The Ogallala Formation is named from exposures around Ogallala in Keith
County, Nebraska. There is a major unconformity between the Trujillo
Formation of the Triassic and the overlying Ogallala Formation of
Pliocene (Late Tertiary) age. Missing here is the geologic evidence for
what may have been some of the more exciting chapters in the canyon’s
history. There is no record, for example, of the Jurassic and Cretaceous
Periods which together encompass almost 120 million years of earth
history. Also missing is any evidence of what transpired during more
than 90 percent of the Tertiary Period, for no rocks of Paleocene,
Eocene, Oligocene, or Miocene age are exposed in the canyon. Together
these four epochs comprise approximately 47 million years of earth
history. It is impossible, of course, to determine how many geologic
formations may have been formed and later eroded during the 167 million
years represented by this unconformity. However, our knowledge of
present-day deposition and erosion suggests that the missing geologic
record undoubtedly represents many thousands of feet of rock.
[Illustration: Fig. 14. The skeleton of _Buettneria_, a large
amphibian, found in Upper Triassic strata in the canyon. (Photograph
courtesy Panhandle-Plains Historical Museum.)]
The lower portion of the Ogallala Formation is composed of a
reddish-brown, fine- to medium-grained sandstone that contrasts sharply
with the underlying red and green shales that are exposed in the top of
the Trujillo Formation. Much of this sandy rock is characterized by
pebbles consisting of a variety of igneous, sedimentary, and metamorphic
rocks. Because it consists of rock and mineral fragments of varied
composition and size, this kind of sedimentary rock is called a
_conglomerate_. The type of rock fragments found in basal Ogallala
conglomerates suggests that they were transported to the
Panhandle-Plains area by streams flowing southeastward from the Rocky
Mountains. As these streams deposited their loads, they left behind a
wide spread blanket of sand, gravel, and mud which formed an extensive
alluvial plain that extended from western Nebraska to northwest Texas.
Although it is less than 100 feet thick in Palo Duro Canyon, in places
this great mantle of _fluvial_ (stream-deposited) sediments is as much
as 900 feet thick.
[Illustration: Fig. 15. The depression in this boulder is a mortar
hole believed to have been used by the Indians for grinding corn.]
[Illustration: Fig. 16. This pedestal rock, located near the
Lighthouse, is capped by a slab of weather-resistant Trujillo
sandstone.]
Most of the Ogallala Formation consists of a mixture of diverse rock
types such as conglomerate, sandstone, siltstone, clay and marl. But the
upper part of the formation is characterized by thick _caliche_
deposits. A dull, earthy calcite deposit, caliche typically forms in
areas of scant rainfall. It is believed to originate when ground
moisture, containing dissolved calcium bicarbonate, moves to the surface
where the moisture steadily evaporates leaving a calcium carbonate crust
on or near the surface (fig. 17).
Caliche, which derives its name from the Latin _calix_, meaning “lime,”
may be firm and compact or loose and powdery. It is also commonly found
mixed with other materials such as clay, sand, or gravel. Caliche
commonly occurs in the Trans-Pecos, southwestern Gulf Coastal Plain, and
the High Plains area of Texas (_see_ fig. 5, p. 8). In the latter area
it typically makes up the “caprock.” Caliche is commonly quarried in
these parts of Texas where it is used as road material and as an
aggregate.
Good exposures of Ogallala caliche can be seen on the surface around the
overlook at Coronado Lodge on the northwest rim of the canyon (fig. 17).
Ogallala strata also crop out along the upper reaches of Park Road 5 as
it starts to descend into the canyon. But probably the most spectacular
exposures of the Ogallala are exposed in the precipitous face of the
Fortress Cliff (fig. 33) which forms part of the eastern rim of the
canyon.
Also located within the Ogallala Formation is a very important
_aquifer_—a porous, water-bearing rock formation. This fine-to
coarse-grained sandstone is very porous and permeable and is the most
important single water-producing formation in the Panhandle-Plains area.
[Illustration: Fig. 17. The white surface in the right foreground
consists of caliche (p. 26) in the Ogallala Formation. Coronado
Lodge can be seen in the right background.]
Opal and chert are locally abundant in the Ogallala conglomerates. The
opal, which is found in small cavities in the conglomerate is not of the
gem variety but it does _fluoresce_. Minerals that exhibit
_fluorescence_ emit visible colors when exposed to ultraviolet light.
For this reason, the Ogallala opal is sought after by rock and mineral
collectors. The chert, a flint-like variety of quartz, occurs as nodules
in the conglomerate and in a well-developed layer near the base of the
formation. Both of these _siliceous_ (silica-bearing) rocks were
apparently prized by the Indians, who used them to fashion knives,
scrapers, projectile points, and other artifacts. The Indians also
learned that flat slabs of caliche were ideal for lining fireplaces and
to construct primitive rock shelters.
A number of Pliocene vertebrates have been found in the Palo Duro area.
Known as the “Age of Mammals,” the Tertiary Period was characterized by
mammals as diverse as were the reptiles of the Mesozoic Era. Among these
unusual creatures were such now-extinct species as the saber-tooth cat
and the elephant-like shovel-jawed mastodon (fig. 18). The remains of
these as well as bones of giraffe-like camels, pony-sized horses, and
sloths have been found in the vicinity of the canyon. The grassy plains
of Pliocene time were also inhabited by large tortoises which reached
lengths of up to 3 feet (fig. 19). Dioramas showing how these animals
might have looked, as well as their actual remains, are on display in
the Hall of Pre-History in the lower floor of the Panhandle-Plains
Historical Museum in Canyon, 13 miles west of the park (p. 35).
[Illustration: Fig. 18. This life-size model of a shovel-jawed
mastodon is typical of the now-extinct, elephant-like creatures that
lived in this area during the Pliocene Epoch. (Photograph courtesy
Panhandle-Plains Historical Museum.)]
[Illustration: Fig. 19. The carapaces of giant tortoises as much as
3 feet long have been collected from Pliocene rocks in the Palo Duro
area. (Photograph courtesy Panhandle-Plains Historical Museum.)]