As you remember, a theme of mine is to remind you readers of the myriad ways that geology affects our lives. Locally this is an easy task. In this column, I simply point your attention a hundred miles to the east.
The Black Hills are a local feature that we often take for granted. How many times have you traveled Rt 212 to Belle Fourche and work your way down to I-90 and zip east from there with out a thought? There is no reason for that anymore. I remind you that the “Black Hills” are not hills of course but a fairly recent (starting near the end of the dinosaur period and taking about 30 million years to occur) domal uplifted range of mountains according to the geologists. These mountains used to be nearly twice as high (15000 feet above sea level) as they are now but erosion has removed a significant amount of material from the “hills”. Domal uplifts are characterized by the oldest rock (these are over a billion years old) being in the center (Near Mt. Rushmore) and the younger rocks surrounding them in a circular pattern. The oldest (PreCambrian which means older than 600 million years old) rocks are granites, rocks similar to granite but changed by heat and pressure called schists, basalts (dark volcanic rocks) and pegmatites (volcanic rocks with large to very large crystals). The granites of the region are famous because they have been carved into sculptures at Mount Rushmore and the Crazy Horse monument. Many of the mineral riches of the region are found in these older rocks toward the center of the dome. 18 Peaks in the Black Hills exceed 7000 feet in elevation. Limestones of the Madison formation are even well known for their cave systems. Several National cave monuments have been establish along with several other commercial caves in the area. Of course there is the wildlife, (not the Casinos!), the scenery, and and recreation. Did I mention skiing when they actually get precipitation?
As you move away from the old rocks at the center in any direction, you travel through progressively younger and younger rocks spanning over 600 million years of time. Rocks of and younger than the Cambrian period are known for their characteristic fossil content which varies depending on the geologic period they were deposited in. Older fossiliferous rocks (Paleozoic which means ancient life) contain simpler fossils than younger (Mesozoic which means middle life) rocks do. The Paleozoic rocks are mostly marine in origin with many of the Mesozoic being marine too but by the end of the Mesozoic, some of the rocks originated terrestrially. These terrestrial rocks contain the dinosaur fauna I am so interested in. Rocks younger than 65 million years ago (when the dinosaurs went extinct), begin to contain exotic extinct mammal species that change as you go up the rock section slowly changing to recognizable, extant (living) forms in the most recent rocks. The Black Hills are even home to remarkable Mammoth fossil deposits (a national treasure) that are only 26000 years old. Most geologic periods are represented in the area with just a few missing being periods of erosion not deposition in this area.
Most things we use in our daily lives are manufactured from materials mined from the earth. The Black Hills are a major source of these materials. Quartzite rock from the Black Hills is used for building material, toothpaste abrasive, rip rap and even gravel for our roads. Gold from the Black Hills makes South Dakota rank second or third in gold production in the nation ever since the days of the gold rush. Bentonite clay is used for many industrial processes and even oil production though locally it is used extensively for lining ponds. Feldspars (the second most abundant mineral on the planet) and micas are used in glass, electrical and the ceramic industries. The limestone is used extensively in the concrete industry. Gypsum is used by wall board companies to make dry wall used widely in construction. Gem quality minerals are often mined from the pegmatites. Of course there is the timber industry which thrives in the mountainous forests. The Black Hills is a major source for all of these materials. The income from all this activity certainly trickles across the border to Powder River Country. We all benefit from the proximity, lower cost and resultant availability of these same resources locally.
Where do you go on vacation when you live in paradise? Next time you travel east, take the time to appreciate the wonders the Black Hills offer. I have always said, see America first and get to know your neighbors sooner. Well, Hawaii was pretty nice. Hey, it is America!