University of Virginia Library

I. Appendix I

Floor Deposition and Erosion in Chaco Canyon

By Donovan Senter

The problem of Chaco Canyon deposition since the period of prehistoric
occupation has caught the interest of every group of archaeologists
working in the canyon. The sharply cut walls of the Chaco
wash show profiles of a former creek bed which may have been that on
the surface at the time the Pueblo people farmed the canyon floor.
Simpson says there was no arroyo in the Chaco in 1849;[1] he found that
"The Rio Chaco, near our camp, (in the upper part of the canyon) has
a width of eight feet, and a depth of one and a half. Its waters, which
are of a rich clay color, can only be relied upon with certainty during the
wet season." Jackson,[2] in 1877, just twenty-eight years later, reports
the arroyo as deep as it is today. Tomasito, an old Navajo still living in
the canyon, claims that there was no arroyo there when he was a small
boy. The combined data indicate a repeated filling and cutting of the
central section of the canyon where the arroyo runs.

In 1928 a pit was sunk at the back wall of Chetro Ketl; the bottom
of the wall, set upon the old surface level, was found 14 feet beneath
the present surface, but this depth was effected by the wall intercepting
material washed from the northern edge of the canyon. Nine miles up
the canyon the caving sides of the wash have broken away to uncover
the profiles of pit houses of Basket Maker III, beneath 10 feet of sandy
deposit. This depth might have been greater, however, except for the
progressive erosion of the upper bank. Neither depth could be used as
an accurate criterion of deposition rate.

The lack of burial grounds for the large ruins has long been a
matter of concern to archaeologists. Were they, perhaps, covered by a
deep layer of sand on the canyon floor? A combination of the two problems,
deposition and undiscovered burials, suggested itself, and dictated
the excavation of a deep trench in the canyon floor near Chetro Ketl, a
trench to be carried down until the old surface level of the Pueblo period
was located. The depth of this would provide a measure of total fill
away from the arroyo bed, minus possible surface erosion, between the
earliest period of Chetro Ketl occupation and the present. The plan

was eventually to carry this trench from wall to wall of the canyon, but
for the first season a small section was selected between points 250 feet
and 300 feet south of Station I, which is four feet from the south wall
enclosing the great kiva of Chetro Ketl. The line of the trench runs
thirty degrees east of south, between that station and the arroyo, 1,164
feet distant. (See map I.)

The cut, fifty feet long and twelve feet wide, was carried down six
feet through the surface sand. At this depth the excavation was narrowed
down to a width of six feet, and the length was shortened to
twenty-five feet, between points 250 and 275 feet from Station 1. The
trench was carried down another six feet. The excavation was now
narrowed to three feet in width and carried down six feet farther, providing
a vertical face eighteen feet below the surface and twenty-five
feet long. At the bottom was sand showing no trace of culture material.
(See fig. 5.)

All potsherds encountered in this trench were saved, marked with
their level, and classified. Sack numbers were entered in their places on
the chart drawn of the wall profile, and classifications of the material in
each sack were listed in the table of pottery. One complete vessel of
Escavada Black on White was encountered at a depth of nine feet below
the surface level in a layer of cienega clay in Deposit 2. (See table of
shards, pp. 137-138.)

The interpretation of the history of the formations uncovered on
the side of this cut was checked by Dr. Ernst Antevs, of the Carnegie
Institution of Washington.

History of Deposition and Erosion in Chaco Cut 1.

• (1) Deposition. (Deposit 1). Lowest and earliest in the cut. Sandy
  silt; little clay. Culminates with a deposit of "cienega clay" about
  5 inches thick. Contains no shards.

• (2) Erosion. (Erosion Surface 1). Erosion has cut Deposit 1, completely
  removing the "cienega clay" except for the north nine feet
  of the face of the exposure. Occupation of Chetro Ketl may have
  begun somewhat before or during this stage of erosion or at the
  beginning of the deposition which followed it.

• (3) Deposition. (Deposit 2.) Silt and clay. At one horizon, at least,
  the silt and clay occur in alternating laminae, perhaps varves,
  indicating deposition in standing water. Within this deposit, at a
  depth of fifteen feet, were found shards of Escavada Black on
  White. (See table, Sack 6.) Above this, in the deposit, Escavada
  Black on White was consistently associated with Gallup Black
  on White. (See sacks 3, 4, 5.) At the depth of nine feet, in
  a layer of "cienega clay," an Escavada Black on White jar (specimen
  Bc52 10/1 was found. The silty clay of this deposit merges into
  

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  Fig. 5—Profile of Chetro Ketl-Chaco Wash Stratigraphic Cut

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  [137

  
  
  
  the "cienega clay" at the depth of nine feet in a layer between two
  and four inches thick. Thereafter the material increases in coarseness
  to fine sand, and then decreases in coarseness to clayey silt.
  This clayey silt seems to have formed a land surface, for it shows
  in place a thin greyish layer of ash, and is distinctly different
  from the overlying material.

