The South Platte River Valley, home to Spinney Mountain and its reservoir, boasts a geological tapestry covered by Wisconsin-aged glacial outwash. This outwash, dating back 15,000 to 10,000 years, resulted from the final glaciation period in central North America, which deposited a mix of sediments: fine silt and clay, alongside larger particles of sand and gravel.

Spinney Mountain, reaching an elevation of 9,524 feet, is predominantly formed of Precambrian rocks, particularly Pikes Peak granite. The Precambrian Era spans from the Earth's formation 4.5 billion years ago to the emergence of abundant hard-shelled animals 542 million years ago at the onset of the Cambrian Era. Pikes Peak granite, a widespread geologic formation in Colorado's front range, is an intrusive, felsic igneous rock with a medium to coarse texture. The granite, formed by cooled magma, presents as tors and rounded massifs, and can exhibit colors ranging from pink to dark gray or black.

The underlying geological layers in the park include Tertiary Denver formation rocks, encompassing Cretaceous Pierre and Benton shales, and the Niobrara formation. Shale, a common sedimentary rock, is characterized by fine-grained layers and is prevalent in the Pierre shale, which was deposited in the Western Inland Seaway during the Cretaceous period.

Benton shale, another marine shale from the Cretaceous, formed from mud deposits and contains numerous fossils of marine mollusks, ammonites and scaphites. The Cretaceous period, approximately 145.5 to 65.5 million years ago, witnessed the end of dinosaurs, the emergence of new mammal and bird groups, and the advent of flowering plants.

The Niobrara Formation, known as Niobrara chalk, was laid down in North America between 87 and 82 million years ago during the late Cretaceous. Comprising two units, the Smoky Hill Chalk Member and the Fort Hayes Limestone Member, it originated from the shells of microscopic golden brown algae in the Western Inland Seaway and is 600 feet thick. The Smoky Hill Chalk Member is particularly fossil rich, preserving well-detailed marine reptiles, while the Fort Hayes Limestone Member displays thick beds of chalk and limestone, interspersed with shale layers.

Its rich geological history, shaped by ancient seas, glaciers and mountain-building events, contributes to the unique and diverse landscape of Spinney Mountain State Park.