Rockd by Walnut Hills

This is going to sound like an advertisement...consider it a recommendation.

Rockd by The Mactostrat lab at the Geoscience department at the University of Wisconsin - Madison is one of my main tools for geological exploration. It's an app on my smartphone. 

Where most geology apps are reference works, like geology textbooks or encyclopedias on your phone, Rockd is equipment. It includes a Brunton compass and information about localities with topographic and geological maps and geology and paleontology profiles. It's pretty impressive how much local information it packs. (The Brunton compass needs a gyroscope and magnetometer to work.)

Let's see what it says about my backyard, Walnut Hills in Centennial, Colorado.

When I open Rockd, I see this, my dash board.

It gives me an overview of my area. 

People that don't live here often mistake Denver for a mountain town. It may be a gateway to the Rockies but it's situated at the western edge of the Great Plains. It's hilly grasslands.

Specifically, Centennial, a town in the Denver metro area, is near the western edge of the Colorado Piedmont region. 

Denver makes a big deal about being the Mile High City but, actually, it's down in a hole carved by the South Platte River. Just about everyone around us is at higher elevations.

We're sometimes called the "high plains" but, technically, the high plains are to the east of us. As explained by Rockd, when I tap the Colorado Piedmont panel, the high plains were formed when the Colorado Plateau was pushed up when the Pacific plate crashed into (and is still crashing into) North America. The Rockies are still growing but erosion is happening faster and the craggy appearance of the Rocky Mountains is due to the deep cutting of millions of years by rain, frost, wind, and chemical action. All the debris that washed out of the mountains piled up to form the high plains, and then streams running from the mountains started cutting the softer sediment down to form the Colorado Piedmont.

How old is the rock around here? Rockd says 0.0117 mA (that stands for "mega annum" or million years) to present. This is fairly new stuff. The late pleistocene began 0.02 million years ago and the Quaternary is the most recent age of geologic time. You probably know that the continents have shifted around a lot in the past. Here's what Rockd says the World looked like in the Quaternary age.

Same as today. Really new stuff.

The colluvium mentioned is "generally unconsolidated material deposited on slopes by gravity and sheetwash". (The original source sited is Trimble, D.E., M.N. Machette, 2003, Geologic Map of the Greater Denver Area, Front Range Urban Corridor, Colorado: U.S. Geological Survey Geologic Investigations Series 1-856-H.

(If I poke the citation, Rockd carries me to the actual paper.)

The layer of colluvium is generally more than 1.5 meters thick. Mind you, that's our bedrock. Above that is the regolith. Regolith is the term for the bedrock after it rots. We have a lot of debris under our feet..

It's sort of hard to tell if the stuff in the Little Dry Creek stream bed is native or if it was placed there to slow erosion, or if it has been washed down from architectural fill upstream, but it could easily be part of the colluvium washed out of the mountains long ago. Some of that is pretty chunky.

So, we're at 5732 feet. Littleton is right on the river at 5351 feet. We're situated right at the top of the rim of the South Platte River valley, which makes the valley 381 feet deep here. The river comes out of the mountains due west of both us and Littleton at Waterton Canyon. Water flows downhill so Denver, through which the South Platte flows is lower in elevation at 5130 feet (Wikipedia says 5139-5690 feet but that would be the whole Denver metro area.)

Here's a geologic map of the area from Rockd.

The lighter stuff is the colluvium I've been talking about. The darker is Dawson formation which is an older Late Cretaceous to Paleocene rock...100.5-56 million years old. It might be worth visiting the next neighborhood over to see if Willow Creek digs into that sandstone.

Most of the fossils listed in this area are nearer the bottom of the river valley, so I don't expect to find any in this neighborhood (though I will look for some microfossils in the creek bank), also most of the minerals will be clays and quartz since most of the other stuff would have broken down long ago.

Rockd is a free download, so install it and see what it has to say about your area. You might be surprised.

