Rock Formation

The geologic story of the Teton Range starts with the formation of the rocks that make up the mountains, rocks far older than the mountains themselves. The process began over 2.5 billion years ago when sand and volcanic debris settled in an ancient ocean. For millions of years, additional sediment was deposited and buried within the earth's crust. Heat and pressure metamorphosed (changed) the sediment into gneiss, the rocks that comprise the main mass of the Teton Range. The stress of metamorphosis caused minerals to segregate. Today, alternating light and dark layers identify banded gneiss, readily seen in Death Canyon and other canyons in the Teton Range.

Next, magma (molten rock) forced its way up through cracks and zones of weakness in the gneiss. This igneous (formed by heat) rock slowly cooled, forming light-colored dikes of granite, inches to hundreds of feet thick. Look for larger dikes as you view the mountains from the Jenny Lake and String Lake areas. Uplift and erosion have exposed the granite that now forms the central peaks of the range.

Diabase, a dark-colored igneous rock, 1.3 billion years ago flowed up through the gneiss and granite, resulting in the prominent vertical dikes seen today on the faces of Mt. Moran and the Middle Teton. The diabase dike on Mt. Moran protrudes from the face because the gneiss surrounding it erodes faster than the diabase. The diabase dike on the Middle Teton is recessed because the granite of the central peaks erodes more slowly than the diabase.

Shallow seas that covered the Teton region 600 million to 65 million years ago have left sedimentary formations, still visible at the north and south ends of the Teton Range and also on the west slope of the mountains. Marine life, especially tiny trilobites, corals and brachiopods, flourished in the shallow seas covering this area. The seas repeatedly advanced and retreated. During retreat of the younger seas, this area became a low-lying coastal plain frequented by dinosaurs. Fossilized bones of a horned dinosaur, the Triceratops, have been found east of the Park near Togwotee Pass.

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