October 2018


Into the Badlands

[Note: This entry was posted on Aug 2, 2019. It is backdated to keep it in chronological order.]

by George Taniwaki

On our trip to Rapid City (see Real Numeracy, Oct 2018), Sue and I saw notable geology and topography. Assuming you believe in evolution, the earth’s history can be seen written in land formations such as mountains, caves, and canyons. On the other hand, if you don’t believe in evolution, then there is no coherent explanation for what you see, it just is.

During our field trip around Rapid City we saw rocks with ages that span 2 billion years. There are not too many places on earth where you can see such a wide range of rock ages and types within a few miles of driving. (Denver, where I grew up is another such place. I didn’t realize how special it was until I moved away.)


Geologic map showing Devils Tower, Jewel Cave, Wind Cave, and Badlands. Image from Mira Costa College

Late Precambrian—Igneous inclusions at Black Hills

The earth consists of a molten ball of iron and other heavy metals called the core. A layer of molten silica and other lighter materials called the mantle floats on top of it. The very top layer is a thin shell, less than 50 km (30 miles) thick that has cooled off and is solid. This layer, called the crust, is all we can normally see.

Geologic time starts about 4.8 billion years ago, when the earth was still a molten mass and the crust had not yet formed. The timeline is divided into 5 eons. The eons are subdivided into 11 eras, then periods, then epochs, then ages.

The story of the Northern Plains starts at the Orosirian period, between 2,050 to 1,800  million years ago. During this period, hot magma rose to the surface and solidified into two igneous rocks, granite and pegmatite. Under heat and pressure, some of these rocks metamorphosed into gneiss (pronounced nice). These form the bedrock making up the Black Hills.

Remember though, we don’t know what the surface of this area looked like back then. It may have been underwater. It probably was not mountainous like it is today, otherwise the inclusions would have eroded away by now.


Granite in the Black Hills. Photo from Deadwood Connections

Paleozoic-Missing layers

There are no rocks aged between 1,800 million years to 225 million years old visible in the area around the Black Hills. They all became buried or eroded away.

Mesozoic—Inland sea and limestone

During the late Triassic through the Cretaceous period, 225 to 65 million years ago, the earth was warmer than now. Dinosaurs ruled the land. The continents began to drift apart and the middle part of what is now the North American continent was below sea level, part of an inland seaway. Salt water bacteria and invertebrate marine animals converted carbon dioxide into calcium carbonate. As they became buried under the sediment the shells became limestone, rich in fossils.

At the same time thin beds of mud, sand, and silt also built up creating layers of black shale, red sandstone, and red siltstone. These striped sedimentary rocks are visible at the base of the Black Hills.

Cenozoic—Inclusions at Devils Tower

During the Paleogene period, 65 to 23 million years ago, the earth began to cool. The dinosaurs became extinct and mammals of all sorts became the predominant land animals.

From the action of plate tectonics, the land to the west rose and the inland sea receded. Magma rising from the mantle circulated to the Earth’s crust forming inclusions and volcanoes. Nearby Devils Tower is a prominent example. The igneous rock is harder than the surrounding sedimentary formations that have eroded away, leaving a butte.


Devils Tower, perhaps once a volcano or an alien beacon. Photo from Pattys-photos

Cenozoic—Volcanic ash at Badlands

Also during the Paleogene period, after the extinction of dinosaurs, a wide variety of mammals lived in the northern plains. Many of the species are now extinct, including saber-toothed cats, rhinos, tapirs, and three-toed horses. Periodically, a volcano would erupt killing them suddenly. Their remains can be found in fossil beds that can be dated by measuring the ratio of isotopes in the layer of ash they are found in.

The ash and lava are harder than the underlying sandstone or limestone. Places where the lava eroded away, the water and wind carved out steep canyons, called badlands.

I’ve been to Badlands National Park as a child. Today, it looks pretty much the same, rugged but beautiful.

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The Badlands in 1967 (top) and today (bottom)

Cenozoic—Supervolcanoes at Yellowstone caldera

During the Neogene period, 23 to 2.5 million years ago, continued tectonic action caused the land to rise just west of Rapid City, ultimately to 2,000m (6,500 ft) above sea level, creating the Black Hills.

