Most of these rocks of marine origin lie at considerable depth beneath
the land surface, concealed by an overlying thick, layered sequence of
rocks laid down by streams, wind, and glaciers. Nevertheless, their
geologic character, position, and form are exceptionally well known from
information gained from thousands of wells that have been drilled for
oil. The initial, nearly horizontal position of the layers of rock
beneath the Interior Plains has been little disturbed except where
mountains like the Black Hills were uplifted about 70 million years ago.
At those places, which are all in the northern and southern parts of the
Great Plains, the sedimentary layers have been warped up and locally
broken by the rise of hot molten rock from depth. Elsewhere in the
Interior Plains, however, earth forces of about the same period caused
only a reemphasis of gentle undulations in the Earth’s crust.
These undulations affected both the older basement rocks and the
overlying sedimentary rocks, and they take the form of gentle basins and
arches that in some places span several States. (See sketch map, figure
7.) A series of narrow basins lies along the mountain front on the west
side of the Great Plains. A broad, discontinuous arch extends southwest
from the Superior Upland to the Rocky Mountain front to form a buried
divide that separates the large Williston basin on the north from the
Anadarko basin to the south.
While the flat-lying layers of the Interior Plains were being only
gently warped, vastly different earth movements were taking place
farther west, in the area of the present Rocky Mountains. Along a
relatively narrow north-trending belt, extending from Mexico to Alaska,
the land was being uplifted at a great rate. The layers of sedimentary
rock deposited in the inland sea were stripped from the crest of the
rising mountainous belt by erosion and transported to its flanks as the
gravel, sand, and mud of streams and rivers. This transported sediment
was deposited on the plains to form the rocks of the Cretaceous Hell
Creek, Lance, Laramie, Vermejo, and Raton Formations. Vegetation thrived
on this alluvial plain, and thick accumulations of woody debris were
buried to ultimately become coal. This lush vegetation provided ample
food for the hordes of three-horned dinosaurs (_Triceratops_) that
roamed these plains. Their fossilized remains are found from Canada to
New Mexico.
[Illustration: _Figure 5.—Geologic time chart and the progression of
life forms. Note Cretaceous_ Triceratops, _Oligocene_ Titanotheres, _and
Miocene_ Moropus.]
GEOLOGIC TIME
The Age of the Earth
The Earth is very old—4.5 billion years or more according to recent
estimates. Most of the evidence for an ancient Earth is contained in
the rocks that form the Earth’s crust. The rock layers themselves—like
pages in a long and complicated history—record the surface-shaping
events of the past, and buried within them are traces of life—the
plants and animals that evolved from organic structures that existed
perhaps 3 billion years ago.
Also contained in rocks once molten are radioactive elements whose
isotopes provide Earth scientists with an atomic clock. Within these
rocks, “parent” isotopes decay at a predictable rate to form
“daughter” isotopes. By determining the relative amounts of parent and
daughter isotopes, the age of these rocks can be calculated.
Thus, the results of studies of rock layers (stratigraphy), and of
fossils (paleontology), coupled with the ages of certain rocks as
measured by atomic clocks (geochronology), attest to a very old Earth!
[Illustration: _Figure 6.—Generalized paleogeographic map of the United
States in Late Cretaceous time (65 to 80 million years ago), when most
of the Great Plains was beneath the sea._]
[Illustration: _Figure 7.—Structural setting of the Great Plains.