• (4) Erosion. (Erosion Surface 2.) As stated under (3), the top of the
  clayey silt seems to have been a land or erosion surface which rests
  fairly horizontally at the present level of about seven feet below
  the surface.

• (5) Deposition. (Deposit 3.) The deposit in the uppermost seven feet
  is of sandy silt and gravel lenses. The gravel lenses represent a
  river deposit. Most of the sand and silt are probably of river
  deposit, but a part of them may have been laid down by the wind.
  It is possible that the river cut into the surface at places, but no
  definite period of erosion is evident. From a lens of gravel lying
  from four to five feet beneath the surface were taken shards which
  were identified as preponderantly Gallup Black on White.

• (6) Erosion. Modern arroyo cutting. The main river, or the tributary,
  which had laid down Deposit 3, disappeared. In late time, the river
  cut its present arroyo.

Potsherds from Chaco Cut 1

Sack No. 1 (Gravel Lens in Deposit 3)

             

Chaco Corrugated	22	42%
Chaco Black on White	17	34%
Gallup Black on White	7	14%
Wingate Black on Red	1	2%
Escavada Black on White	3	6%
Red Mesa Black on White	1	2%
	51	100%

Sack No. 2 (Gravel Lens in Deposit 3)

           

Gallup Black on White	12	38%
Escavada Black on White	7	22%
Chaco Corrugated	6	19%
Chaco Black on White	4	14%
McElmo Black on White	2	7%
	31	100%

Sack No. 3 (Deposit 2)

       

Escavada Black on White	12	80%
Exuberant Corrugated	1	7%
Gallup Black on White	2	13%
	15	100%

Sack No. 4 (Deposit 2)

           

Escavada Black on White	11	50%
Gallup Black on White	3	13%
Exuberant Corrugated	3	13%
Red Mesa Black on White	2	11%
La Plata Black on White	3	13%
	22	100%

Sack No. 5 (Deposit 2)

       

Escavada Black on White	37	55%
Exuberant Corrugated	27	39%
Gallup Black on White	4	6%
	68	100%

Sack No. 6 (Deposit 2)

       

Escavada Black on White	39	91%
Exuberant Corrugated	3	7%
Affinis Gila Redware	1	2%
	43	100%

Chaco pottery types have[3] been dated from the charcoal found
associated with the shards in the Chetro Ketl refuse mound[4] and from
the types found associated with the successive building periods uncovered
in Mound 50. The dates listed in the following table must be understood
to be flexible and to represent periods of popularity of these types
but neither the beginning nor the end of the period during which such
vessels were made.

         

Chaco Black on White	1050-1130 A. D.
Gallup Black on White	950-1100 A. D.
Escavada Black on White	850- 950 A. D.
Red Mesa Black on White	800- 850 A. D.
La Plata Black on White	700- 800 A. D.

Conclusions

Deposit 1, at the bottom of the cut, was sterile of cultural material.
This was cut by Erosion Surface 1, likewise sterile. In Deposit 2 were
shards of Escavada Black on White, indicating that the period of erosion
represented by Surface 1 probably came sometime before, or early
in, the tenth century.

Erosion Surface 2 was apparently caused by a period of cessation
of deposition if not of extreme erosion, antedating the deposition of
Gallup and of Chaco Black on White shards in Deposit 3, directly above
it. The date for Erosion Surface 2, then, would appear to be somewhere
in the eleventh century.

This approximate chronology of dry and wet periods
in the Chaco as indicated by the profile of erosion and of deposition
surfaces and as dated by the incorporated potsherds coincides
with the tree ring chronology for the area. Much of Chetro Ketl
remains to be excavated, but it seems probable, from the evidence of
wall types and pottery, that it was occupied some time before the
extreme drouth of 900 to 907 A. D. This drouth may be that responsible
for the erosion of Surface 1, after which the shards of Escavada Black
on White made in the pueblo were carried or washed onto the surface
now listed as Deposit 2 but which was the ground level about 950 A. D.

Between then and 1035 the years, as indicated by the tree rings,
were of average rainfall, with some dry seasons, but between 1035 and
41 there was another drouth, less severe than the one in the early 900's.
It does not appear to have affected the expansion of the pueblos. Erosion
Surface 2 is probably representative of this period.

Deposit 3 represents a post-drouth period, probably from about
1059 or 1100 A. D. onward. It contains the Gallup and the Chaco Black
on White, which were both being made at the end of the century.

The successive erosion and deposition surfaces suggest increasing
dryness in the canyon, although the tree ring growth indicates that
there was no change in weather but merely a succession of wet and of
dry periods. The increasing denudation of the canyon floor was, perhaps,
because of deforestation and of farming, perhaps because of farming
alone. Interpretation of the profiles on the cut of the canyon floor
accords with the Douglass theory of recession of the forest border, after
human despolation, and of consequent erosion of the light soil by wind
and water.[5]