Summerville Formation facts for kids

Quick facts for kids Summerville Formation Stratigraphic range: Oxfordian 161–151Ma PreЄ Є O S D C P T J K Pg N
Summerville Formation at the type locality at Summerville Point, east side of the San Rafael Swell, Emery County, Utah
Type	Geological formation
Unit of	San Rafael Group
Sub-units	Bluff Sandstone
Underlies	Morrison Formation
Overlies	Todilto & Curtis Formations
Thickness	100 m (330 ft)
Lithology
Primary	Mudstone, sandstone
Other	Gypsum
Location
Coordinates	39°12′58″N 110°32′38″W / 39.216°N 110.544°W / 39.216; -110.544
Approximate paleocoordinates	31°54′N 54°42′W / 31.9°N 54.7°W / 31.9; -54.7
Region	Four Corners, (Southwestern United States)
Country	United States
Extent	Henry Mountains, Paradox Basin
Type section
Named for	Summerville Point, Emery County, Utah
Named by	James Gilluly & J.B. Reeside Jr.
Year defined	1928
Summerville Formation outcrops in Utah, Colorado and New Mexico

The Summerville Formation is a special layer of rock found in parts of the Southwestern United States, including New Mexico, Colorado, and Utah. This rock layer formed a very long time ago, during the Late Jurassic period, specifically in a time called the Oxfordian stage.

Discovering the Summerville Formation

The Summerville Formation is a fascinating part of Earth's history. It helps scientists understand what the landscape was like millions of years ago. This rock layer is part of a larger group of rocks called the San Rafael Group.

What Does the Summerville Formation Look Like?

This rock formation is mostly made of red mudstone, which is like hardened mud. You can also find thin layers of green and red sandstone, which is rock made from sand. The entire formation can be up to 100 meters (about 328 feet) thick.

In the lower parts of the formation, you might see cracks that look like dried mud. You can also find shapes left behind by salt crystals. The upper parts often contain a lot of gypsum, a mineral used to make plaster. These features tell us that the rocks formed in a very flat, salty area, similar to a desert near a shallow sea.

Where Can You Find These Ancient Rocks?

You can see the Summerville Formation in many places. It's visible in areas like the San Rafael Reef and the Waterpocket Fold. You can also find it in the Henry Mountains and scattered across regions like the Paradox Basin. It even appears in north-central New Mexico.

The thin, even layers are a key feature of this formation everywhere. However, the gypsum is not found in all locations, like the San Juan Basin. In some southern and southwestern areas, you might find a type of rock called conglomerate, which is made of larger pebbles cemented together.

The Summerville Formation sits directly on top of older rock layers, such as the Curtis Formation or Todilto Formation. Above it, there's a gap in the rock record, called an unconformity, before you reach the younger Morrison Formation. This gap means some time passed, and maybe some rock eroded away, before the Morrison Formation began to form.

How Did the Summerville Formation Form?

Scientists believe the Summerville Formation formed as an ancient sea, called the Sundance Sea, slowly pulled back towards the north. At the same time, two ancient lake basins, the Curtis and Todilto, were filling up.

This created a very flat landscape. Sometimes, during dry periods, wind-blown sand would cover the area. Then, when temporary salty lakes appeared, waves would spread this sand into thin layers. This process created the distinct, thin layers we see today. The Morrison Formation above it shows that the climate later became more humid.

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  Evenly layered Summerville Formation rocks along Utah Highway 24, west of Hanksville, Utah.

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  A close-up view of the Summerville Formation showing its distinct, even layers, west of Hanksville, Utah.

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  Summerville Formation rocks with cracks filled by gypsum. A U.S. quarter coin shows the size.

Traces of Ancient Life

The Summerville Formation does not contain many body fossils, like bones or shells. However, it is famous for something else: dinosaur tracks! Scientists have found tracks left by theropod dinosaurs in this formation in Utah, New Mexico, and Colorado. These tracks give us clues about the animals that walked across this ancient landscape millions of years ago.

How Geologists Learned About Summerville

Scientists first officially named and described the Summerville Formation in 1928. They studied the rocks where they were visible in a place called the San Rafael Reef. Similar rock layers in northeastern Arizona and the San Juan Basin are sometimes called the Wanakah Formation. However, in north-central New Mexico, these layers are still known as the Summerville Formation.

See Also

• List of dinosaur-bearing rock formations