Walnut Hills

Be forewarned that geology, like astronomy, and paraphrasing Gershwin, is a sometimes thing. Weather, hydrology, and economic factors and play havoc with your plans. That said, I always encourage lifelong learners to explore close to home first. That's the easiest and most accessible focus and everyplace is interesting.

My geology laboratory will be the Little Dry Creek valley in the Walnut Hills neighborhood in Centennial, Colorado. I have been assured that there is, in fact, not a single walnut tree in Walnut Hills. Also, I have never seen Little Dry Creek without water flowing through it. And it is "little" not because of it's size. It was named after the same engineer from New Hampshire, Richard Sullivan Little, that Littleton, Colorado was named after. Little settled in the area with his family in 1862. The town of Littleton is about 15 miles due west of Walnut Hills.

Two creeks have sculpted the Walnut Hills area, Little Dry Creek and Willow Creek. The soil is deep and soft so the creek valleys are fairly deep and steep. They are tributaries of the much larger South Platte River valley in which the Denver metro area is located.

This is a view of Little Dry Creek from Uinta Street in the Walnut Hills neighborhood where it flows behind Walnut Hills Elementary School. The hilliness is directly due to the erosive action of the creek.

The creek begins as runoff from the area just east of Walnut Hills and is tracked by the Little Dry Creek Trail, part of the Centennial Connector Trail, which parallels it throughout the neighborhood. I take the trail often on trips to the several shops in the area, so I have plenty of opportunities to study the area.

In general, road cuts, newly graded dirt roads and other architectural features that clear dirt off the rock below are a geologist's friend...not necessarily here.

It is an urban area, which means that I will have to be careful to distinguish between the actual geology and landscaping. Granite is not indigenous to this area so the rocks in the above photo were brought in. Here is a picture of the natural materials that inhabit the area.

The dirt and mud goes pretty deeply.

That doesn't sound too interesting, geologically, but I think you'll find that not to be the case. The place in the photo is at the western border of the neighborhood and is the only place on Little Dry Creek that shows a well defined soil profile, so I will certainly be coming back here.

On a walk down the trail to a local grocery, I used my AllTrail app to get an idea of the shape of the valley. I started at the eastern border of the neighborhood and walked to the western border. Here's the map and elevation profile.

Over 1.2 miles, the creek drops about 130 feet. Of course, this is the elevation profile of the trail, but the trail begins and ends at close to the elevation of the creek. The grade is pretty much linear. The hump at mile 0.6 is where the trail runs up the shoulder of the valley above the creek. 

On the return trip, I walked up the main road and cut down Uinta for a cross section of the valley down from the rim. Here's an elevation profile of that.

I'll take some other cross sections later. Of course, they change from the top of the valley to the bottom. There are five places that roads cross the creek on the neighborhood. 

Notice above that AllTrails provides topographic maps. The gray lines in the above maps are contours. Google Maps, Google Earth and the Rockd app also gives you topographic maps. When hiking, a topographic map is important. When exploring the geology of an area, they are necessary. A topographic map shows you the shape of the land.

Rockd also provides geological maps of areas. Those show you the kind of rocks and history of an area. I'll look at what the Rockd app has to tell me about Walnut Hills in the next blog.

It's possible that you have been walking around your home territory for years without considering what's under your feet. The next time you're out, look around and ask yourself how the land became the way it is. What were the shaping forces? What are the rocks and dirt like and why are they like that?

Wolf on the rocks

Here's my "backyard".

That's the Rockies in the distance. Little Dry Creek carves a nice valley down from this point. One of the sources is a marshy area across Yosemite, here:

When I walk to the top of the rise to the east, I see this:

a different world with an actual horizon.

This area is a geotourism dream...the Rockies with canyons cutting into their granitic hearts, rich paleontological sites, ancient volcanoes weathered down to their basaltic innerds, carved sedimentary bluffs. I am fortunate to live in this area.

But, y'know. Everywhere I have ever lived has been geologically fascinating. Earth is geologically fascinating.

