In Late Jurassic time the sea to the west eventually dried up, either
because it was filled with sediments or because the land rose above sea
level, or both. This brought about a change from the parallel bedding in
the marginal marine environment of the Summerville to irregular
stream-channel sandstones, flood-plain silts and muds, and freshwater
lake deposits.
Streams from higher lands to the south brought in mud, silt, and sand
that piled up hundreds of feet thick over thousands of square miles,
including the Monument. These sediments were later compacted into the
brightly colored siltstone, mudstone, sandstone, and limestone now known
as the Morrison Formation. The colors are about the same as those of the
Summerville. Algae and other microscopic organisms extracted calcium
carbonate from the lake waters, and when they died this material settled
on the lake bottoms to make limestone.
The soft siltstone and mudstone of the Morrison Formation weather
rapidly into steep or fairly steep slopes, but the harder beds of
sandstone, most of which are in the lower third of the formation, known
as the Salt Wash Member, are sculptured into bold ledges or low cliffs.
The generally softer upper two-thirds of the formation is called the
Brushy Basin Member. The Morrison is best exposed in and southeast of
The Redlands, where the bare rocks are carved into badlands like the
famous ones of South Dakota. Both the Fruita Canyon and No Thoroughfare
Canyon approaches to the Monument pass typical badlands in the Morrison.
The entire 600 feet of this formation is best seen in the high bluff on
the east side of the mouth of No Thoroughfare Canyon (fig. 21).
[Illustration: MORRISON FORMATION, on east side of mouth of No
Thoroughfare Canyon. Forty feet of Summerville Formation at base is
concealed by slope wash, but underlying white- and salmon-colored
members of Entrada Sandstone are clearly exposed at lower left.
Protective caprock at upper right is lowermost sandstone of
Cretaceous Burro Canyon Formation. Upper two-thirds of Morrison is
typical of the Brushy Basin Member; lower one-third is not typical
of the Salt Wash Member, which generally contains more and thicker
lenses of sandstone, some of which are just around the corner to the
right. Mesa on left skyline is above Serpents Trail in the Monument.
Looking west from Little Park Road. See also figures 55 and 60.
(Fig. 21)]
In parts of the Colorado Plateau southwest of the Uncompahgre Plateau,
the sandstone lenses in the Salt Wash Member of the Morrison contain
uranium and vanadium ore associated with carbonaceous matter, including
coalified wood. No ores have been found in or near the Monument
presumably because such carbonaceous matter, which helped precipitate
the ores, is lacking on the northeastern side of the Uncompahgre
Plateau.
Some of the beds of siltstone and mudstone in the Brushy Basin Member of
the Morrison shown in figure 21 contain bentonite, a clay derived from
the decomposition of volcanic ash, which indicates the presence of
active volcanos in or near the Plateau at the time these beds were
deposited. Bentonite swells when wetted, so it is widely used in
well-drilling muds, sealing canals, etc. Some bentonitic material has
been dug from the Brushy Basin along the Little Park Road just south of
the point from which the photograph in figure 21 was taken and was used
for sealing irrigation canals in the Grand Valley.
The Morrison is not well exposed in the Monument, as the formation is
restricted to the higher parts where most of it is hidden by vegetation.
The lower part is seen in roadcuts and outcropping ledges along a high
stretch of Rim Rock Drive between Artists Point and the head of the west
arm of Ute Canyon, where sandstone lenses in the Salt Wash Member are
especially thick.
The climate during Morrison time was wet enough to support abundant
vegetation along the many lakes and streams—at least enough to feed the
hungry dinosaurs and other reptiles that roamed the area. Many bones and
parts of several skeletons of dinosaurs have been found in the Morrison
at several places in The Redlands not far northeast and northwest of the
Monument.
The most famous dinosaur locality near the Monument is Riggs Hill where,
in 1900, the late Elmer S. Riggs of the Field Columbian Museum (now
Field Museum of Natural History) dug out part of the first known
skeleton of a huge _Brachiosaurus_ (fig. 22). This discovery made quite
a splash in the scientific world, for it was the first and only type of
dinosaur found whose front legs were longer than its hind legs. The
fossilized thigh bone (femur) alone is 6 feet 8 inches long and weighs
549 pounds; the arm bone (humerus), though incomplete, is even longer.
