From the appearance of the rugged, mountainous terrain of Yellowstone
National Park, it is difficult to visualize a time when this region lay
close to sea level, at times even below sea level. Yet the evidence is
clear that from the Cambrian Period to the latter part of the Cretaceous
Period, a span of about 500 million years, vast stretches of western
lands were flooded repeatedly by broad shallow seas that often reached
from Canada to Mexico (fig. 8). During these great floodings, widespread
horizontal beds of sand, silt, clay, limy mud, and other sediments were
deposited on the ocean floors, along the adjoining beaches and wide
tidal flats, and across the broad flood plains of large rivers that
emptied into the seas. All of these ancient sediments have now hardened
into compact well-layered sandstones, shales, and limestones (figs. 9
and 10). These _sedimentary rocks_ have been divided into 25 or more
distinct formations in the Yellowstone region (fig. 5), where they
locally attain a combined thickness of more than 10,000 feet.
The first Paleozoic sea to reach the Yellowstone region, some 550
million years ago, brought with it the earliest abundant signs of life
on earth. Small hard-shelled animals that lived mainly on the shallow
sea bottom are now preserved as fossils in rocks deposited during the
Cambrian Period. Many of these animals were _trilobites_, long-extinct
organisms resembling today’s crabs and spiders. Each younger set of
rocks or formations contains a different group of dominant fossils, each
diagnostic of that period of geologic time in which they lived (fig.
11).
[Illustration: MIDDLE PERMIAN SEAS. Distribution of sea (blue) and
land (red) during the middle part of Permian time (approximately 250
million years ago). Only a part of the Yellowstone National Park
area (black) was flooded during this period. (Fig. 8)]
Fossils indicate the kind of environment in which the animals lived
(fig. 12). Some species thrived in the open oceans; others thrived only
along the beaches and in nearby lagoons. Still others, such as the
incredibly large dinosaurs of the Jurassic and Cretaceous Periods, could
survive only on the land or in swamps. From studies of the fossils and
of the physical characteristics of the rocks in which they are now
found, the shoreline patterns of the shifting seas can be determined.
Studies show that the seas advanced and retreated across the Yellowstone
Park region at least a dozen times during the Paleozoic and Mesozoic
Eras.
Toward the end of the Mesozoic Era (in the latter part of the Cretaceous
Period), the metamorphic basement rocks of Yellowstone lay covered by
the vast blanket of flat-lying sediments. Today, these sedimentary rocks
are exposed along the Snake River and its tributaries in the
south-central part of the Park, over much of the Gallatin Range in the
northwest corner, and at several places in the north-central and
northeastern parts (pl. 1). Elsewhere, either they are hidden from view
beneath volcanic debris—ash and lava—that later buried them, or they
have been removed by erosion. But wherever exposed, the original
horizontal layers of sedimentary rocks have been severely twisted and
broken by later mountain-building movements.
[Illustration: CROWFOOT RIDGE in the southern Gallatin Range, as
viewed from the road along the Gallatin River near the northwest
corner of Yellowstone National Park. The rocks, chiefly Paleozoic
limestone, sandstone, and shale, were deposited in broad shallow
seas that covered all of the Yellowstone region several hundred
million years ago. The original layers were horizontal, but they
have since been tilted and broken by giant mountain-building forces
originating deep within the earth. (Fig. 9)]
[Illustration: MOUNT EVERTS, as viewed toward the northeast from the
road south of Mammoth Hot Springs. The mountain, about 1,500 feet
high above the plain, is formed by gently tilted sedimentary rocks
of Cretaceous age, chiefly sandstone and shale of the Frontier,
Cody, and Everts Formations (fig. 5). The conspicuous rimrock at the
top of the mountain to the right is composed of the Yellowstone
Tuff. When the tuff was deposited (by explosive eruptions from the
south), there was no valley along the edge of the mountain. (Fig.
10)]
[Illustration: FAUNAL SUCCESSION in sedimentary rocks. The different
animals are now preserved as fossils, which are diagnostic of the
period in which the animals lived. (Fig. 11)]
Man
CENOZOIC QUATERNARY and TERTIARY Mammals
CRETACEOUS
MESOZOIC JURASSIC Dinosaurs
TRIASSIC
PERMIAN Reptiles
PENNSYLVANIAN Amphibians
MISSISSIPPIAN
PALEOZOIC DEVONIAN Fishes
SILURIAN Sea scorpions
ORDOVICIAN Nautiloids
CAMBRIAN Trilobytes
PRE-CAMBRIAN Soft-bodied
creatures
[Illustration: LIMESTONE OF MISSISSIPPIAN AGE along Pebble Creek at
the Pebble Creek campground, northeastern Yellowstone National Park.
(Fig. 12)]
[Illustration: Closeup A shows one of the highly fossiliferous
layers within the limestone.]
[Illustration: Closeup B shows some of the fossils and their casts.
Most of the fossils are of a variety of shelled sea animals
(brachiopods) that lived on the ocean floors approximately 300
million years ago.]