Geology of Lake Pueblo

Lake Pueblo Park has a rich geologic history. The rock layers visible throughout the park reflect a number of interesting geological processes. According to Brad Sageman of Northwestern University three of the most important processes are the following:

1. DEPOSITION OF SEDIMENTARY ROCKS. During the Cretaceous Period a vast inland sea connected the Arctic region with the Gulf of Mexico and a massive mountain range bordered this sea on the west (present day Nevada). The shallow sea extended from Utah to Kansas and covered the area that is now Lake Pueblo Park. Sediments shed from the western mountains accumulated in the broad basin and preserved numerous plant and animal fossils. As these layers were buried deeper in the basin and compacted they turned into rock layers of sandstone, shale, and limestone.
2. UPLIFT AND MOUNTAIN BUILDING. Toward the end of the Cretaceous, the uplift of the present Rocky Mountains began. This uplift created the impressive ranges seen to the west of Lake Pueblo, and it also elevated the high plains east of the Front Range. The geologic period following the Cretaceous is called the Tertiary, and it witnessed the deposition of non-marine sediments on the high plains, shed from the uplifting Rocky Mountains.
3. EROSION AND LANDSCAPE EVOLUTION. As the Tertiary progressed, the river systems seen to this day were developed, draining the uplifted mountains and eroding the high plains. One of these rivers, the Arkansas, carved out the valley now filled by Lake Pueblo. This erosion created the steep bluffs on the north and south sides of the lake, and these bluffs just happen to be one of the best exposures of sedimentary rocks deposited during the Cretaceous found anywhere in the world.

Geologists give names to rock layers that can be traced over large areas. The layers visible in Lake Pueblo Park include, in ascending order, the Dakota Sandstone, Graneros Shale, Lincoln Limestone, Hartland Shale, Bridge Creek Limestone, Fairport Chalky Shale, Blue Hill Shale, Codell Sandstone, and Ft. Hayes Limestone. These rock units can be seen along the dam spillway, through Rock Canyon and up into the Liberty Point area on the North side of the dam.

Geologists also define geologic time by designating specific locations that serve as the standards for all other rocks of the same geologic age. Because Lake Pueblo's Rock Canyon area has one of the best exposures, fossil records, and other characteristics it was chosen as a GSSP (Global Boundary Stratotype Section and Point) - an internationally agreed upon stratigraphic section which serves as the reference for a particular boundary on the geologic time scale. Geologists from around the world travel to Lake Pueblo to study this site.

Fossils are quite common on the park, but please remember that collection of rocks or fossils is prohibited.