Topo

The International symbol for geologist might well be a geologist's pick and a rolled map. These are the primary tools of the field geologist. (Like Rabbit says in Twister, the movie, "don't fold the map," There was a big crease through Wichita.)

There are several kinds of maps that a field geologist may be interested in, but the two main types are topographic maps and geologic maps. Actually, any time you go into the field, for geology, hiking,camping, etc., a topographic map is your friend. It tells you at a glance how the land is shaped.

And topographic maps are not expensive. You can order them from the National Geological Survey for a few bucks. In fact, you can download them here:

https://ngmdb.usgs.gov/topoview/

For free.

Other countries will have other government departments that take care of distributing topographic maps.

Printed maps are usually clearer than digital maps, but they're also awkward. A typical topographic map is about half your height or more and... don't fold it...roll it...right?

Actually, you can clean up a digital map and get a lot of mileage from it. Here's the 2019 1:24000 scale Highlands Ranch topographic map (GNS). It shows my neighborhood, Walnut Hills.

Eh, it also shows a lot more than I need, so I took a screenshot of it, blew it up and cropped it to just Walnut Hills to get this.

I opened that in Google Photo and used the image processing tools to make the contour lines clearer. Specifically, I used the Highlights, Pop, and Sharpen tools and got this. I don't want to be too specific. Different maps will need somewhat different approaches and the Google Photo app provides a variety of tools. For instance, an old yellowed map may need a blue filter to tame some of the yellow background. The trick is to play with an image until it does what you want it to do. Other apps like Photoshop or Gimp has a lot more to work with but the basic photo touch-up utilities do pretty well for maps.

That made me very happy! Now, I have a topographic map of my study area in my phone (that I can actually read!) and I can show you a few tips on how to read one.

What a topographic map adds to, say, a road map, are the light brown squiggles. They're called "contours" and they represent lines of equal elevation. For instance, look at this section.

One of the contours is broken by a number, 5750. That indicates that every point on that line represents a point in the real world where the ground is approximately 5750 feet above sea level. (This is the US, so topographic maps here use the Imperial system of measurement.)

Notice that there are two different shades of contour lines. The ones with numbers are called "major" contours. The lighter ones are not numbered and are called "minor" contours, but you can tell what they represent by counting them. 

The next major contour over is at 5700 feet and there are four minor contours between so each pair of minor contours have a difference in elevation of ten feet. That will be constant on this map. Adjacent major contours represent a difference of fifty feet. Adjacent minor contours differ by ten feet.

But that may differ between two different maps. This is a hilly area. Out on the plains, a few miles to the east, minor contours may only differ by two feet with major contours of ten feet difference. In the Rocky Mountains to the west, major contours may differ by one hundred feet.

The blue and dashed lines near Davies Avenue are Little Dry Creek and Little Dry Creek Trail. Notice how the contour lines form Vs along the creek with the creek flowing down the points of the Vs. But which way does the creek flow?

If you were walking the creek, you would see that streams always flow with the Vs pointing upstream.

Here's another section of the map.

The little rectangle with a flag on top is Walnut Hills Elementary School. The creek runs behind the school in a deep (about 30 feet) valley. You might have noticed that the contour lines are bunched closer together on the north side of the creek than the south. The closer contour lines are on a topographic map, the steeper the gradient is. The north slope of the valley behind the school is considerably steeper than the south side.

You might think of a contour as where the water line would be if the land flooded to that level. By looking at a topographic map, you can easily "see" what the shape of the land is. Can you "see" where the big hill is behind the school? It's where the contour lines loop around South Ulster Circle.

Pick up or download a topographic map of your area and carry it with you when you go out. How well does it show the shape of your area?

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.