CRACKING ROCKS AT TEEPEE CANYON:AGATES

BISMARCK LAKE NEAR CUSTER, SOUTH DAKOTA-VIEW FROM THE CAMP SITE.

I have visited the Teepee Canyon agate area a couple of times in the last few years, but without much success.  On my initial visit I did not have the proper equipment and my undersized rock hammer was of little use.  I found a few agate fragments but certainly nothing to write home to momma about.  On my second visit I ran into a rain storm and the roads did not look very passable and I am somewhat terrified of lightening.  My momma did not raise no fools!

TEEPEE CANYON AGATES AT THE AMERICAN FEDERATION 2012 SHOW.

 But, I am persistent and in summer 2012 I was bound and determined to go back to the Canyon and pound on the rocks.  Teepee Canyon is located approximately18 miles west of Custer, South Dakota, about 2 miles west of Jewel Cave National Monument off U. S. 16.  As soon as travelers leave the Monument they should look to the west, up slope, and begin to spot piles of broken rocks, big pieces.  Sawmill Spring Road, (FS 456) leads off to the west and about a mile further West Teepee Canyon road takes off.  My best advice is to follow one of these roads/tracks and look for quarries where past prospectors have tried their luck.  The land is managed by the U.S. Forest Service and all previous claims have expired (as I understand it).  The gentleman in the local USFS office in Custer told me that only hand tools were allowed and to stay in the area of previous mining (I left the big fircrackers in the vehicle).  It is my understanding that after the fire of 2000 other agate producing areas external of the main Teepee Canyon site had been discovered.  I don’t know if these are claimed.  What I do know is that there are enough “big rocks” at Teepee Canyon to last me a lifetime!

THE DIGGINGS AT TEEPEE CANYON. 

 The agates are located in chert nodules housed within the lower Minnelusa Formation.  I suppose these nodules are the result of silica-rich meteoric waters circulating through the unit with resulting diagenesis producing the chert.  Why some nodules are agatized—I don’t have the slightest idea.  Just as I am uncertain how/why agates really form!  The formation of agates in several types of rocks is extremely complicated, even for the “experts”.

The Minnelusa sits on top of a widespread carbonate unit termed the Mississippian-age Madison Limestone, or the Pahasapa Limestone as it is generally known in South Dakota.  In the early part of the Pennsylvanian (younger than the Mississippian) the climate was warm and quite wet in the future South Dakota and a red soil developed on the Madison.  Later in the period marine waters returned and rocks of the Minnelusa were formed. The Minnelusa is an interesting rock unit composed of beds of sandstone, dolomite, and thick beds of anhydrite.  Braddock (1963) noted that dissolution of some of the anhydrite had caused numerous collapse structures and collapse breccias in the Black Hills.  Originally designated as Pennsylvanian in age, fossils in the Minnelusa indicate both a Pennsylvanian and Permian age.  I presume that since the agates seem to be in the lower part of the formation, they belong to the Pennsylvanian.  I was able to locate brachiopods in some of the beds at Teepee Canyon but am uncertain as to their exact age.

CHERT NODULES IN MINNELUSA FORMATION.

BRACHIOPODS IN MINNELUSA FORMATION.

The nodules at Teepee Canyon are composed of chert, often red to tan in color, and of various sizes.  Very few of these nodules are agatized so locating a good agate is “hard work”.  First of all, the enclosing carbonate matrix is mostly a fine-grained dolomite that is extremely hard and quite difficult to “break”; hence, the need for a large crack hammer, eye protection, heavy clothes, and preferably steel-toed boots.  Many prospectors/miners at Teepee Canyon have cut into the outcrop and removed “really large” pieces of dolomite to crack open.  I decided early on that sort of mining was not on my agenda and simply cracked open smaller pieces mined by others.  The nodules in the carbonate were quite numerous and it appeared, at least to me, that some of these would polish quite nicely.  But, I was after agates!  I found numerous smaller pieces, all fragments less than one-half inch long, but then—there it was, a real Teepee Canyon agate.  Now, it is nothing spectacular, perhaps 1.25 x 1.0 inches in size, but never-the-less it was mine, and it satisfied my yearning!  So, after about two and one-half hours of cracking rocks in a hot sun I decided to take my find and return to camp on beautiful Bismarck Lake by Custer State Park.

AGATE ROUGH COLLECTED SUMMER 2012.

RED SOIL DEVELOPED ON TOP OF MADISON FORMATION NEAR JEWEL CAVE NATIONAL MONUMENT.  SOIL FORMED IN A WET AND WARM CLIMATE.

 mike

REFERENCES CITED

Braddock, W.A., 1963, Geology of the Jewel Cave SW Quadrangle, Custer County, South Dakota: U.S. Geological Survey, Bulletin 1063 G.