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Spinney Mountain, East face

​​​​​​​​​Spinney Mountain State Park features a 2,520 surface-acre reservoir along the path of the South Platte River in South Park. The South Platte River Valley is mantled with Wisconsin-aged glacial outwash material. Spinney Mountain is composed of Precambrian rocks, mostly Pikes Peak granite, and at 9,524 feet it over shadows the north side of the reservoir. The park is underlain by Tertiary Denver formation rocks (Cretaceous Pierre and Benton shales) and the Niobrara formation. The latter two are of the Colorado Group. 

Wisconsin-age refers to the final period of glaciation within the current ice age, as well as to being geographically located within central North America. This age began about 110,000 years ago and ended between 15,000 and 10,000 years before the present. Glacial outwash is simply soil or sediments (also called alluvium) deposited by flowing water that is associated with a glacier and it is typically made up of a variety of materials, including fine particles of silt and clay, and larger particles of sand and gravel.

Precambrian rock is rock that was formed during the Precambrian Era. The Precambrian Era spanned a period of time from the formation of the Earth (around 4.5 billion years ago) to the evolutionary point of abundant macroscopic hard-shelled animals, which marked the start of the Cambrian Era, some 542 million years ago.

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Igneous rock is formed by solidification of cooled magma and it is intrusive if the crystallization or solidification occurred underground.  Felsic is a term referring to silicate minerals, magma and rocks that have been enriched with lighter elements like silicon, oxygen, aluminum, sodium and potassium. Granite has a medium to coarse texture and can be pink to dark gray or even black. Outcrops of granite tend to form tors and rounded massifs. Granite is nearly always massive, hard and tough.

Pikes Peak granite is a widespread geologic formation found in the front range of Colorado, including around the Pikes Peak region. It was created by cooled magma but was the result of different geologic events.

Shale is a fine-grained sedimentary rock whose original constituents were clay minerals or muds. It is characterized by thin laminae breaking with an irregular curving fracture, often splintery and usually parallel to the often-indistinguishable bedding plane. Shale is the most common sedi​mentary rock.

Pierre shale is of marine origin and was deposited in the Western Inland Seaway during the Cretaceous period. It is a geologic formation that occurs east of the Rocky Mountains from North Dakota to New Mexico and is about 700 feet thick, lying above the Niobrara formation. It is a dark-gray shale, that contains many fossils, and has veins and seams of gysum, and concretions of iron oxide, contained within it.

Benton shale is also a marine shale representing the first deposits formed after the Western Inland Seaway invaded the interior of the North American continent during the Cretaceous period. It was formed from mud deposits and contains many fossils of marine mollusks, ammonites and scaphites.  

The Cretaceous period occurred approx. 145.5 to 65.5 million years ago. It was the last age of dinosaurs, but at the same time, saw new groups of mammals and birds as well as flowering plants appear. At the end of the Cretaceous period one of the largest mass extinctions in Earth history took place and many species, including dinosaurs, pterosaurs, and large marine reptiles disappeared.

The Western Inland Seaway was a body of water that cut the North American continent in half during most of the mid and late-Cretaceous period.

The Niobrara Formation, also called Niobrara Chalk, is a geologic formation in North America that was laid down between 87 and 82 million years ago during the late Cretaceous period. It is about 600 feet thick and is composed of two structural units, the Smoky Hill Chalk Member that overlies the Fort Hayes Limestone Member. The chalk was formed from calcium carbonate that was once contained in the shells of microscopic, one-celled golden brown algae living in what was once the Western Inland Seaway. The algae scavenged calcium that was dissolved in the water in order to build their protective shells which were periodically shed to eventually come to rest on the sea bottom. Over time a layer of these decomposing shells formed the Niobrara Chalk layer which underlies much of the Great Plains of both the U.S. and Canada. Evidence of vertebrate life is common throughout the formation and includes specimens of plesiosaurs, mosasaurs, and pterosaurs as well as several primitive aquatic birds.

The Smoky Hill Chalk Member of the Niobrara Chalk formation is fossil rich and known primarily for its exceptionally well preserved marine reptiles. It is the uppermost of the two structural units of the Niobrara Chalk with the other unit, the Fort Hayes Limestone Member lying beneath it. Pierre Shale overlies the Smoky Hill Chalk. It's major constituent is chalky shale.

The Fort Hayes Limestone Member of the Niobrara Chalk formation is recognized by its thick massive beds of chalk and limestone that are separated by small, thin beds of shale.