As we review Black Hills geology, we may wish to refer occasionally to our geologic map of the South Dakota Black Hills, showing the trails and to our stratigraphic column diagraming the formations. To gain a perspective on geologic time, an outline is furnished by the University of Oklahoma.

Below are eight examples of rock formations through which the great majority of trail mileage passes in the Black Hills. We will begin with the geologically older formations and proceed toward the more recent formations. Click the link in each category for a map, details, and photographs.

1) Arkosic, metamorphosed sandstone, of the Centennial Trail from Norbeck north to Sheridan Lake, and away from the Centennial all the way to Pactola Lake. Map, details, and photographs.
2) Schists, slates, and quartzite in a broad central area from Deerfield Lake Loop in the west to the Flume Trail in the east and from Pringle in the south to near Deadwood in the north. Map, details, and photographs.
3) Amphibolite located in the vicinity of Boulder Hill and the Flume Trail and in narrow areas elsewhere. (The view here is atop Boulder Hill though not necessarily of amphibolite itself.) Map, details, and photographs.
4) Granite and pegmatite from Norbeck Wildlife Preserve and Custer State Park Map, details, and photographs.
5) The Deadwood Formation, which the Mickelson, Bear Mountain, Centennial, and other trails cross briefly. Map, details, and photographs.
6) The Pahasapa and Englewood limestone formations of Eagle Cliff, 76 Trail, Bear Mountain, Big Hill, Little Spearfish, the Mickelson Trail, Centennial Trail, and so on.  Map, details, and photographs.
7) The Minnelusa sandstone of the Fanny Peak area, Bear Lodge Trails, Dugout Gulch, Wind Cave National Park, and so on. Map, details, and photographs.
8) The Cenozoic igneous of the Northern Hills, Bear Lodge Mountains, Old Baldy Trail, Mt. Roosevelt, and Bulldog Gulch and Bear Butte along the Centennial Trail. Map, details, and photographs.

Our review of Black Hills geology will not provide a treasure map for gold-panners or gem collectors. Our purposes are threefold: 1) to provide safety awareness associated with predominate rock types in trail areas, 2) to acquaint the hiker with the sort of scenery to be expected near various trails, and 3) to give us an idea of what sorts of rock "furniture" may be associated with the trails we may hike.

The Forming of the Black Hills

Were it not for erosion, the Black Hills might form something like an egg-shaped bulge upon the earth's surface. Toward the center of this "egg" would be the older rocks. Toward the perimeter of the egg-shape would be the younger rocks.

Erosion has in a sense sliced into the top of the egg-shaped bulge so that as one progresses from the foothills toward the center of the Hills, the rocks tend to become older. Were it not for the uplift that created the bulge of the Hills, these older formations would be deeper within the earth's crust. The uplift has tended, however, to cause the older formations to be higher in elevation toward the center of the Hills than are the younger formations in the foothills. The notable exception is provided by the erosion-resistant, limestone plateau, which yeilds some of the highest elevations of the Black Hills, rivaling Harney Peak itself.

The oldest type of outcrops are of schist and slate, or of arkosic sandstone. They are metamorphic. Next is amphibolite, also metamorphic. These three metamorphic formations extend perhaps half the length and half the width of the South Dakota portion of the Black Hills. Next oldest are the granite areas, igneous rocks that, as with the schist, arkosic, and amphibolite, are from the Precambrian Eons (that is, from geologic times previous to the Cambrian Period of the Paleozoic Era).

Next are three sedimentary rock outcrops, formations from the Paleozoic Era. Finally, we mention an igneous outcrop of the Cenozoic Era.

Our discussion is limited. Not nearly all the best information on Black Hills geology is located online, yet we provide a few links at the bottom of this page and in each of the eight sections above.

Hiking Safety and Rock Type

Generally when we hike, we will stay on the trail. But most of us do wander a few yards away from the trail for various reasons. Perhaps we are camping, seeking water, or simply gaining a better view of our surroundings. When we depart from the trail for reasonable purposes, we often find ourselves interacting with the rocks and boulders near the trail. Some of them present a serious hazard to us.

Every type of formation in the Hills has large rocks that are so precariously balanced that the slightest pressure could cause them to tumble in our direction. But that tendency is far more prevalent in some formations than in others. Climbing among rocks requires us to test each of them for stability before we place ourselves in a vulnerable position.

Harney Peak granite is generally the most secure of outcrops in the Black Hills. Occasionally a hand grip will pry off small rocks from a boulder, but often we can climb on granite with boot toes and finger-grips—testing each grip before relying upon it. Generally speaking, granite provides us as much security as can be found upon any boulders. This is the rock climbers' favorite rock.

On the other extreme are the loose and unstable schist and slate outcrops and the "brickwork" of limestone and sandstone. Such rocks can not be trusted very far, if at all. A simple hand held to such a rock for balance may bring a sharp and heavy anvil down upon our feet—causing serious injury. Standing aside, we may jolt the rock from one direction, suppose it to be safe, and then find that pressure applied from a different direction quickly dislodges it.

In-between these extremes are the arkosic and amphibolitic formations that in the Black Hills are neither so secure as granite nor so unstable as schist, slate, limestone, or sandstone. And of course, we do well to never take a rock outcrop of any sort for granted. But in a general way we become more alert and cautious with certain types of formations.

Links for More Information

South Dakota Geology —South Dakota Dept. of Environment & Natural Resources
Geology of Mount Rushmore —National Park Service
Virtual Geology Field Trip —South Dakota State University
South Dakota Geological Survey —Martin J Jarrett
The Black Hills Mineral Region —G. M. Schwartz, University of Minnesota
Spearfish Canyon Geology —Spearfish Canyon Foundation
Spearfish Canyon Mileposts —GORP guide to outdoor recreation
Geologic Driving Tour —Chadron State College