This was accompanied by significant volcanic activity in the Rocky Mountains. We didn’t visit Yellowstone National Park on this trip as it is 600 km (360 miles) away, but it is worth mentioning. As you travel west through Wyoming, the mountains get taller and the volcanoes larger. The largest volcano is Yellowstone, the caldera of a supervolcano that had its last supereruption very recently on the geologic timescale, about 630,000 years ago. In that event, it ejected 1000 km3 of rock, dust, and ash, or about  250 times as much debris as the 1980 Mt. St. Helens eruption. An earlier supereruption 2.1 million years ago is believed to have been 10 times larger than that.

Today, the Yellowstone volcano is quiet, but the area is teeming with geysers, hot springs, and mud pots.


Notable geysers with live-stream webcams. Photo from National Park Service

Cenozoic—Limestone caverns at Jewel and Wind Cave

As the glaciers advanced and receded over the past 2.5 million years, the current period called Quaternary, groundwater percolated through the limestone. The water reacted with the calcium carbonate, dissolving it and carrying it away, leaving hollow areas called karsts. Further erosion caused the karsts to grow larger and more numerous. Eventually they formed sinkholes on the surface and caves underground.

Mineral saturated water dripped from the ceilings of caves. Some of the carbonates precipitated out, leaving stalactites and stalagmites. (Simple mnemonic to remember which is which, stalactites hang tight to the “c”eiling, stalagmites are like mites on the “g”round.)

There are two well-known cave systems in the Black Hills, Jewel Cave National Monument and Wind Cave National Park. If you only have time or inclination to visit one, here’s a guide to help you choose. (TL;DR, it’s a tie.)


Jewel Cave on a bright day. Photo from National Park Service

For more geology field trips, see Science field trip–North Cascades (Aug 2017) and Science field trip—Bay of Fundy (Jun 2012).

* * *

Dinosaur Park

Back in the city, we drove along Skyline Drive, a winding road that leads out of the city. At the top of the hill is a large parking lot and gift shop. Across the road is a staircase that leads to Dinosaur Park. There are six life-size, but not realistically shaped, concrete dinosaur sculptures painted in garish green and white. The whole site was built in the 1930s as a WPA project. There are spectacular views of the city, and it’s free, so it is worth spending an hour to visit.


A brontosaurus or something like it

SDSMT campus

South Dakota School of Mines and Technology has a Geology Museum that has an excellent collection of fossils, many found at the Badlands. You can take a virtual tour here.

SDSMT also has a well equipped Industrial Engineering lab. Stuart Kellogg, the department chair, was nice enough to give me a tour. The lab includes 3D printers, laser engravers, and CNC machines.

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A nearly complete brontothere; IE department video on YouTube

Finally, no trip to an engineering college campus is complete until you find the bent monument that honors the local Tau Beta Pi chapter.


Get bent. Photo by Susan Wolcott

[Update: Rearranged the sections to be in geologic chronological order. Moved some of the text to related Aug 2017 blog post.]


Main Street Square at night with Hotel Alex Johnson in background

[Note: This entry was posted on 5 June 2019. It is backdated to keep it in chronological order.]

by George Taniwaki

My wife, Sue, was a member of an advisory committee for the Department of Industrial Engineering at South Dakota School of Mines and Technology (SDSMT) in Rapid City. On her last meeting in October, I tagged along for a quick vacation.

I’ve been to Rapid City before, in 1967, back in the days when every summer my father would pack all of us into a car without air conditioning and drive us to a cheap motel close to a fossil bed in the western U.S. where he would go ask a rancher for permission to prospect. More on the geology of western South Dakota in a separate Oct 2018 blog entry.

I’m looking forward to see Rapid City again after a 51 year absence. I’ll include a few pictures from my childhood trip in this blog entry.

Hotel Alex Johnson

Unlike my childhood trip, Sue and I flew to Rapid City and stayed at the fanciest hotel in town, the Hotel Alex Johnson (see picture at top). It was built in 1927  and is the tallest building in town. It has been renovated, though the elevators are still small and slow. The rooms are nice and there are great panoramic views from the bar on the top floor. Be sure to buy Chubby Chipmunk chocolates from the store in the lobby.

