Great Plains geology – a personal journey

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).

Great Plains Geology Fig. 2
Fig. 1. John Wesley Powell’s coloured map of the US physiographic provinces (1895). The area shaded in a deeper blue just to the left of centre is Powell’s Great Plains.

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.

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Fig. 2. Bison grazing in western South Dakota. (Photo by Anne Diffendal.)

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.

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Professor Emeritus Robert F Diffendal, Jr (USA)

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