"South Dakota, the land of infinite variety..." This phrase is especially
true when one considers the wealth of geologic
diversity that lies beneath our feet. From the glacial sediments underlying most of Eastern South Dakota to the igneous and
metamorphic rocks of the Black Hills area, our state's geological deposits range in age from a few thousand years to over a
billion years old! (See cross section below.)
Geologists normally describe the rocks of any area in chronological
order, i.e., from the oldest to youngest. However, this
fact sheet first will concentrate on surface rocks, examining the geology along the east-west line illustrated by the cross
section below, from Sioux Falls to Harney Peak. Then, the older rocks beneath will be discussed. By coincidence this
east-west transect starts with some of the youngest rocks, in Eastern South Dakota, and ends up with some of the oldest rocks,
in the Black Hills area.
Eastern South Dakota - The Pleistocene Epoch
The Glaciers Advance
The unconsolidated rocks making up most of the surface east of the Missouri River are of glacial origin. Evidence suggests
that these sediments were laid down through numerous glacial advances and retreats beginning some 1.5 million years ago,
and ending 10 thousand years ago. Rock particles of many types, shapes, and sizes were left behind by the glaciers, and this
material is collectively called drift . Many of these rock fragments originated from Canada and other points north and east of
their present positions. The average thickness of the glacial sediments in Eastern South Dakota is about 100 feet (30.5
meters), although in the northeastern part of the state thicknesses can sometimes exceed 900 feet (274.3 meters).
Two of the most noticeable land forms left behind from this glacial
activity are moraines and kettles. Moraines are often
long, rounded ridges that create rugged, boulder-strewn upland areas. Kettles are usually small, concentric lowlands,
commonly called sloughs. Some of the largest kettles form lakes, the best known probably being Lake Poinsett.
Central South Dakota - The Cretaceous Period
The Great Interior Seaway
Beneath most of the glacial drift east of the Missouri River, and forming the land surface of large tracts of Central and
Western South Dakota (see Figure 2), are Cretaceous age shales, sandstones, and limestones. These sedimentary rocks were
deposited over 65 million years ago in a vast inland sea that covered most of this state. Many fossils of giant marine reptiles
that swam in those seas have been found, although other marine fossils, such as fish and sea shells, are generally easier to
locate. (The future Marine Vertebrate Fossils of South Dakota fact sheet will have more information).
South Central S. Dakota - The Tertiary Period
Badlands And Sand Dunes
Extreme South Central South Dakota is an area of pine-covered buttes, rolling sand dunes, and badlands topography. The
dunes are a northern extension of the Nebraska Sand Hills, resulting from the deposition of fine-grained sediments originating
from the mountains to the west. During the Tertiary Period, about 2.5 to 65 million years ago, and on into the Quaternary,
sediments that later became the buttes and badlands were deposited. Concurrently, rich and diverse fauna and flora existed in
this area, resulting in fossil deposits and scenic vistas today. Anyone visiting the Badlands National Park can attest to this!
Western South Dakota - The Cretaceous Period
The Black Hills Uplifted
Toward the end of the Cretaceous or during earliest Tertiary times, the Black Hills were thrust up during a period of
mountain building. At that time the highest elevations were probably over 15,000 feet above mean sea level. Over time, this
uplifted dome was eroded down to its present elevation. Today, the Black Hills area gives us the most complete geologic
record of rock history in South Dakota. The oldest rocks are found in the center of the uplifted, eroded dome. These rocks are
metamorphic, mostly slates and quartzites, and are over 2 billion years old. Intruded into these are granites, such as one sees
at Harney Peak and Mount Rushmore. Moving out from the central core of the Black Hills, one encounters progressively
younger formations encircling one another. Looking at the stratigraphic column (Figure 3 fold out), one can see the many
formations that are represented in the over 400 million year time interval between the Cambrian Deadwood Formation and
the Jurassic Morrison Formation. All of these formations are visible in the Black Hills.
The Precambrian Era
Lying beneath the sedimentary rocks of South Dakota are found a variety
of very old Precambrian igneous and metamorphic
rocks. Most are over a billion years old. Where the overlying sedimentary rocks have been eroded away, as in the core of
the Black Hills, and in the Sioux Falls, Dell Rapids, and Milbank areas, rocks such as granite and quartzite can be seen at the
surface. Both of these rock types have played significant roles in the economy of this state.
Milbank "mahogany granite" is world renowned as an ornamental stone.
From Tokyo to London one can find this beautiful
granite used as facia stone on many buildings. The granite in the Black Hills is too fractured to be useful for building.
Many older buildings around the state are built of the incredibly hard
and weather resistant pink "Sioux quartzite." Over the
years, quartzite has seen many diverse applications. Toothpaste, engine block manufacturing, and road construction are just a
Why Is Geology Important?
Almost everything we use in our daily lives revolves around earth materials.
