueblo Bonito, Judd's test pit No. 3 was deepened and more late pottery was discovered. Next, a trench previously dug through the west refuse mound as a means of studying its composition was extended out into the flat. In this extension the north bank of the buried channel was clearly profiled. The channel filling, marked by coarser material, rests unconformably on the edge of the old refuse mound and on strata of the main valley fill.

Farther east, an outcropping in the north wall of the present arroyo gave still another point and thus enabled us to indicate the course of the buried channel as it passed Pueblo Bonito. In 1936 Senter (1937) dug pits near Pueblo Chettro Kettle and likewise exposed a buried channel containing potsherds of late date. From the position given, however, I would judge his buried channel to be a branch of the arroyo produced by the stream which drained the reentrant in the cliffs back of Chettro Kettle.

A careful reading of Jackson's account shows that there was in 1877 an abandoned side channel 8 feet deep, or half that of the main arroyo, between the latter and the ruins of Pueblo del Arroyo. This side channel ("old arroyo" of his map) still existed, in part at least, in 1925. It must be distinguished from Jackson's buried channel which he describes as 14 feet deep and marked by "an undulating stratum of broken pottery, flint chippings, and small bones firmly embedded in a coarse gravelly deposit." This stratum he first observed below masonry walls exposed by the side channel above referred to—walls that did not show on the surface. He traced the stratum upstream "several rods" in the vertical north bank of the main arroyo, noted its presence also on the opposite side, and followed it thence farther upstream to the small ruin shown in plate 6, upper. The stratum reached its lowest level 14 feet below the surface, or 2 feet above the bed of the main arroyo, a fact that led to Jackson's conclusion: "Since the desertion of this region the old bed has become filled to the depth of at least 14 feet, and through this the arroyo has made its present channel." (Jackson, 1878, p. 444.)

In 1924 and 1925 the walls of the "side channel" were much weathered and waste from the Society's excavation of Pueblo del Arroyo had been dumped into the main arroyo southeast of the ruin in an effort to check further erosion. The exposures described by Jackson had been destroyed in the widening and deepening of this arroyo but in an excavation made for a storage cellar back of the old Wetherill "hotel," the northern border of a channel containing late Bonito pottery could still be seen.

When considering the significance of the buried channel described above, I have often thought of it as the "post-Bonito channel" because late Bonito potsherds on the bottom of it identify the channel with the final years of Pueblo Bonito or even later. In 1925, by means of test pits in the plain that lies in front of Pueblo Bonito, we traced this old channel eastward from the exposure near Pueblo del Arroyo for more than 1,000 feet. The lenses of clayey gravel and other materials found in those pits were thoroughly characteristic of channel deposits. Late Bonito pottery fragments were found from 10 to 18 feet below the surface in test pit No. 3 and from 7 to 15 feet below in pit No. 4. Additional exposures enabled us to plot the channel's course with a high degree of accuracy for several miles.

That this buried channel is part of a continuous arroyo once extending the full length of Chaco Canyon seems definitely established. The existence of such a through channel is proof that there intervened during alluviation of the valley a period of arroyo formation and dissection similar to the present. To put it another way, an early period of alluviation, represented by the main valley fill, was followed by a period of dissection that included our post-Bonito channel. After an unknown interval alluviation was again resumed, the arroyo system was filled up and some slight addition made to the valley deposits, and alluviation, or at least a balance between alluviation and erosion, was continued down to the year 1860 to be interrupted in the ensuing decade by formation of the present arroyo.

Such a history appeals to the imagination by reason of its symmetry and because of the nice correlation with human history that can be made, as outlined in a later part of this paper. However, the importance of the issues involved requires that available evidence be examined with care.

The buried channel, as exposed in various places, is round-bottomed although vertical side walls have been detected in a few instances. Vertical walls are to be expected in any through-flowing arroyo. However, as previously pointed out, local conditions often prevented a critical study of side contacts of the old channel. The present arroyo, which coincides very closely with this post-Bonito channel throughout much of the area under examination, has, by reason of its greater width, removed all traces of the buried channel over considerable distances. Sections of the old channel have been discovered largely through search for lenses of gravel containing late-Bonito potsherds, and in many localities the side contacts of the old channel are poorly exposed. The best evidence found that however round-bottomed the channel may have been, its walls were generally vertical, is its known length and presumed extension both ways.

Supporting evidence that the post-Bonito channel is part of a continuous arroyo is given by the existence of lateral channels representing its important tributaries. A mile downstream from Pueblo Bonito there is an indentation of the north canyon wall which we called "the Rincon del Camino." The present road to Farmington and Aztec goes this way. Drainage from a considerable area falls over the cliffs at the head of this rincon and reaches the main arroyo through a half-mile-long tributary (pl. 8, upper). The material exposed in this tributary consists of irregularly bedded yellow sand, numerous lenses of dirty gravel, and a few lenses, 2 to 6 inches thick, of laminated dark clay. All this material, except the clay, is similar to that now carried by the tributary stream and was undoubtedly derived exclusively from the drainage of Rincon del Camino. Similar sand and gravel are exposed in the north wall of the main arroyo for about 300 yards east of the tributary.

There is thus a triangle of material derived from the alluvial fan of the side stream extending from the main arroyo to the head of the rincon, as outlined in figure I. This triangular mass contains no material from upstream except the clay beds, and evidently the area could not have been occupied by the main stream of the canyon during deposition of the fan. All other evidence, however, leads to the conclusion that during the period of alluviation the main stream wandered at will over the canyon floor. If, however, an arroyo similar to the present one had been formed, contemporary drainage from Rincon del Camino might easily have excavated a tributary large enough to carry away and destroy the main valley fill over the entire triangular area shown on the map. If conditions of alluviation were restored thereafter and the main arroyo began to fill up, the excavated area would also be filled. The waters of the main arroyo could, however, enter the area only as a gentle overflow or backwater from which clay similar to that of the clay lenses would have been deposited. Most of the filling would consist of sand and gravel brought in by the tributary stream. It seems obvious, therefore, that the fan of Rincon del Camino is later than the main valley fill and was deposited after a period of erosion.

That the area was occupied by human beings during this episode is evident from discovery, at the place marked section 9 on figure 1, of several hearths from 10 feet to 12 feet 8 inches below the surface (pl. 9, lower). A short distance away two potsherds were found in gravel lenses, but they are undecorated ware of indeterminate age. A small sherd collection, gathered among lumps fallen from the bank of the main arroyo but derived from the fan of Rincon del Camino, consists also of fragments indeterminate as to age.

The human remains before us, therefore, do not afford reliable evidence of the synchronous erosion and refilling of the post-Bonito channel and the triangular area at Rincon del Camino. However, it seems impossible to account for the type of sand and gravel found here except on the theory that it was deposited in an area excavated during an earlier period of erosion. The alluvial fan of Rincon del Camino presents general evidence, if not conclusive proof, of a period of erosion and sedimentation contemporaneous with the cycle postulated from a consideration of the post-Bonito channel.

The sequence of geologic events in Chaco Canyon, if the post-Bonito episodes of erosion and sedimentation be accepted, may be summarized as follows:

Recent geologic events in Chaco Canyon

The twofold character of the valley fill is thus well established. The buried channel has been traced from near Una Vida to a point about 1-1/2 miles below Pueblo Bonito where it becomes so large that remnants of the main valley fill cannot be identified.