Region 3: The Rocky Mountains

Late Precambrian rocks are exposed in northwestern Montana (especially in Glacier National Park) and northern Idaho. They include limestones formed from carbonate sediments deposited on a warm, shallow sea floor. These rocks contain fossils called stromatolites, layered domes formed by mats of bacteria known as blue-green algae or cyanobacteria (Figure 3.51).

Figure 3.51: Stromatolites lie exposed on the surface of the Grinnell Glacier cirque in Glacier National Park, Montana. These fossils were previously covered by ice and have only recently been exposed. Large specimens are greater than 0.6 meters (2 feet) in diameter.

Shallow marine waters continued to cover most of this area through the early part of the Paleozoic (Cambrian-Silurian), supporting a great diversity of life including trilobites, graptolites, brachiopods, and cephalopods. The sea retreated briefly during the middle Devonian, exposing the earlier rocks to erosion and resulting in unconformities in the geological record. Sea level rose again in the late Devonian, covering nearly all of Montana, Wyoming, and part of Idaho. These late Paleozoic seas were filled with diverse and abundant fusulinid foraminifera (see Figure 3.3) as well as crinoids, conodonts, mollusks, sponges, brachiopods, graptolites, and fish, including sharks.

Mesozoic rocks in eastern Idaho contain abundant marine invertebrates, especially clams, snails, and ammonoids, of Triassic and Jurassic age (Figure 3.52). Fossils from the early Cretaceous include those of fish, turtles, crocodilians, gastropods, bivalves, and plants (Figure 3.53). In addition, a variety of dinosaur fossils (bone, teeth, and eggshell fragments) have been found from ceratopsians, Ankylosaurus, and theropods. Similarly, late Cretaceous deposits of Idaho contain coal, leaves, and freshwater clams.

Cretaceous rocks are well exposed in many parts of Wyoming, particularly around the edges of the Bighorn Basin. Notable fossils here include flat clams, such as Inoceramus, some of which reached enormous sizes (see Figure 3.28), and heteromorph ammonoids including Didymoceras (see Figure 3.29B).

During the Paleogene, the Western Interior Seaway advanced across the continent for the final time before tectonic uplift caused it to drain away. The warm, humid climate allowed the growth of lush forests. Plants that grew in these forests, including magnolia, ginkgo, sequoia and cypress, were preserved as the coal that is mined today in places such as Wyoming. Lakes were widespread, and were sites of deposition for thick, organic-rich sediments. The most extensive and well-known of these is the Green River Formation, a layer of cream-colored shale 600-2000 meters (1970-6560 feet) thick, with occasional layers of chert and limestone.

The Green River Formation outcrops across a large area of southwest Wyoming, northwest Colorado, and northwest Utah, and composes the largest known accumulation of lacustrine sedimentary rock in the world. Its sediments accumulated in a system of lakes that covered this area during the Eocene, between 58 and 40 million years ago (Figure 3.54). The Green River is famous for the great number of well-preserved fossils found in its lake and river sediments, especially aquatic organisms such as fish, gastropods, and algae, but also many terrestrial plants and animals, including the oldest known bat (Figures 3.55 - 3.57). Well-preserved specimens of the fish Knightia are commonly found in the Green River Formation, and it is one of the most abundant vertebrate fossils in the world. A member of the herring family, the average Knightia is 7 - 12 centimeters (3 - 5 inches) long. Knightia are thought to have fed on algae, tiny crustaceans, and insects, and they were a major source of food for many of the larger fish in these Eocene lakes. They are commonly found in mass mortality or “death bed” layers because they swam in schools. The abundant plant and animal life preserved in the Green River Formation is also the reason for its status as a major oil shale deposit.

Figure 3.54: The Eocene Green River Formation. This map shows the size and location of the various lakes in which Green River sediments were deposited at different times during the Eocene epoch.

The Eocene was a time of extensive volcanism in the Rocky Mountain region. This is reflected in the occurrence of silica-rich layers in the Green River Formation, which formed from weathered volcanic ash, as well as the famous Yellowstone Petrified Forest (Figure 3.58). This extraordinary assemblage of multiple layers of volcanic ash contains numerous upright-standing, petrified tree trunks and abundant transported logs and stumps. It formed when ash was repeatedly eroded off of volcanoes and re-deposited in braided streams and rivers.

Figure 3.58: Specimen Ridge, overlooking the Lamar River, Yellowstone National Park, Wyoming. These fossil tree trunks are preserved in the position they occupied when they were alive, around 48 million years ago during the Eocene epoch, before they were buried suddenly in a volcanic eruption. This photo was taken around the year 1887. Note man standing at bottom for scale.

During the Paleocene and Eocene epochs (65 to 34 million years ago) a number of archaic groups of mammals arose and went extinct (see Figure 3.39), and many of today’s modern mammal groups evolved. Fossil bones of these mammals occur in several areas of northern Idaho, including the Tolo Lake Fossil Site in Idaho County. Abundant leaf and plant remains from this time period can also be found in northern Idaho (Shoshone and surrounding counties), where an ancient lake (approximately 15 million years old) provided ideal conditions for the fossilization of soft plant parts. Fossils in the Miocene Clarkia Fossil Beds (Figure 3.59) are so well preserved that some leaves even retain their original color; most are yellow, orange, and brown since they were shed during fall months (although they rapidly oxidize and turn black when exposed to air). The lake formed when a basin was dammed by basalt flows on the Columbia River Plateau. Although best known for its plants, the Clarkia Beds also contain well-preserved fossil fish, snails, and insects. Like most lagerstätten deposits, the Clarkia Beds probably formed in a low-oxygen sedimentary environment, which slowed decay of the organic remains.

Figure 3.59: A slab of leaves from the Clarkia flora, about 13 centimeters (5 inches) across.