Shoshone National Forest - Geography and Geology

Geography and Geology

The altitude in the forest ranges from 4,600 feet (1,400 m) near Cody, Wyoming, to 13,804 feet (4,207 m) at the top of Gannett Peak, an elevation gain of over 9,200 feet (2,800 m). Of the three major mountain ranges found in the forest, they are geologically distinct from each other. All of the mountains are a part of the Rockies and are at the transitional point between the central Rockies and the northern Rockies. The Absaroka Mountains were named after the Crow Indian tribe, although they only inhabited the far northernmost part to the mountain range. The majority of the Absaroka Mountains are contained within the forest, with the highest peak being Francs Peak at 13,153 feet (4,009 m). Stretching north to south through the northern and eastern sections of the forest, they span over 100 miles (160 km) from the Montana border to south of Dubois, Wyoming.

Important passes through the Absarokas include Sylvan Pass, which leads to the eastern entrance of Yellowstone National Park; and Togwotee Pass, which provides access to Jackson Hole and Grand Teton National Park. The peaks of the Absaroka are basaltic in origin, having been the result of volcanic activity estimated to have occurred 50 million years ago during the Eocene epoch. The rocks themselves are relatively dark and consist of rhyolite, andesite and breccias. Because of the erosional influences of glaciers and water and the relative softness of the rocks, the Absarokas are quite craggy in appearance. Gold was mined from the slopes of Francs Peak until 1907, and the small ghost town of Kirwin is still visited. Few lakes exist in the Absarokas, but the headwaters of both the Bighorn and Yellowstone Rivers are found there.

The Beartooth Mountains in the northernmost section of the forest are intrusive igneous and metamorphic in origin, and at 3.96 billion years old, some of these exposed Precambrian rocks are among the oldest found on the Earth. Although often considered a part of the Absarokas, they are distinct in appearance and geologic history. Uplifted approximately 70 million years ago during the Laramide orogeny, the Beartooths consist of vast windswept plateaus and rugged peaks with sometimes sheer cliff faces. The granite, gneiss and schist rocks are rich in minerals such as chromium and platinum. Iron and magnesium are found in the biotite, amphiboles and pyroxene minerals throughout the range. Quartz and feldspars are also commonly found. Geologists believe that the Beartooths were at one time at least 20,000 feet (6,100 m) in altitude, but subsequent erosion for tens of millions of years has reduced them to an average of 12,000 feet (3,700 m) for the higher peaks. There are an estimated 300 lakes in the Beartooth region of Shoshone National Forest, some of them left behind by the receding glaciers of the last glacial maximum known as the Pinedale Glaciation, which ended roughly 10,000 years ago. The Beartooth Highway (U.S. Highway 212) crosses 10,974-foot (3,340 m) Beartooth Pass, and from there descends to the northeast entrance to Yellowstone National Park.

The Wind River Range is in the southern portion of the forest and is composed primarily of granitic rock, gneiss and schist. Gannett Peak is the highest peak in Wyoming, and another seven peaks also exceed 13,500 feet (4,100 m). At one time, Fremont Peak was thought to be the tallest mountain in the Rocky Mountains due to its prominence when viewed from the Oregon Trail. Over 230 mountains rise above 12,000 feet (3,700 m). This range is also popular with mountain climbers from all over the world because of its solid rock and variety of routes. The Cirque of the Towers in the Popo Agie Wilderness is one of the more popular climbing and hiking destinations, and an estimated 200 different climbing routes are located within the peaks that surround the cirque. Hundreds of lakes are located in this region as are the headwaters of the Wind River.

There are over 500 lakes in the forest, and 2,500 miles (4,000 km) of streams and rivers. The Clarks Fork of the Yellowstone River is designated as a National Wild and Scenic River for 22 miles (35 km) through the forest, with cliffs towering up to 2,000 feet (610 m) as the river winds through a gorge. The forest is on the eastern slopes of the Continental Divide, and the rivers flow into the Atlantic Ocean basin.