Landscape History - Mountains and Plains: The Ecology of Wyoming Landscapes - page 12

Chapter 2 Landscape History

Millions of Years Ago ERA PERIOD EPOCH

Holocene

AGE

Rocks exposed in canyons, escarpments, and moun-
tains provide the data geologists need to interpret the
development of western landscapes. 1 Unimaginable
changes have occurred. About 350 million years ago,
during the Paleozoic Era (fig. 2.1), the area that would
become Wyoming was covered by seawater and the
climate was tropical. the water receded and advanced
many times, depositing sand in some places and more
finely textured silt and clay elsewhere. Sandstones, silt-
stones, claystones, and other kinds of sedimentary rock
were formed. Skeletal fragments from coral and other
marine organisms, rich in calcium carbonate, became
limestones. An example is the Madison Limestone that
forms many escarpments in the region today.
Also during the Paleozoic, shifting seas created enor-
mous swamps and tidal flats that were frequented by
dragonflies with 30-inch wingspans. Primitive vascular
plants were abundant, including tree-sized clubmosses,
ferns, and horsetails. 2 Fossilization was less frequent on
the uplands, but there were woodlands that included
primitive gymnosperms, such as the ancestors of cycads,
ginkgo, pine, spruce, fir, and sequoia. Mountain ranges

0.01

Quaternary

Pleistocene

2.6

Pliocene

5

Neogene

Miocene

23

Oligocene

34

Paleogene Eocene

56

Paleocene

65

Cretaceous

145

Jurassic

200

Triassic

251

Permian

299

Pennsylvanian

318

Mississippian
Devonian
Silurian
Ordovician
Cambrian

359

416

444
488

Fig. 2.1. Geologic time chart showing the ages (millions of
years ago) of eras, periods, and epochs. the Paleogene and
neogene periods of the cenozoic were previously known as
the tertiary. Adapted from the international commission on
Stratigraphy (http://www.stratigraphy.org).

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