Great Plains geology: A personal journey

Professor Emeritus Robert F Diffendal, Jr (USA)

I grew up in the 1940s and 50s in the eastern US state of Maryland and went to cinemas on my own from the age of six, mostly to see what were then to me exciting western movies. In 1962, I was off to graduate school in the Great Plains state of Nebraska, a place that I pictured in my mind as it had been depicted in some of those films. Imagine my surprise when it looked nothing like the outdoor scenes in most of those films. Silly me, to have thought that films were made as closely as possible to the real subject area.

From graduate school in 1962 to now, I achieved my goals and became a geologist and professor, travelling and doing research in the Great Plains and western Central Lowland physiographic provinces, and looking at geology in exotic places like the UK, China, Australia and New Zealand. Fast forward to 2013. I had enough experience and expertise on Great Plains geology by then that I was asked to write a short book of about 35,000 words on the geology of the Great Plains by the director of the Center for Great Plains Studies at the University of Nebraska, Dr Richard Edwards. After visiting and studying sites in Alberta and Saskatchewan in Canada, and in south-western Texas that I had not previously studied, I started working on the book now titled Great Plains Geology that is reviewed in this issue of Deposits on the page opposite (Fig. 1).

I may be wrong, but I think that few people from the UK have much of a mental image of the Great Plains or know its boundaries. Certainly, that is true of most of our citizens in the USA. The area of land included in the Great Plains has been much debated since the late 1800s, when the physiographic region was defined and its area probably drawn for the first time on a map by the second director of the US Geological Survey, John Wesley Powell (Fig. 2; 1895). Powell only included the part of the Great Plains in the US on his map, but wrote that the place extended north into the Canadian prairie provinces of Alberta and Saskatchewan, and south into a small part of northern Mexico. I have included descriptions of some sites in those areas of the Great Plains in my book.

Readers of Deposits have a background in the jargon of geology and palaeontology, so I do not need to spend time defining too many terms. Instead, let me tell you that much of the Great Plains is beautiful and that most of the people are friendly. The place has spots that are spectacular ecotourism sites where, if you plan ahead and are lucky like my wife, Anne, and I, you can see wildlife, such as bison, moose and coyotes (Figs. 3 to 5), and migrations of vast flocks of geese, ducks, white pelicans, Sandhill cranes and other wonderful birds.

Much of the Great Plains is semiarid: that is, it receives on average less than 50cm of precipitation a year, although along the eastern border, there is somewhat more on average. Precipitation from year to year and from one part of the region to another year to year can vary greatly. Temperatures also vary greatly during the year and even during the day. Visitors or residents need to watch weather forecasts and radar regularly every day, and to look for changing weather conditions when in the countryside.

As you travel across the Great Plains, you can observe and interpret many geological features. One feature is easy to interpret if you know what to look for in the landscape. There are many places on the Great Plains where buttes and mesas are capped by gravelly river deposits. These mark the former low spots on the landscape, now high and dry because the rivers and streams that carried them to those places have shifted courses and eroded much more deeply into the adjacent more easily eroded, finer-grained sedimentary strata. This left an inverted topography with the river deposits high above those now accumulating on the floors of the new valleys.

The Cypress Hills of south-eastern Alberta and south-western Saskatchewan in Canada, and Castle Rock, Colorado in the US are two such places described in my book (Figs. 6 to 8). Even though the capping river deposits are discontinuous, sometimes unusual rocks in them were eroded from narrowly confined outcroppings of rocks in distant uplands, so at least parts of former drainage systems can be traced out. This form of comparison and matching of rock types to work out former drainage paths has been applied by geologists successfully since at least the late 1800s. In the case of the Cypress Hills, researchers studying the gravel types have linked them to erosion from the Sweet Grass Hills, a small, isolated mountain area with about 900m of relief located about 80km to the southwest in the state of Montana. On the other hand, the cemented gravels at Castle Rock contain large pieces of volcanic tuff eroded from outcrops of that rock along the valley side of the ancient valley, as well as key rocks transported from more distant places in the adjacent Southern Rocky Mountains.

I included outstanding Great Plains archaeological and paleontological sites in addition to or combined with ecotourism and geologic features sites in my book. Head-Smashed-In Buffalo Jump in southern Alberta in Canada, located about 194km south of the Calgary airport, is one such place. This spectacular UNESCO World Heritage Site, so designated in 1981, is on the east-facing side of the Porcupine Hills, near the southern end of these hills and adjacent to the Old Man River. The jump site is due to exposed thick river-deposited sandstone beds that form a discontinuous escarpment near the top of the hills (Fig. 9) over which Native Americans drove bison to their deaths at many times over at least the last 5,700 years. Anne and I first visited this site decades ago with our Canadian friends from Alberta, John and Karen Hillerud. Since then, the visitor centre, built into the hillside, has been greatly improved and now has outstanding interpretive displays that explain the archaeology, geology and other features of the site. There is a wonderful replica of the cliff with bison poised to race off of it and of the archaeological dig site below (Fig. 9).

