Date of this Version
Meriwether Lewis and William Clark undertook their journey with the Corps of Discovery in 1804-1806 in order to explore the area that the United States had purchased from France in 1803. Then known as Louisiana, this region included almost everything west of the Mississippi to the continental divide (fig. 1).
In order to find the best route across the continent, President Thomas Jefferson charged Lewis with following the Missouri River to its headwaters and then locating rivers flowing down the west side of the Rocky Mountains to the Columbia River and into the Pacific Ocean. Jefferson's written instructions further specified that the members of the expedition collect and describe plants and animals new to science; enter the latitude and longitude of the rivers, mountains, and other features; and note the land's potential for farming, as well as the climate, timber, and wildlife. They were also to record the occurrences of volcanic features and minerals of all kinds, but especially metals, limestone, coal, and saline and mineral waters.
Jefferson, an avid fossil collector, had contributed to scientific knowledge about fossil mammals. He joined many naturalists of the time in the hope that living examples of animals, known only by their petrified remains, might yet be found in unexplored regions of the Earth.
Their journals, notebooks, and maps indicate that Lewis and Clark took their charge seriously and gathered a great deal of information on these topics. The explorers were astute observers of the land through which they traveled. In most cases, however, they did not know what had caused its appearance. At that time, few, if anyone, would have because the discipline of geology was in its infancy.
Because of the great interest in the Lewis and Clark expedition, large numbers of people have visited or will visit its sites, tracing some or all of its route. A large part of this route follows the Missouri and the Yellowstone rivers through the Great Plains, a region with a rich geologic history. This publication introduces the general reader to an explanation of the geology of the Great Plains as it is understood today. Quotations from the journals of Lewis and Clark describe geologic features of the landscape seen by the explorers and largely still visible to the traveler today.
This educational circular is divided into two sections. The first describes the general geology of the Great Plains. The second directs the modern traveler to some of the places where Lewis and Clark recorded geologic observations as they made their way across the Great Plains. It follows the expedition up the Missouri River in 1804-1805 and Clark's return down the Yellowstone River in 1806. Appendixes include suggestions for further reading and a list of U.S. Geological Survey topographic maps that trace the Missouri and Yellowstone rivers across the Great Plains.