Th e Ge ol o g i c Story Groundwater is in rocks. It is not in subterranean pools or in underground rivers. It occurs inside rocks, within pore spaces and between mineral grains. Porous rocks are the storage vessels for groundwater; when these rocks are water-saturated they become aquifers. Have you ever seen the rain “wet” a rock? Once water saturated, many rocks appear darker. Even if the surface water is wiped off, the rock remains damp. A rain100 million years ago
70 million years ago
wet rock provides an illustration of water seeping into the pore spaces of the rock. It is easier to extract or drain water out of coarse-grained rocks like gravels or sandstones, because the pore spaces tend to be large and well connected. The rock is said to have high porosity and permeability. By contrast, fine-grained rocks like mudstones, clays and shales yield water at much lower volumes and slower rates, because the smaller less-connected pore spaces drain less efficiently. Fine-grained
66 million years ago
65 million years ago
rocks yield little water and may even produce impermeable or confining layers separating aquifers. The rocks that make up the groundwater aquifers in the Denver Basin are sedimentary rocks, deposited as beaches and river channel sands as an ancient sea retreated and the Central Rocky Mountains rose. The rocks occur in logical patterns that reflect how and where they were deposited. Because the rock types influence groundwater yield, these depositional patterns directly influence the aquifer’s
55 million years ago
37 million years ago
behavior. These many-million year old beaches and deposited river channel sands create the geologic formations that geologists have labeled, in descending order, the Dawson, Denver, Arapahoe and Laramie Formations, and the Fox Hills Sandstone.
These paintings at left depict Ancient Denvers, scenes from the Front Range as they may have appeared over the past 100 million years. Images from the Denver Museum of Nature & Science. 34 million years ago
16,000 years ago
Citizen’s Guide to Denver Basin Groundwater
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