The Geologic Story of Yellowstone National Park

by William R. Keefer

The deposits of the shifting seas

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]).

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.

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)

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)

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

LIMESTONE OF MISSISSIPPIAN AGE along Pebble Creek at the Pebble Creek campground, northeastern Yellowstone National Park. (Fig. 12)

Closeup A shows one of the highly fossiliferous layers within the limestone.

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.