Wednesday, January 25, 2012

Geoclub trip - The Geology of South Dakota, abridged.

Hi all!

Today I'm going to write about the geology we saw in our few days in South Dakota. As I think I said in one of my last two posts, there's so much to see in this area, it was impossible for us to see all the great stuff that's there, or even see everything at the places we did stop. This was a constant frustration for students throughout the trip who wanted to see everything at every stop. I tried to see it as a whirlwind tour, so that I would know where I wanted to return to in the future and how much time I would need to do it justice!

On day 2 of the trip we arrived at Badlands National Park in the mid-afternoon. The Badlands were so named because when French fur traders were traversing the northern part of the US, the rugged topography and barren landscape were "bad lands to cross." We now refer to "badlands" in the general sense as any sort of rugged landforms featuring sharp ridges. Typically, badlands are formed in relatively soft,sedimentary rocks (ex. mudstones) due to erosion by rainfall and runoff. The picture below is characteristic of the South Dakota Badlands. Today with park roads winding through it, it is not difficult to cross, but you can imagine that the first settlers to cross through this territory may have found it intimidating!



The Badlands are composed of many many layers of soft sedimentary rock, but the layers can be grouped into three sections based on rock type and erosional characteristics. In geology the technical term for this is "lumping." Students are often encouraged to "be a lumper not a splitter!" because it makes large-scale geologic interpretation more manageable. So if we "lump" the rocks, we see three groups with easily distinguishable characteristics. The lowest "lump" is composed of weathered shale, a fine-grained, mud-rich, rock. The shale weathers to a variety of colors, including gray, white, red, and yellow. Because shale is so soft, when it erodes, it forms rounded mounds. This is me giving some sort of lesson about horizontal layering of sediments, with the brightly-colored shale mounds in the background.



The clay in the shale forms a cool texture on the surface when it absorbs water and expands then contracts. The texture is called "popcorn texture." Yet another highly technical geology term :) Here is a close-up of the clay "popcorn" in the lowest lump.



The middle lump is also composed of a lot of mud and clay, but rather than being weathered shale, these are sediments that were carried east from the rising Rocky Mountains several tens of millions of years ago. The middle lump features thin, almost perfectly horizontal layers, in pastel colors. The rocks do not form mounds as they erode, but the rainfall does create sharp gullies that carve the slopes of the rocks and cuts across the layers. Some layers are more resistant than others due to a larger grain size or natural cements. These layers stand out against the more mud- and clay-rich layers. The middle lump is the thickest section of the Badlands and is what you see in most pictures:



Note how sharp and narrow the ridges are! Alan Howard says "you can't drink too many beers and walk on those things!" So true. The uppermost lump is fairly unique in the set. It is the thinnest lump by far and is composed of some layers of the mudstone, but has several layers of volcanic ash as well. The ash layers are bright white and more resistant to weathering than the mud, making the uppermost lump the most rugged. The bright white horizontal layers near the tops of the rocks in this picture are the ash layers. The ash was carried by wind from the volcanoes on the west coast of the U.S.



The Badlands have preserved a great collection of fossils from roughly the Eocene epoch, about 50 millions year ago. These fossils are about the same age as the fossils at the Gray Fossil Site near Gray, Tennessee. Similar animals are found at both locations, primarily mammals. The prehistoric creature that Nicole found most interesting at the Badlands Visitors' Center was the "pig dog." Pig dog is an actual animal from that time period, and fairly ferocious, at least based on the appearance of the fossilized form!

We camped in the Badlands that night - this was our campsite, how beautiful!!!!



The next day (only day 3!) we woke up early and drove further west to the Rapid City/Black Hills area of South Dakota. Kevin made us AWESOME burritos for breakfast on the campstove with eggs, cheese, onions, green peppers, and salsa. Yum!!

Our first stop of the day was at the SDSM&T geology museum on campus. Having been a SDSM&T student for a few weeks the summer before, I was excited to see the campus for the first time! It's very small, but their geology museum is really great! It takes up the whole floor of a pretty large building and has tons of fossils and great mineral, rock, and meteorite samples. I wish we could have a collection even half as big at McClung!! We spent about 1.5 hours looking around. We had some very intellectually inclined students on this trip, they made us so proud everywhere we went! :)



From the SDSM&T geology museum we headed over to the town of Hot Springs to the Mammoth Site. The Mammoth Site has the best collection of preserved mammoths anywhere in the world! The museum there is really cool, as it is also an active dig site that is staffed by seasons of volunteers. Anyone can volunteer to participate in the dig, you just need to go to an orientation! The museum was built on top of/around the dig site, so the dig is completely enclosed in a building that houses paleontology labs and the museum. The reason there are so many mammoth fossils here is because there used to be a watering hole at the location of the dig site during the Pleistocene epoch. However, the sides of the watering hole were steep, and the rock was shale (lots of mud, remember!), so the banks of the watering hole were steep and slick. Animals would try to get down to the water's edge but would not be able to climb back out, so they would drown. Over time, the lake filled in with mud and other sediments, forming a giant mammoth tomb. This is the location of the current museum. Below is a picture of a volunteer doing some excavation around a mammoth skull.



The last stop of day 3 was at Crazy Horse National Monument. It was too foggy to see the actual carving of his face, which was too bad, but we did watch the video of how the carving was started, and where they are now in terms of progress. Answer: not very far. When the sculpture is finished it will be the largest stone carving in the world, however, only a very small amount has been completed, as it is funded entirely by donations. There are two nice museums there, however, one with lots of Native American history and one about the designing and beginnings of carving the monument. Crazy Horse is carved from the same stone as Mount Rushmore: the Harney Peak granite. They save the smaller pieces when they blast and give them away to visitors. I'm not sure how many visitors want to take home a chunk of granite, but all of us swarmed the big crate of rocks and were very competitive about what pieces we dug out! It was pretty cold that evening and starting to drizzle, so the free coffee was also definitely welcome! The weather that evening should have tipped us off...continue to the next post (once I write it) to see why!

Got to go do a lab report!

Happy trails!
Sarah

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