The ribs are 9 feet long. A bronze plaque now marks the site of the
excavation (fig. 39).
In 1901, Riggs removed all but the forepart of a skeleton of
_Apatosaurus_ from the southeast side of a large hill of the Morrison
Formation just south of the old Fruita bridge. Riggs also found remains
of _Diplodocus_, _Camarasaurus_, and _Morosaurus_, and, in 1937, Al
Look, prominent writer and amateur paleontologist of Grand Junction, and
Edwin L. Holt, an instructor in Mesa College at Grand Junction, found
the closely associated remains of _Allosaurus_, _Stegosaurus_, and
_Brachiosaurus_ at the western end of Riggs Hill. Dinosaurs generally
are thought of as huge creatures—many were huge indeed (fig. 23)—but
they came in various sizes and some were quite small.
An interesting Late Jurassic vertebrate fossil locality in the Salt Wash
Member of the Morrison Formation, about 3 miles northwest of the West
Entrance of the Monument and about 3 miles southwest of Fruita, was
discovered in June 1975 by George Callison, Associate Professor of
Biology and Research Associate in Vertebrate Paleontology at the
California State University at Long Beach. During the summers of 1975
and 1976 Dr. Callison and his assistants removed the closely associated
skeletal remains of many small, primitive mammals and both small and
large dinosaurs and other reptiles. Part of the results were presented
in an unpublished manuscript.[28] During the summer of 1977 and later,
additional mammalian fossils were removed by Callison and assistants and
additional reptilian fossils were removed by Lance Erickson,
paleontologist of the Historical Museum and Institute of Western
Colorado (fig. 2). Hopefully, the work will be continued with the aid of
grants from several sources.
[Illustration: EXCAVATING TYPE SPECIMEN OF _BRACHIOSAURUS
ALTITHORAX_ RIGGS from south side of Riggs Hill. Photograph taken in
1900, reproduced by permission of the Field Museum of Natural
History (Chicago). See also figure 39. (Fig. 22)]
The locality, which covers parts of about 180 acres of public land
administered by the Bureau of Land Management, has been fenced and
posted to discourage vandalism, and has been designated the Fruita
Paleontological Area.
In order to evaluate the importance of the locality and to make plans
for its future development and protection, the Bureau of Land Management
held the Fruita Paleontological Workshop on March 28-30, 1977, to which
were invited several renowned vertebrate paleontologists and
archaeologists together with interested local personnel of the Bureau,
the National Park Service, and the Museum. All remarks and prepared
speeches were taken down by a shorthand reporter and were reproduced for
the attendees in the 83 page unpublished “The Fruita Paleontological
Report.”
[Illustration: SKELETONS OF TYPICAL DINOSAURS OF MORRISON
FORMATION.[29] _A_, _Camptosaurus_, a small dinosaur about 11 feet
long; _B_, _Apatosaurus_, a gigantic dinosaur about 76 feet long;
_C_, _Allosaurus_, a large carnivorous dinosaur about 30 feet long;
and _D_, _Stegosaurus_, a large armored dinosaur about 24 feet long.
(Fig. 23)]
The close association of Late Jurassic mammalian and reptilian fossils,
as found at the Fruita site, is of considerable interest and importance,
but is by no means unique, for similar associations occur elsewhere in
Colorado, and in Wyoming, Europe, and Africa. Of those in the United
States, the quarry at Como Bluff, near Laramie, Wyo., is considered by
most of the experts to be the most outstanding. Of the material
unearthed at Fruita thus far, which includes bones of some of the large
dinosaurs found earlier by Riggs, remains of some of the smaller
dinosaurs and a complete skull of the moderately large flesh eater
_Ceratosaurus_ are considered the most important.
Freshwater clam and snail shells abound in some beds of the Morrison,
particularly in limestones, and occur sparingly in other types of beds.
The shells occur mainly in The Redlands, particularly about 1½ miles
west of the Fruita bridge. Some of these shells that are filled with
agate are sought by rockhounds.