Sandstone rock showing underlying Geology of Willow Spring Open Space

Geology of Willow Spring Open Space

The Geology of Willow Spring Open Space is not as obvious as it is in the nearby Rocky Mountains, however, it forms the foundation of the area.

In Colorado, geological forces are very visible in the mountains, with different layers of rocks and fault lines sometimes visible. The major features of the Willow Spring Open Space are the relatively flat flood-plain of Willow Creek, with the man-made Englewood Dam at the North end, and steep slopes rising up to the surrounding neighborhoods. Those features don’t automatically bring thoughts of geology into people’s minds like the dramatic rock formations in the Rockies.

lake at Englewood dam after a heavy rain
Floodplain lake of Willow Spring at the base of the Englewood Dam

However, the geology of a location determines whether it is hilly or flat, rocky or sandy, and soil type, along with many other characteristics. Some plants depend on a certain type of soil to grow and thrive, which is frequently based on the underlying rocks. The geology of this area was studied in detail as one of the factors in deciding how the Englewood Dam should be built.

sandstone rock with path nearby

As you walk along the paths, you will see that the ground below you is mostly sand or clay with occasional rocky spots showing sandstone layers. The sand and clay is from existing sandstone rocks breaking down and also from materials that have been washed in by the creek and down the hills from the neighborhood.

The Denver Basin Geology

This area is a part of a geologic basin known as the Denver Basin, which is a sag in older sedimentary layers that was filled in at the base of the earliest mountain chain known as the Ancestral Rocky Mountains. It is known for having thick deposits of gravel and sand which eroded from those mountains.

The specific formation below Willow Spring Open Space is the Dawson-Arkose formation from the Tertiary period of the Cenozoic Era, between 23-66 million years ago. This was after the time of dinosaurs; small mammals were able to survive and evolve without the fear of the dinosaur predators. Flowering plants were also starting to develop during this era.

The Dawson-Arkose formation consists of delta and river sediments, with layers of sandstones, siltstones and claystones, which are all differentiated based on the particle size.

Sand is made of particles of rock and hard minerals with each particle is visible to the naked eye. The particles have sharp edges and don’t stick together.

Silt is in between sand and clay and, when it is wet, forms runny puddles of mud.

Clay is made up of particles that are the finest of all soil particles, from the chemical decomposition of rocks. Clay particles stick together and form a sticky texture when they are wet or dry. High quality clay has been used to make pottery over the centuries.

sandstone rock showing variety of sediments
Exposed section of sandstone rock showing variety of sediments

The Impact of Geology on Willow Spring Open Space

The clay that is prevalent in Willow Spring Open Space contributes greatly to the stability of the Englewood Dam. The sand, silt and clay mixture also have contributed to the evolution of which native plants will grow here. The plants and geography then determine what insects, birds and other animals will be drawn by the combination of the landscape and the plants. That is how the Geology of the Willow Spring Open Space influences everything we see there.

References and Further Reading

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