The Geology P.A.G.E.: Geology of the National Parks Through Pictures

My next series of posts about the Geology of the National Parks Through Pictures will follow along on a birthday trip that we took back in March of 2019 hitting up some of the parks in southeastern Utah and southwestern Colorado.

You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.

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Hovenweep National Monument
After leaving Arches NP, we hit up a small park on the border of Utah and Colorado, Hovenweep, which preserves several ancestral Puebloan buildings within a local canyon. The park has a nice short trail that leads around the canyon allowing you to get a 360 degree view of the canyon and all of the buildings located within it.

Entrance sign highlighting the high desert of the Colorado Plateau.

Hovenweep is represented by several buildings within this area that are estimated to have been built between 1230 and 1275 CE by the ancestral Puebloans. The name "Hovenweep" means "deserted valley" in the Ute/Paiute language and was given that by William Henry Jackson, a pioneer photographer who was here in 1874. Hovenweep itself is located on top of the Cajon Mesa, which is tilted towards the southwest. There are other districts and buildings of the Hovenweep National Monument across the border in Colorado, however they were not accessible at the time we were down there.

There are two rock units within the park. The primary rock unit is the Dakota Sandstone. This is the rock that forms the rim of canyon. The Dakota Sandstone is Early Cretaceous in age, ~100 million years old, and represents the western shore of the very large Cretaceous Interior Seaway. The Dakota is made up of yellow to grey sandstones, mudstones, and a few thin beds of coal. These were deposited within a wide range of coastal environments including deltas, alluvial fans, and coastal deposits.

Cross bedding is definitely visible within the Dakota Sandstone, indicating prehistoric sand dunes.

The Dakota Sandstone was so sturdy, that the ancestral Puebloans used it as the building stone for all of their buildings in Hovenweep. Below the Dakota Sandstone is the other rock unit within the park, the Burro Canyon Formation. The Burro Canyon is also early Cretaceous in age and is made up of conglomerates, shale, mudstones, and sandstones, all deposited within river and floodplain environments.

Here is part of the Hovenweep Castle, showing of the Dakota Sandstone building blocks.

Another part of the Hovenweep Castle from a different angle.

Here is a view down into the canyon, looking at the Square Tower. The Dakota Sandstone is a rather porous sandstone and therefore rain water has a tendency to soak up into it. The underlying Burro Canyon Formation, however, is impermeable and therefore the water will not go through the Burro Canyon and flows off the top of it. The result is a series of springs and seeps along the canyon walls next to the buildings, which is likely the reason that the ancestral Puebloans settled here in the first place.