Williston basin and Anadarko basin are separated by a midcontinental
arch._]
[Illustration: Table 1.—Generalized chart of rocks of the Great Plains]
Geologic age Missouri High Plains—Plains Pecos
Millions of Plateau—Black Border—Colorado Valley—Edwards
years ago Hills Piedmont Plateau—Central
Texas
Quaternary
Pleistocene Glacial deposits, Alluvium, sand Piedmont, terrace,
alluvium, and dunes, and loess and bolson
terrace deposits deposits
2 erosional surface
Tertiary
Pliocene EROSION
5 Flaxville Gravel Ogallala formation
and Ogallala
Formation
Miocene Arikaree Formation Arikaree Formation
22-24 erosional surface
Oligocene White River Group White River Group Mostly missing
because of
erosion or
nondeposition
37-38 erosional surface
Eocene Wasatch and Golden
Valley Formations
53-54 Dawson Arkose
Paleocene Fort Union Denver, Poison
Formation Canyon, and Raton
Formations
65
Cretaceous Hell Creek and Vermejo and Laramie
Lance Formations Formations
Fox Hills Sandstone Trinidad and Fox
Hills Sandstones
Shales, sandstones, and limestones
deposited in Late Cretaceous sea
Dakota Sandstone Dakota Sandstone
and Lakota
Formation
Glen Rose and
Edwards Limestones
136
Jurassic Sundance Morrison Formation Jurassic rocks not
Formation, Ellis present
Group, and
Unkpapa Sandstone
190-195
Triassic Dominantly red rocks
225
PALEOZOIC Paleozoic rocks, undivided
570
PRECAMBRIAN Precambrian rocks, undivided
As the mountains continued to rise, the eroding streams cut into the old
core rocks of the mountains, and that debris too was carried to the
flanks and onto the adjoining plains. The mountainous belt continued to
rise intermittently, and volcanoes began to appear about 50 million
years ago. Together, the mountains and volcanoes provided huge
quantities of sediment, which the streams transported to the plains and
deposited. The areas nearest the mountains were covered by sediments of
Late Cretaceous and Paleocene age (table 1)—the Poison Canyon Formation
to the south, the Dawson and Denver Formations in the Denver area, and
the Fort Union Formation to the north (fig. 8). Vegetation continued to
flourish, especially in the northern part of the Great Plains, and was
buried to form the thick lignite and subbituminous coal beds of the Fort
Union Formation (fig. 9). The earliest mammals, most of whose remains
come from the Paleocene Fort Union Formation, have few modern survivors.
Beginning about 45 million years ago, in Eocene time, there was a long
period of stability lasting perhaps 10 million years, when there was
little uplift of the mountains and, therefore, little deposition on the
plains. A widespread and strongly developed soil formed over much of the
Great Plains during this period of stability. With renewed uplift and
volcanism in the mountains at the end of this period, great quantities
of sediment again were carried to the plains by streams and spread over
the northern Great Plains and southeastward to the arch or divide
separating the Williston and Anadarko basins (fig. 8). Those sediments
form the White River Group, in which the South Dakota Badlands are
carved. In addition to the _Titanotheres_, huge beasts with large, long
horns on their snouts who lived only during the Oligocene (37 to 22
million years ago), vast herds of camels, rhinoceroses, horses, and
tapirs—animals now found native only on other continents—grazed those
Oligocene semiarid grassland plains.
[Illustration: _Figure 8.—Progressive southeastward expansion of areas
covered by Paleocene, Oligocene, and Miocene-Pliocene sedimentary
deposits._]
Powder River basin
Denver basin
Raton basin
PLAINS
Margin of Oligocene deposition
Margin of Miocene-Pliocene deposition
[Illustration: _Figure 9.—Big Horn coal strip mine in Fort Union
Formation at Acme, Wyo. Photograph by F. W. Osterwald, U.S. Geological
Survey._]
Sometime between 20 and 30 million years ago the streams began
depositing sand and gravel beyond the divide, and, for another 10
million years or more, stream sediments of the Arikaree and Ogallala
Formations spread over the entire Great Plains from Canada to Texas,
except where mountainous areas such as the Black Hills stood above the
plains. Between 5 and 10 million years ago, then, the entire Great
Plains was an eastward-sloping depositional plain surmounted only by a
few mountain masses. Horses, camels, rhinoceroses, and a strange
horselike creature with clawed feet (called _Moropus_) lived on this
plain.