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Geology, Landforms, and Drainage:

All of the rocks exposed in the Chaco Canyon are sedimentary in
origin, ranging in age from the Allison member of the Mesaverde
group, through the Chacra sandstone member, to the Lewis shale—all
belonging to the Upper Cretaceous. The Chaco Canyon has been eroded
mainly out of the Allison and Chacra members, as the Chaco River
leaves the Lewis shale about five miles below Pueblo Pintado. Back of
Pueblo Bonito, across the canyon from Tseh So, the canyon walls rise
nearly sheer for 125 feet. Including the back slopes, the canyon at this
point is more than 350 feet deep. The upper and greater portion of the
cliff scarp is made up of the massive buff Chacra sandstone, whose
counterpart the Cliff House sandstone forms imposing scarps in the
Mesa Verde. Due to a dip of the beds down toward the north, more of
the underlying Allison member is exposed in the southern cliffs than
in the north wall. The Allison member is made up of interbedded
sandstones and carbonaceous shales, with stringers of white clay, argillaceous
shale, selenite, and coal. In the vicinity of Casa Rinconada
the coal seams are thin, and the coal varies from lignite to subbituminous.
Progressing westward the seams increase in thickness, and the
quality of the coal improves. The characteristic profile of the Allison
exposure is that of a concave talus slope, littered with fragments of
shale and occasional large angular blocks of sandstone from the nearly
vertical face of the superior Chacra sandstone. This may be seen best
in the pediment of the Mesa Fajada, and along much of the southern
cliff wall.

Few fossils are found in the walls of the Chaco Canyon. These
are mainly casts of the giant fucoid alga Halymenites major, and shells
of Inoceramus barabini. It is normally assumed that the Allison shales
and sandstones were laid down during a period of oscillation of the
Cretaceous seacoast, some of the sediments having been formed in fresh

water, and others in coastal waters of the sea. The Chacra sandstone
is definitely marine in origin. Some shark's teeth have been collected
from the Chacra sandstone, in addition to the above mentioned algae
and shells. Plant rests are fairly common in the coal beds.

Within fifteen miles of Pueblo Bonito, advancing northeastward,
successive exposures of Lewis shale, Pictured Cliffs sandstone, Fruitland
formation, Kirtland shale, Ojo Alamo sandstone, and Puerco and
Torrejon formations are encountered. The Escavada Wash and its
tributaries the Kimbetoh Arroyo and Alamo Arroyo pass across all of
these formations. With the exceptions of the Kirtland and the Puerco
and Torrejon, the exposures are narrow (though continuous), with an
average width of less than two miles in the area to the north and east
of the Chaco Canyon ruins. The Lewis shale is of marine origin, as is
the succeeding Pictured Cliffs sandstone. Although calcareous rocks
are rare in all this area, some thin layers of impure brown limestone
are found in the Lewis shale. The Fruitland formation, of sandstones,
shales, and clays, was laid down in waters that changed from brackish
to fresh. Extensive badlands have been formed in the Fruitland, especially
of the weird monumental type. Various fossils of Dinosauria,
Chelonia, and Pisces have been obtained from the Fruitland formation.

The Kirtland shale, of fluviatile origin, is noted for its badlands
which normally assume a rounded billowy form. In the strata of the
Kirtland shale occur barite, gypsum, aragonite, siderite, petrified wood,
and numerous remains of dinosaurs, turtles, crocodiles, and fish. Succeeding
the Kirtland shale is the Ojo Alamo sandstone, whose geologic
position has been given variously as terminal Cretaceous and basal Tertiary.
The shale of the Ojo Alamo is fairly rich in remains of Reptilia;
and the conglomeratic sandstone contains silicified logs (up to three feet
in diameter), and pebbles (up to six inches in length) of red jaspery
quartz, brown and grey chert, vein quartz, pink and white quartzite,
rhyolite, andesite, felsite, porphyrite, granite, gneiss, schist, and obsidian.
Also there are found pieces of lignitized wood, concretions of
manganese, and limonitic concretions. The Puerco and Torrejon formations
possess a large vertebrate fauna—mainly archaic placental
mammals, and an abundance of turtles and crocodiles, but no dinosaurs.
Calcite crystals are found in bedding planes in the Puerco; and pebbles
of chert and quartz up to one inch in diameter are found in the
Torrejon.

Upon the horizontal to gently dipping Cretaceous rocks of the area
ephemeral torrents and prevailing winds have sculptured a landscape
of alternating dales and swells, with here and there ridges, knolls,
buttes, and mesas rising up to as much as a hundred feet above the
general plateau level. Only the Chaco River, and the lower portions of
its principal tributaries, has deeply incised the surface. Despite a

meager rainfall, the Chaco River collects enough water, during the torrential
precipitations of late summer, from its drainage basin (estimated
between 4,200 and 4,800 square miles) to maintain an ever-enlarging
channel some 150 miles in length. (Estimated runoff about
65,000 cubic feet.) Although normally the upper Chaco River is only
a sandy wash in the bottom of a narrow channel (70 to 150 feet in
width), in the rainy season a swirling flood of ill-smelling chocolate
colored water will fill its bed from bank to bank.[2] The load of the river
is made up of mud and sand, with a few small sandstone pebbles carried
in the train. No pebbles larger than one inch in diameter are to be
found in the canyon sector of the river.

Between Shabik'eshchee and Peñasco Blanco (the main area of prehistoric
settlement in the Chaco Canyon) the river describes a sinuous
course which results in the cutting away of tons of bank fill with every
rise of water. Although there is some deposition on the inner slackwater
margins of the bends, each season sees a fairly complete evacuation
of the material eroded from the banks. This was seemingly not
the case during all of the past millenium, as there are traces of an older
arroyo (now completely filled in) which wriggled its way across the
canyon floor, intersecting the present arroyo in a number of places.[3]
This filled arroyo, which was fifteen to eighteen feet deep, contains
shards from the latest Chaco period. There have evidently been several
cycles of filling and cutting in the Chaco Canyon since Basket Maker
days (some twelve hundred years ago) as pithouses and hearths of the
Basket Maker period have been revealed to a depth of more than thirteen
feet in various places along the present Chaco River bank.[4]

The work of the wind, prevailingly from the west and southwest,
is less spectacular than that of summer rains, but it has left its imprint
on every portion of the landscape. Sand blasting of the cliffs and
isolated rocks goes on continually, leaving polished or striated surfaces
here, and niched or honeycombed rocks there. Sandstone blocks at the
feet of southwest-facing cliffs are exposed to the greatest action of the
wind, and frequently one finds such boulders converted into sponges of
stone. Undercutting of cliffs, in the first three feet above the canyon
floor, contributes in no small fashion to the downfall of huge masses of
cliff rock. This is best observed between Kin Kletso and Chetro Ketl.

Silt and sand, collected from hundreds of square miles of shale and
sandstone, are harried by the wind—filling the air in the windy months
of spring and summer, or piling up in drifts along the larger washes.
One of the largest aggregations of sand dune and drift in the Chaco
Canyon is in the ox-bow around the Peñasco Blanco.