Main Street Square

The Main Street Square is cater-corner from the hotel. On the day we arrived the roads were blocked off and there was an outdoor beer festival in progress. Rapid City must consist of optimistic planners and hardy folks since it is mid-October and snow was in the forecast. Brrr.

Mt. Rushmore

It was unusually cloudy and snowy the week we were in Rapid City. We drove into the Black Hills and stopped at the Mt. Rushmore visitor center. But we were disappointed that we could not see the monument through the fog.

Despite the fog, I was excited to learn that the Mt. Rushmore sculpture has a trail and tramway that was used to haul workers and equipment to the top. But I was disappointed again when I was told that you cannot climb the mountain. Darn. I really wanted to slide down Washington’s chest to re-enact the famous scene from North by Northwest.

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I’m more excited to (not) see Mt. Rushmore as an adult

Crazy Horse Memorial

My family stopped at the Crazy Horse Memorial in 1967. It had already been under construction for 20 years. But without stable funding there was very little progress. It was just a large rock with a hole. It was so unremarkable that my dad, who photographed everything, did not take a picture of it.

Today, the sculpture actually looks like a person, so I am told, which has greatly improved fund raising efforts. But unfortunately, it was so foggy today that we didn’t bother going.


Crazy Horse Memorial circa 1977, ten years after my family visited it, still not a lot of progress

Dahl’s Chainsaw Art

As you approach Mt. Rushmore, there is a small town called Keystone. It has a full-time population of about 300 and consists of a long string of shops, restaurants, and hotels. Most of them having at least one American flag in front. Yet one store stands out. You must stop at Dahl’s and check out the fantastic carved bears, eagles, and signs.


Sue getting photobombed by artist Jarrett Dahl

Rushmore Borglum Story

Across the parking lot from Dahl’s is the Rushmore Borglum Story, a small museum dedicated to the sculptor Gutzon Borglum. It is only open in the summer, so we weren’t able to visit.

Reptile Gardens

Driving back into town, we passed by Reptile Gardens and had to pull in. As a child I saw a stuntman wrestle an alligator, then show off his trained cockatoos, and finally help us kids ride Aldabra tortoises. What a guy!

They still have snapping alligators, poisonous snakes, exotic birds, and giant tortoises at Reptile Gardens. But today the alligator show no longer includes wrestling and kids can’t sit on the tortoises. Oh well, that’s probably for the best. As a child, I felt bad for the captive animals.

Incredibly, I learned that at least one of the tortoises is a hundred years old, so may have been around the last time I was here. I wonder if they remember me.

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Fun at Reptile Gardens in 1967

Wall Drug

For miles around the city of Wall, you see the signs telling you how close you are to free ice water, fresh donuts, and western knick knacks. Welcome to Wall Drug Store, quite possibly the most famous family store in America.

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Wall Drug sign in France;  A jackalope statue (not to scale); Four presidents not obscured by fog

Minuteman Missile National Historic Museum

Just east of Wall Drug is a national park that tells the history of the Cold War. The park consists of three former missile installations along I-90. This museum did not exist when my family came to Rapid City in 1967. The concrete hardened missile silos were active then and were considered secret military sites. But if they were open to the public I’m sure my father would have insisted we see them.

People my age will remember that there were once over 10,000 nuclear warheads loaded on Minuteman missiles scattered around the U.S., ready to launch at a moment’s notice. There were another 20,00 warheads in storage, just in case things got serious. Each warhead was between 20 to 200 times more powerful than the one dropped on Hiroshima.

The silos were not randomly scattered. They were clustered, with one set running north from Colorado (where I grew up), Nebraska, South Dakota, to North Dakota. Knowing the world could end any minute made for some dark humor as captured below.

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Morbid mementos from the front lines of the Cold War

Well, that’s it for the non-geology part of the trip. In part 2 of this blog entry, I describe the earth activity that formed Devil’s Tower, Jewel Cave, and the Badlands. I also visit the SDSMT campus and tour its Geology Museum.