My earliest hobby was mineral collecting. When I was three, my family moved to Lagrange, Georgia, built over a gem rich pegmatite lens. My father and I visited a quarry about five miles outside the city and we knew nothing of the "space age" minerals that were being mined there so we picked up a couple of chunks of pretty rose quartz after googling at the towering walls of rock and then we left.

Years later, a couple if guys broke into the site late at night with a pickup truck and jack hammer. The owners closed the quarry to the public. From then on, the rock was crushed and sold as land fill. The site has produced beryls the size of barrels. Some are in the Smithsonian. Garnets, rutiles, tourmalines abound there...to be crushed and used as land fill 

We went back many years later and picked around the sides of the highway. Just before I moved to Colorado, after my father had died, the site opened back up to the public.

I didn't see much of my father during my childhood. He had to work (a lot) to support our family. (I didn't realize until much later how truly poor we were. Dad did a good job.) When I was in high school, he decided that we needed something we could do in common and his answer was mineral collecting. 

We spent a lot of time on the rocks. Any local construction project was fair game. Whenever we traveled, we looked for holes in the ground to explore. We connected with college professors and hermits that also collected.

Our personalities were different. He was meticulous, pouring over a single boulder on the dike of a dam, while I would start at one end and quickly traverse the granite, making a mental note of rocks I would revisit. But it tied us together. 

When he found out that he was dying of lung cancer, the last year and a half of his life involved joyously panning for gold and gems and chipping away at boulders.

Georgia intrudes into Alabama with a wedge of igneous and metamorphic rock that always delivers surprises. When I was ten, we moved from Lagrange to Valley, Alabama. During my high school years, we lived on a street that dead ended at a power line easement where I spent a lot of my free time. It was forested and sculpted by several small creeks. I once scooped some sand from one of the creeks for examination. When I got it home, I found that it was full of grains that glowed bright orange under my black light. When I looked at it under a microscope, I found sharp, octahedral crystals of thorite. Thorite isn't common.

I'm not crazy about fossils, but most of Alabama is fossil country. I was surprised though to find big, textbook quality crystals of pyrite popping out of the clayey banks of the Alabama River in Selma, where I lived for twenty years.

Alabama isn't known for it's ragged landscape. It's not known for the network of canyons that stretch across it's northern border or the waterfalls the dot the landscape from Lookout Mountain in the northeastern corner of the state to the forested area thirty miles north of the Gulf of Mexico. I was glad that it was an "undiscovered" outdoor paradise.

Geology is accessible but that's not the only thing that recommends it as a hobby. It's inexpensive. You don't really need much equipment to pursue it. Here's a list:

Small rock pick: has a hammer at one end and a pick at the other.

Chisel (or set of chisels) and rubber mallet.

Safety goggles: for when you make rock chips fly.

Small dropper bottle of dilute hydrochloric acid (or vinegar).

Small magnet.

Mohs Hardness Scale: either the nine mineral samples or common materials such as an iron nail, glass microscope slide, emery paper square, and your finger nail.

Magnifying glass

Freezer bags and tissue paper for specimens.

Something to carry it all in.

There are some incredible phone apps for geologists. In addition to the regular science stuff like Google maps (or Google Earth), Vieyra's Physics Toolbox, Walter Stubb's MC50 Programmable Calculator, and the Arduino Science Journal, check out Workshop512's Dioptra and University of Wisconsin Microstrat's Rockd, an all in one utility for geologists. The last app provides a major necessity for geological exploration...maps, both geological and topological. You can also poke an area on a map and it will give you the geological lowdown on the area. Definitely check it out!.

If you want to try something special, that's usually not too expensive either. Ultraviolet prospecting is a cool nighttime activity. A combination long and short wave ultraviolet light is available at Home Science Tools for less than $50. An added benefit is that scorpions and rattlesnakes also glow under a blacklight.

Earth is our home and it is exquisitely carved. Anyone can find adventure in their back yard.