All geologic periods have left traces in Montana. During the Archaean, the entire region was the bottom of an arm of the Pacific Ocean. It shared the heavy vegetation of the later Paleozoic, and was the swampy residence of Mesozoic reptile terrors. During the mountain building at the close of Cretaceous time, the predecessors of the Rocky Mountains were formed and Montana assumed something like its present surface pattern. Volcanic action upthrust lava in the form of conical hills ranging to several thousand feet in height.
During the Pleistocene epoch four great ice sheets plowed down from the northern part of the continent. Each erased most of the effects of its predecessor; thus the fourth, or Wisconsin, sheet had the most easily traceable influence. Its vast bulk (of an estimated 10,000-foot thickness in places) smoothed out the plains, filled in valleys, and created new stream courses and lakes. It deposited silt in piles hundreds of feet thick and many miles long. But it came only as far as the Missouri River and only east of the Rockies. Similar effects in western Montana were due to the action of piedmont glaciers independent of the Wisconsin sheet.
Great dams or moraines heaped up by the mountain glaciers created hundreds of lakes, two of the largest being the long-dry Missoula Lake, formed by the blocking of Clark Fork of the Columbia, and Flathead Lake, now one of the largest fresh-water bodies in the United States. Other remaining glacial lakes dot the Glacier Park region.
Passing, the glaciers left the surface substantially as it is today. Their less spectacular effects appear in the composition of the State's soils.
An outstanding geologic phenomenon is the Boulder batholith, an intrusive mass of igneous rock, 40 miles in mean width, extending southwest from near Helena to the Big Hole River. Formed at the beginning of the Rocky Mountain building period, it apparently occupies a huge basin whose dissected sides contain remnants of the entire series of sedimentary rocks from pre-Cambrian shales to late Cretaceous sandstones. Its principal rock is a dark coarse granite.
Seventy percent of Montana's exploited mineral wealth is concentrated in Silver Bow County, a division of this region. Gold and silver first brought Butte to the Nation's attention, but copper, zinc, and lead are now of first importance.
Ancient forms of life have left their signatures abundantly in Montana's rocks. The first were one-celled algae, followed several million years later by metazoa, tiny worms. Their marks are found in the Algonkian strata of the Proterozoic era, in the Little Belt Mountains, and in several ranges of the Rockies. Fossil mollusks, snails, corals, and trilobites of Paleozoic age are found throughout Montana. Extensive coal deposits, remains of the luxuriant forests of the Cretaceous and Tertiary periods, are, broadly speaking, the State's most valuable fossils.
Several important discoveries have been made near Harlowton, Wheatland County, including some Paleocene mammals, especially seven species of the condylarth, and the oldest primate remains known to science. Several dinosaur skeletons, usually the most publicized of fossils, have been unearthed in Montana. Dr. Barnum Brown discovered an almost complete skeleton of Tyrannosaurus, largest and fiercest carnivorous dinosaur, on Hell Creek north of Jordan, Garfield County. In Wheatland County he found one of the smallest dinosaurs known to science. Triceratops, an armored brute larger than a modern rhinoceros, was first found 14 miles south of Glendive, Dawson County. This grotesque animal had a 3-foot horn projecting over each eye; another jutted from its snout; and a collar of bone enveloped its neck like an Elizabethan ruff. A second skeleton, almost complete, was found in Treasure County.
Remains of Stegosaurus, weirdest of all armored dinosaurs, were found in 1924 at Sheep Creek, 25 miles north of Great Falls. Besides having hindlegs that boosted its rear skyward while its head was within two feet of the ground, the stegosaur had thick armor plates that stood erect in a staggered row along its back from head to tip of tail.
Partial skeletons of Ichthyosaurus and Plesiosaurus, seafaring reptiles of the Jurassic, have been found in Cascade and Wibaux Counties. A trachodon jaw is in the Larimer collection at Glendive, together with such oddities as gizzard stones--rocks worn smooth in saurian digestive processes.
Hoplitosaurus, a 15-foot horned toad found 32 Miles south of Billings, had been broken into 20,000 pieces by earth movement and exposure. Many other reptile fossils have been taken from the Yellowstone Valley, including two camptosaurs, two nodosaurs, and a tenantosaur.