The Dawson formation - and a bit about safety

Still looking for an outcropping of the Dawson formation, I headed down to the Englewood Reservoir. The geologic map of the area showed that the bedrock of a couple of sites there was Dawson sandstone. Here's the Rockd map I was looking at.

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.

https://couponfollow.com/research/disaster-preparedness-on-a-budget

Take a look. It might save your life one day.

The final terminal hike: Thornton

The title sounds ominous but it's okay.

A few years ago, I started a series of hikes around the Denver light rail system. The first year I hiked along the C line from  Mineral Station in Littleton to the Auraria West Station in downtown Denver in station-to-station sections. The next year, I decided to hike around all the terminal stations (terminal = end of the line)...until Covid hit and I stopped using the RTD. I wasn't afraid of catching the virus. I didn't want to give it to a member of my family who would then be knocked out of work for a time. I completed all the hikes except one.

So last Sunday, I took the train to the last terminal in Thornton.

These hikes are not technical hikes. They're pure sightseeing. I go. I see. I take photos. And, now, I'll share the photos with you 

The Arapahoe Marketplace Shopping Center is very near the summit of the eastern rim of the South Platte River valley, so it's a good place to view the Front Range of the Rocky Mountains. I see that Mount Evans (the snow cap in the distance) is being renamed Mount Blue Sky due to the first governor of the Colorado Territory, John Evans', involvement with the Sand Creek massacre and other atrocities against Native Americans.

Arapahoe Marketplace is about halfway between our house and the nearest train station, Arapahoe at Village Center Station so I'm treated to this view whenever I leave the area and return.

The station is nestled between I25 and the cluster of buildings that make up Village Center and Fiddler's Green.

It's often my point of departure for day hikes.

The light rail offers some spectacular views of the Rockies throughout.

Union Station is the hub for the Regional Transportation District, RTD, and is a gateway to downtown Denver and it's many shops, restaurants, and other points of interest. The cables in the center of the photograph above are the suspension cables for the famous Millennium Bridge. It is a footbridge that spans a train yard. It's innovative design allows it to fit between a cluster of skyscrapers. 

It's one of several interesting bridges in the area. Some of them are mentioned here in the January 13, 2019 blog.

The northern branches of the light rail system has newer trains that look more like traditional passenger trains. They're heftier, silver electric trains with larger capacities. 

The Eastlake terminal is one of the newest on the light rail, servicing the Eastlake community of Thornton.

My hike destination was the East Lake Nature Reserve, which surrounds the lake that gives the community it's name. The area is short grass high plains. It's not very picturesque during the winter. The only wildlife I noticed was geese on the lake, but it looks like it might be a hotspot for birders during the warmer parts of the year.

Like most of the prairie, any trees will likely be around lakes and streams, or will be planted there by the inhabitants. The prominent vegetation is grass, which dies back in the winter to add to the thick, rich prairie soil. The scientific name for the soil is mollisol (mollis is Latin for soft, which, if you've ever had to dig a hole in prairie soil might seem a little ironic. Dead grass gives the dirt a tough, fibrous texture).

The bedrock is much the same as that where I live, Arapahoe/Dawson formation and colluvium. Of course it's covered by a thick cover of mollisol, so I didn't see any outcrops.

Longs Peak, one of the prominent mountains of the Rocky Mountain National Park, is visible from Eastlake Station. Much of the head waters of the Colorado River drains from it's slopes.

A last look at the Eastlake Station before heading back home.

Another view of Mount Blue Sky from the top of the hill at Arapahoe Road.

It was a good day for a hike, clear skies, and not very cold. People were out with their families. Overall, a nice last hike for my terminal hikes.

GPS check

I had a dentist appointment on the eighth of March, so I took a train and, while I waited at Arapahoe at Village Center station, I compared my GPS coordinates as measured by Google Maps and the Physics Toolbar app. Both are easy. To get the coordinates from Google Maps, you just set a pin on the map where you are...poke your screen at the spot and hold it there until the teardrop shaped pin appears. Then you can read your coordinates in the search bar. Here's what I got.

Physics Toolbox is just as easy. The menu button at the upper left gives you a list of options. Choose GPS and the screen gives you the data. Here's mine.

So what I have from Googles Maps is:

Latitude: 39.600337

Longitude: -104.888418

and from Physics Toolbox:

Latitude: 39.600331

Longitude: -104.888407

That's pretty close, but how does it work out in distance?

There's a formula that gives you the distance between two points, specified by global coordinates on the Earth's surface. In the Astronomy LabBook, it looks like this:

I have it saved in my MC50 Programmable Calculator app so I don't have to crunch the numbers from scratch and my data has the same number of significant digits, so I can punch them right into the calculator. What I get is 0.000024 kilometers. That's 0.024 meters or 2.4 centimeters. Assuming that Google Maps is accurate, Physics Toolbar's stated precision of ± 5.36 meters is pretty conservative. 

Of course, the precision changes according to atmospheric conditions and the number of satellites in range (here we have 20 satellites.) 

GPS is important in any field science. It's easy to note your position in a field notebook when you have a pair of accurate global coordinates, and you should. At each observation you should note your position (if you can't get your coordinates, describe your position as best you can by giving the position in respect to some named landmark) and the date and time.

The smartphone has become a great companion for the field scientist. I never leave home without mine.