SOUTH DAKOTA LAKE SUPERIOR AGATE

I recently had the opportunity to rummage through South Dakota, stopping at sites of geological interest and collecting some interesting material.  Information on the Black Hills and the western part of the state has been described in several previous blogs (for example see 7-2-13 posting).  I am certain many readers/rockhounds have fond memories of observing the many geological features out west, driving through the badlands, and even collecting a few specimens in the Hills.  I always enjoy my expeditions.

   

General geologic map of South Dakota.  Glacial sediments dominate East River.  West River has the Black Hills and numerous outcrops of Tertiary and Cretaceous rocks. X is Milbank, Y represents the Sioux Quartzite, Z is Glacial Lake Dakota.  Map courtesy of South Dakota Geological Survey.

The eastern part of the state most likely seems less interesting to travelers and I suppose most drivers set their cruise control on 80 MPH and zip through the fields of soy beans and corn on I-90.  And in fact, collectors will have a difficult time even locating bedrock unless they happen to explore areas along some of the major streams (for example, the Precambrian Sioux Quartzite cropping out along the Big Sioux River: Y on map) or “hills” of the Precambrian Milbank Granite (sticking up through the glacial drift; X on map).  Besides the Precambrian rocks there are excellent exposures of the Cretaceous Niobrara Chalk and Pierre Shale in the Missouri River Trench (southeast border of state).   What it lacks in collecting opportunities, however, is partially made up by rockhounds understanding the fascinating glacial history during the Pleistocene Epoch.  A good place to start with this understanding is by reading the Roadside Geology of South Dakota (2009) publication authored by John Paul Gries. 

 

Shaded relief map of South Dakota.  Map courtesy of Ray Sterner, John Hopkins University.

The U. S. Geological Survey places landforms of South Dakota into either the Great Plains or the Central Lowlands Physiographic Regions. Each of these regions is further subdivided into smaller sections.  However, I find it much easier to think of the state divisions as: 1) East River (the Central Lowlands) where the landscape is generally subdued and is covered by a variety of glacial sediments; 2) West River (the Great Plains) and its magnificent outcrops of Cretaceous and Tertiary rocks; and 3) the Black Hills, a Laramide mountain range that is part of the Great Plains but more closely related to the Rocky Mountain ranges.  Of course, the "river" here is the mighty Missouri River bisecting the state in a generally north-south direction.

So, eastern South Dakota is essentially covered with glacial drift or meltwater sediments and the landforms are sometimes difficult to recognize.  The James River Lowlands is currently occupied by an underfit river (small river, big valley) but during parts of the Pleistocene served as a major conduit for ice advances and meltwater releases. West of the James River is a highland area known as the Coteau de Missouri (French for Missouri Hills!).  The hills are underlain by Cretaceous bedrock but surficial rocks are mostly glacial till.  However, some small streams close to the Missouri River expose outcrops of the Cretaceous-age Pierre Shale and/or Niobrara Formation.  

East of the James River Lowlands is a region known as the Coteau des Prairies (Prairie Hills). The escarpment leading from the Lowlands to the Coteau is one of the more noticeable areas of relief in East River (~500 feet).  The bedrock again is usually the Pierre Shale but surficial rocks are glacial till.  Dakotans know this area as “Lake Country” since prairie potholes, many times the result of ice blocks left behind (kettles), are scattered across the area.

The Minnesota River Lowlands was the site of a major, really large, Pleistocene flood as waters from the massive Lake Agassiz broke a barrier and flowed southeast in the Glacial River Warren---again creating a modern underfit river. 

 

Lake Superior Agate collected from South Dakota (width ~8.5 cm).

Eastern South Dakota is not a hotbed for collectable minerals; however, the glacial deposits have some really nice cobbles of rocks from the northeast, especially those igneous rocks exposed around Lake Superior.  And, if these Lake Superior rocks were transported south and east there is always a chance to find Lake Superior Agates (Lakers). I did not realize it until much later in life that Dakotans find these agates in substantial numbers.  South Dakota’s best known rockhound, June Culp Zeitner (1916-2009), noted in her best-selling collecting book Midwest Gem, Fossil, and Mineral Trails:  “Lake Superior-type banded agates are found along many of the streams and glacial lakes in the eastern part of the state.  Pickeral Lake and Big Stone Lake are some of the places nice agates have been found especially in the northeastern part of the state.”  The specimen in my collection came from gravel deposits (Pleistocene) associated with the Big Sioux River.  This is not surprising since the river has its headwaters ~100+ miles to the north deep in the heart of glacial debris.

I wish to thank Ray Sterner of the John Hopkins University Applied Physics Laboratory for allowing me access to his files of state relief maps.