In the know about Seminoe: A geological study

WYOMING – The Wyoming State Geological Survey (WSGS) has released a new publication about the geology in Seminoe State Park in south-central Wyoming. The pamphlet highlights the park’s rock formations, sand dunes, and hydrogeology.

“Seminoe State Park has some of the most spectacular geology in Wyoming,” says WSGS Director, Dr. Erin Campbell. “It’s rare to have such a complete geologic section, from Precambrian to Cretaceous rocks, so beautifully exposed and easily accessible.”

Readers of the “WSGS Seminoe State Park brochure” will learn that the area’s oldest rocks—granite and other metamorphic rocks—are at the north end of the park and make up the core of the Seminoe Mountains.

They will also learn that Seminoe Reservoir can hold more than one million acre-feet of water (1 acre-foot=326,000 gallons), most of which originates as snowpack in the nearby Medicine Bow, Sierra Madre, and Shirley mountains. The reservoir’s largest inflow is the North Platte River, which is an important tributary to the Missouri-Mississippi River System and a major source of municipal water for several Wyoming cities.

The pamphlet is the second in a series about the geology of Wyoming’s state parks. The first focuses on Curt Gowdy State Park in the southeastern corner of the state.

“The WSGS is pleased to provide these geology pamphlets for the state parks,” says Campbell. “We hope they will help increase visitation to the parks, as well as help visitors understand the geology around them.”

The pamphlets are free and available at the WSGS office on the University of Wyoming campus in Laramie, as a download from the agency’s website, and at their respective park’s visitor centers. Pamphlets now in production will focus on Glendo and Guernsey (eastern Wyoming), Buffalo Bill (northwestern Wyoming), and Keyhole (northeastern Wyoming).

Red Goose Egg Formation in contact with white Tensleep Sandstone.

Geological history

The oldest rocks in Seminoe State Park lie at the north end of the park near Seminoe Dam. These rocks consist of granite and metamorphic rocks as old as 2.7 billion years (Precambrian). These Precambrian rocks make up the core of the Seminoe Mountains and serve as the anchor for Seminoe Dam. Within the park, you may find dense cobbles or boulders with red, yellow and black stripes; this rock is called “banded iron formation.” These eroded from the Seminoe Mountains and are part of the Precambrian Seminoe iron-ore deposits explored for mining between 1870 and the 1960s, although no significant iron production occurred.

Around 550 million years ago, these oldest rocks were uplifted from deep in the earth’s crust and overlying rocks eroded away. The Cambrian Flathead Sandstone (~540 million years old, or Ma) was deposited in seas and shallow streams on top of the older uplifted rocks, forming a geologic contact that represents a gap in the rock record known as an “unconformity.” Approximately 2 billion years of time is missing at this unconformity.

Traveling south from the dam, you pass through increasingly younger rock units, such as the Mississippian- through Pennsylvanian-age Madison, Amsden, and Tensleep formations (~360–310 Ma), which form distinctive, steep ridges of light-colored rocks throughout the area. These rock units originally formed in oceans and vast sand dune fields.


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