Those darker beige splotches at either end of the dam embankment are Dawson formation bedrock. The satellite image looks like this.
Geologic maps are the primary tool of the field geologist. It's not always obvious what's beneath your feet but, when you hold a geologic map, you hold the collected knowledge of the geology of an area. Add a satellite image and a topographic map and you're set...well, assuming you know how to read them.
For instance, it doesn't take much imagination to see that, before the dam was built, Willow Creek had carved a valley right through the Dawson sandstone. The surrounding materials are much more recent stuff laid down by local streams...the colluvium I've been talking about.
The dam changed the face of the area. Again, it would be difficult to tell how much of this material was moved in, and how much was just rearranged dirt from the site. The plaque above tells a lot about the dam. It's purpose is flood control. At flood stage, Willow Creek changes from it's current swampy state to a small lake the embankment is 55 feet high and the reservoir can store 652 million gallons of storm runoff. After the devastating floods early in Denver's history, the city really took flood control to heart. Other examples in the area are the Holly Reservoir, Bear Creek Lake and the Mount Carbon dam, and Strontian Springs dam in Waterton Canyon.
Englewood Reservoir dam
The sandy colored ridge in the distance is underlain by the arkose sandstone of the Dawson formation. Most sandstone is composed almost entirely of quartz sand, laid down in the past by erosion and deposition by water or wind. The parent stone was often granite or gneiss but quartz is the only thing left after the feldspar, mica, and other materials are weathered away. Arkose sandstone isn't rare. About 15% of the sandstone you see will be arkosic.
The stuff washed out of the Colorado plateau to form the Rocky Mountains was dumped quickly onto the plains and buried so that the feldspar didn't have the chance to decompose into clay. Arkose sandstone is the result. Here's a stream tumbled piece that I found on the site.
The broken face doesn't have any tool marks, so this is not likely an artifact. There was probably a weak plane between the granules that make up the rock and it just broke apart. Microphotographs show that the mineral grains are still pretty much intact.
You can see clear quartz, white feldspar, and black amphibolite. Sandstone with more than 25% feldspar is considered arkosic. This specimen is loaded with feldspar.
I use a clip-on microscope with my smartphone for the photomicrographs in this blog.
This is the hill on the west side of the dam where the Dawson formation reemerges. A formation is a group of rocks that consistently appear together and can be distinguished from neighboring rocks. The Dawson formation underlies most of the Denver area and is composed mostly of arkose sandstone with some mudstone (shale) thrown in.
This is the ridge on the eastern side of the dam over the Dawson sandstone. If the dam were not maintained, it would wear away, so grasses and other plants with deep roots are encouraged to grow there. The top soil they create is why the bedrock isn't visible and it would be rude (and illegal) for me to dig around in it. But it is interesting to see how the hard sandstone under the surface influences the surface features of the land.
The clay's here expand when they get wet and when they dry out, they contract and crack. I saw these cracks with something growing in them. Looks like writing, doesn't it?
From a distance, the plants looked like they might be a moss so I got on my belly to take some photomicrographs and, turns out they're grass seedlings.
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I've talked about safety and disaster preparedness on this blog before.
I think there are several reasons that many people are unconcerned about what they would do in an emergency. It's likely that they have never had a major disaster happen to them...yet, so it's not in front of their minds. Also, familiar surroundings don't feel dangerous.
But can you think of anyplace in your home that would be hard to escape from in the event of a fire? I bet you could. Do you even know how you would get out of your bedroom if you awoke one night to the smell of smoke?
Can you list all the dangerous events that could occur in your area and would you know how to survive them? Are they the same hazards you might encounter In your favorite vacation spot?
Blake Kelly is a Boy Scout that's interested in safety and disaster preparedness. I thank him for sharing the following link.
Take a look. It might save your life one day.
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