July 11, 2023

Where Ancient Seas and New Mountains Meet 

Almost every reservoir in Colorado has been formed by placing a dam at one or more ends of a valley and using the hillsides to store water. Most of these valleys have been carved by geologic activity, flowing water and weathering, meaning the exposed rocks on each side will be similar to each other along the length of the reservoir. 

Chimney Hollow is much different, and that brings both benefits and challenges for construction of the new reservoir. 

If you were able to see current-day Colorado as it appeared in the Mesozoic Era 66 million years ago and longer, you would have seen an area of shallow seas, where sediment deposits formed sandstones, shales, limestone and more into horizontal layers. About 66 million years ago, however, an abrupt change occurred. A mountain building era known as the Laramide Orogeny thrust peaks of granite into the sky in central Colorado, creating the Rocky Mountains and the Front Range. 

Rock layers on the main dam plinth

As those new mountains rose over millions of years, they forced the existing rocks to be tilted upward. Erosion and other weathering effects created a series of north-south valleys along the Front Range where the ancient sedimentary rocks formed the two sides. These are visible in places such as Red Rocks, Horsetooth Reservoir and Carter Lake. 

What makes Chimney Hollow a special place is that it sits at the boundary between those ancient sedimentary rocks and the newer Rocky Mountains. That means the rocks that constitute the strength and mass of the dam can be quarried from the harder and stronger crystalline rocks on the west side of the valley, while the tunneling to create the inlet/outlet works is through less-brittle sedimentary rocks on the valley’s east side. 

The geology does pose some challenges, however, as the dam crosses many layers of rock that have different properties. Through our dam design partners at Stantec, builders have been able to meet those challenges.  

When the reservoir is complete, visitors will be looking at an open book that tells the story of billions of years of Colorado geology – right in one single valley.