Three paleontological sites in the US – Hot Springs Mammoth Site, Agate Fossil Beds National Monument and Ashfall State Historical Park – ought also to stimulate your interest in visiting the Great Plains. Hot Springs Mammoth Site is located in the city of Hot Springs in South Dakota, slightly more than 100km south of the Rapid City, South Dakota airport. In 1974, a contractor was having the site cleared for a development when his crew bladed off parts of a mammoth skeleton and stopped work to assess the situation. My colleague, Dr Larry Agenbroad, was asked shortly thereafter to take a look at the site and found more bones. Larry, and his students and colleagues, mapped out the area where the bones occurred. Over the years since 1974, the site has been preserved and greatly improved with the still-being-worked excavation site enclosed now in a very nice visitor centre.

The mammoth skeletons (Fig. 10) and those of many other species of fossil animals died about 26,000 years ago and are preserved in ancient, thinly-bedded sinkhole deposits. These are surrounded on all sides by red, clay-rich Permo-Triassic beds. The sinkhole must have been filled with water in the Late Pleistocene and was a natural trap for unwary animals because of its steep and slippery clay sides. Most of the mammoth skeletons found so far are reportedly of young males. Make of that what you will, but that is interesting.

The fossils at what is now Agate Fossil Beds National Monument were discovered in 1885 by James Cook and his soon-to-be wife, Kate, on conical buttes (Fig. 11) near a ranch house on a ranch along the side of the Niobrara River in western Nebraska that would soon be theirs. Over the years since then, palaeontologists from many famous universities, such as Yale University, the University of Nebraska and Carnegie University, have excavated fossils there. My friend, Dr Bob Hunt, Professor Emeritus of the University of Nebraska and his students and colleagues, have done most of the recent major work there and at adjacent parts of Sioux County in Nebraska.

The Early Miocene fossils found at Agate include: small rhinos; camels; horses; corkscrew-shaped burrows and the skeletons of fossil beavers that dug them; species of an extinct lineage of herbivores called oreodonts; chalicotheres (animals that looked like a big horse with clawed feet); entelodonts (the so-called “giant hogs” that filled the omnivore/scavenger niche); and amphicyonids or bear-dogs. What a wild and crazy menagerie that must have been.

When I first went to the park in the 1960s, the only roads to there were unpaved and the one from Mitchell to Harrison in Nebraska, had signs up saying “No Services for the next 60 miles” (about 100km). Today, that road is paved to the turnoff to the National Monument but that sign has stayed the same. Gas up before you visit.

Ashfall Fossil Beds State Historical Park is located on the extreme eastern side of the Great Plains in Antelope County, Nebraska, just a few miles west of the former western margin of the Early Pleistocene ice sheets. The site was discovered in 1971 by my long-time friend and colleague, Dr Mike Voorhies, while he was searching for fossils near his home.

In Late Miocene times, the largely river deposits of the Ash Hollow Formation there were laid down on the floodplains of rivers shaded by trees. The ground in the valleys was irregular and small ponds formed in low, poorly drained spots. Many animals were drawn to these water sources. A supervolcano erupted on what is now the Snake River Plain in the state of Idaho, some 1,600km to the west of the park site about 11.93Ma. Volcanic ash from that eruption was carried to the east and rained down on the land from the volcano eastward at least onto the park site, leaving behind correlative ash deposits in low protected spots like the former pond. The ash at the park is up to three metres thick and contains many fully articulated skeletons of such animals as rhinos (Fig. 11), several genera and species of three-toed horses, camels, primitive deer, carnivores, rodents, a snake and a giant land tortoise. Also recovered are skeletons of a bird similar to a secretary bird, crowned cranes and an eagle-like vulture.

All of these animals died due to the damage that the ash caused to their lungs during and sometime shortly (a few days to weeks) after the ash from the eruption filled the pond. The rapidity of the burial of the animals in ash in quiet water is demonstrated by the lack of evidence of predation to the skeletons and the presence between the teeth of the rhinos of fossil grass seeds from plants that they were eating just before their deaths.

The Great Plains is also the site of many historical events commemorated by parks and monuments, and popularized in films and books. These include the sites of the Battle of Little Big Horn in Montana, the site of Fort Union, traces of the Oregon and Mormon trails, parts of the Santa Fe Trail, the home of William “Buffalo Bill” Cody, and many others. All are worth your time for a visit.

Please read my book (Great Plains Geology) and come for a visit to any of these or other sites in the Great Plains. If you have questions about specific sites, feel free to contact me by mail at 605 Hardin Hall, University of Nebraska-Lincoln, Lincoln, NE 68583-0996, USA or much faster by email at: rfd@unl.edu. If I cannot answer your questions, I will try to find someone who can.