If we don't grow it, then we probably mine it. In
South Dakota, we use our glacial gravels for the building and maintenance of roads. The crushed Sioux quartzite is also used
extensively for roads and other construction projects. Much of the riprap along our dams is also Sioux quartzite.
Ever since the gold rush days, the Black Hills area has been an important
gold and silver mining area. South Dakota has
recently ranked second or third among the states in the production of gold. Many semiprecious gem stones can also be found
in the area.
Other materials mined in Western South Dakota include limestone and
gypsum for cement, bentonites for drilling muds, and
feldspars and micas for the glass, electrical and ceramics industries. The production of oil in the northwest and natural gas in
the southwest are also contributors to our economy. In certain other areas of the state, significant reserves of uranium and low
grade manganese ore may also be found.
A thorough understanding of geology also enables a better understanding
of our aquifer systems. As the majority of our state's
population relies on ground water for drinking water, the better the data concerning this critical resource, the better the
management and protection of these supplies. (See S.D. Aquifers fact sheet.)
The scientific study of South Dakota's geology is ongoing. Just correlating
the geology from one part of the state to another is
an immense task (see generalized stratigraphic column ), but the more we understand the history of the rocks beneath us, the
more treasures we uncover!
The geology of South Dakota is a truly unique asset. Not only does it
help build our homes and roads, run our cars, and create
jobs, but its beauty attracts thousands of tourists to our state every year.
Aquifer - a body of rock or sediment that is sufficiently permeable
to conduct ground water and to yield economically significant quantities
of water to wells and springs.
Bentonite - soft clay, formed by the weathering of volcanic ash, with the unique characteristic of swelling to several times its original volume when in contact with water.
Drift - collectively, the variously sized and shaped rock particles left behind by glaciers.
Igneous - rocks formed by solidification from a molten or partially molten state. Can either be formed beneath the earth's surface (plutonic), or by volcanic activity (extrusive).
Mean - in statistics, the average of a group of numbers.
Metamorphic - rocks formed in the solid state in response to pronounced changes of temperature, pressure, and chemical environment.
Riprap - foundation or wall made of broken stones, loosely or irregularly thrown together.
Sedimentary - rocks formed generally in one of three ways, either by accumulation and solidification of eroded rock fragments, i.e., sandstone; by precipitation from a saturated aqueous solution, i.e., rock salt; or by the secretion of organisms, i.e., coral limestone.
Stratigraphic column - the vertical succession of rock and soil layers at a particular location.
Unconsolidated - sediments that are loosely arranged or with particles not cemented together.
S.D. Geological Survey, 1964. Geologic Map of S.D.: S.D. Geological
Survey Education Series Map 1.
Schoon, R.A., 1974. Generalized Stratigraphic Column Of Central And Northwestern South Dakota: South Dakota
Geological Survey Educational Series Map 6.
Publications available through the S.D. Geological Survey, USD, Vermillion,
Geologic Map of South Dakota : South Dakota Geological Survey Educational Series Map 1 by South Dakota Geological
A Geology of South Dakota; Part 1: The Surface by Rothrock, E.P., 1944, South Dakota Geological Survey Bulletin 13.
Minerals and Rocks of South Dakota: South Dakota Geological Survey Educational Series 5 by Petsch, B.C., and
McGregor, D.J., 1973.
Record of life: S. D. Geological Survey Educational Series 2. by McGregor and Petsch, 1968.
South Dakota's Rock History: South Dakota Geological Survey Educational Series 3. by Petsch, B.C., and McGregor, D.J.,
The Black Hills: Geological Gem of the West, a video describing the events that created the Black Hills and the Badlands by
Gerald Teachout, available through the Petrified Forest, HC-80, Box 766, Piedmont, SD 57769, Grades 6- adult.
Fossiliferous Cenozoic Deposits of Western South Dakota and Northwestern Nebraska edited by J. E. Martin, 1985.
Museum of Geology, SD School of Mines and Tech., Dakoterra, vol. 2 no. 2.
Geology of the Black Hills, South Dakota and Wyoming edited by F.J. Rich, 1985. Geological Society of America, Rocky
Mountain Section Guidebook, American Geological Institute.
Earth Sciences and Physics Department, University of South Dakota, Vermillion,
Geology Department, S.D. School of Mines and Technology, Rapid City, SD 57701.
Museum of Geology, S.D. School of Mines and Technology, 501 E. St. Joseph, Rapid City, SD 57701
South Dakota Geological Survey, Akeley Science Center, USD, Vermillion, SD 57069.
U.S. Geological Survey, Rm. 408 Federal Bldg., 200 4th Street SW, Huron, SD 57350.
Martin J. Jarrett, South Dakota Geological Survey, Vermillion, SD 57069. 1994.
Dr. James E. Martin, S.D. School of Mines and Technology, Rapid City, SD 57701.
Publication of the Geology of South Dakota fact sheet was funded
through a Natural Resource Conservation Education
Grant, USDA, Forest Service.