226 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I47 



SILT DEPOSITS 



In every deliberate test pit, within the ruin or outside, we invari- 

 ably came upon one or more of these silt layers. My field notes fre- 

 quently refer to them as "pavements" because they were so smooth 

 and hard as to seem man-made. When more than one was encoun- 

 tered only the amount of the overlying materials set each apart. 

 For example, 35 inches under the floor of Room 225 we exposed 

 the pavementlike surface of a 5-inch-thick adobe stratum with a 

 slight downward slope to the east and south; beneath it were three 

 lesser surfaces separated from one another by from 9 to 20 inches 

 of sand, clay, or occupational debris. A similar, or perhaps the same, 

 southward-sloping surface was noted 37 inches under the floor of 

 adjoining Room 241. 



Another silt layer 4 inches beneath the northeast steps. East 

 Refuse Mound (pi. 76, lower), may be a record of the same flood 

 that deposited a like layer 19 inches below the present surface at 

 the northeast corner of the West Mound enclosure despite the fact 

 nothing comparable appeared in the extended East Mound trench 

 (fig. 24). Although the main valley fill ofifers no evidence that the 

 surface gradient ever differed appreciably from what it is today, I 

 neglected to test this possibility fully. I neglected, for example, to 

 run a level between the two silt layers last noted or between that under 

 Station 1, Northeast Foundation Complex, and the silt surface sur- 

 rounding Pueblo del Arroyo and underlying at a depth of 5 feet 

 the small P. Ill ruin Jackson saw on the opposite bank. 



That buried silt layers should appear more clearly defined in some 

 trenches than in others is understandable, considering the character- 

 istics of flood waters and flood-borne silt. That the overburden should 

 be sand in its infinite variety is obvious, knowing that Chaco Canyon 

 is bordered by sandstone whose disintegration provided the canyon 

 floor. Thin clay streaks, gravel lenses, pellets, and chunks of water- 

 washed adobe identify in this overburden places where floodwaters 

 ponded. 



As Bryan (1954, p. 25) observed, these pavementlike silt beds 

 are generally of limited area although an occasional one may extend 

 a quarter-mile or more. They are the leavings of runoff wandering 

 laterally from the main flow — silty sand retarded by grassy ground 

 cover. In lesser measure we noted this same retarding effect at 

 present-day Navaho fields where a chico bush, a handful of bunch- 

 grass, or a chance rock sufficed to check or turn the advance of a 

 muddy rivulet. Unretarded, floodwaters in greater volume might 



