By late Triassic time the Monument was part of a nearly flat plain cut
on the dark Proterozoic rocks, but there were hills or low mountains to
the northeast. Streams from these hills dropped mud, silt, sand, and
some gravel on this plain and into many small lakes that occupied the
gentle depressions. Later, these deposits hardened mainly into red
siltstone and sandstone, but thin beds of gravel were cemented to form
conglomerate, and thin beds of limestone formed in some of the shallow
lakes by the precipitation of the mineral calcium carbonate. These
rocks, which comprise the Chinle (pronounced Chin-lee) Formation, are
only 80 to 100 feet thick in the Monument but are as much as 700 feet
thick near Moab, Utah, southwest of the Uncompahgre Plateau, where the
entire formation is present. There, the Chinle rests on still older
Triassic and Paleozoic rocks—all absent in the Monument for the reasons
noted previously. In some parts of the Plateau, sandstone or
conglomerate beds in the lower part of the Chinle yield uranium ore, but
these beds were not deposited in or near the Monument.
The red color of the Chinle and some of the overlying rocks is caused by
minute amounts of iron oxide—the same pigment used in rouge and red barn
paint. Various oxides of iron, some including water, produce not only
brick red but also pink, salmon, brown, buff, yellow, and even green or
bluish green. This does not imply that the rocks could be considered as
sources of iron ore, for the merest trace of iron, generally only 1 to 3
percent, is enough to produce even the darkest shades of red.
Because it is soft, the Chinle is easily eroded into steep slopes at the
foot of high sandstone cliffs in all canyons of the Monument and on top
of the high bluffs that face The Redlands. It also forms the broad base
of Independence Monument. Rim Rock Drive crosses the Chinle three times
in the lower part of Fruita Canyon and twice in No Thoroughfare Canyon.
Fossil reptile bones, petrified wood, and freshwater shells come from
the Chinle in parts of Arizona and Utah. Reptiles probably roamed the
Monument in Chinle time, but their remains have not been located.