WISCONSIN LAKE SUPERIOR AGATES

I have been out fishing and hunting agates again back in the Midwest, at one time my home for several years.  As an avid fisherman I am always searching for the elusive Northern Pike and Walleye.  As a geologist I always have my eyes turned down looking for agates in shoreline gravels!  I am not the only one with this dual passion as I ran on to a website (www.hotspots.com) with combination stories of fishing and agate hunting.  My kind of place!  I am probably better at catching fish that finding Lake Superior Agates (Lakers)! However, during this recent trip I was able to procure a couple of agates from Wisconsin, not an easy task (at least for me).  

As I understand the situation, Lakers have a fairly widespread distribution with the “type” hunting areas in shoreline gravels of Lake Superior and adjacent sand/gravel pits.  But the Laker name comes, not from Lake Superior, but from the Lake Superior Till, a covering of glacial debris.

However, glacial action (Pleistocene) moved specimens into adjacent Wisconsin, Michigan, Iowa, Missouri, Nebraska, and South Dakota and there are reports of a Laker or two from Kansas.  In addition, Pabian and others (2006) noted Lake Superior-type agates have been found in deposits of the Mississippi River as far south as Louisiana.  Essentially any “gravel pit” in the glaciated region, or along the Mississippi River, has the possibility of producing a Laker.

My interest in Lakers dates “way back” to the 1970’s when I taught a combination canoeing/geology field course in Minnesota (my idea of a perfect class).  Traveling from western Kansas the class visited many interesting geological localities including some of the iron mines and taconite concentrators in northern Minnesota.  The “high point” of the class was the week we spent taking a close look at the old Precambrian rocks exposed in the Boundary Waters Canoe Area.  And did I mention sunsets, bears, campfires, and smallmouth bass?  At any rate, on the way back to Kansas we stopped and searched for agates along the north shore of Lake Superior.  And, we usually had some success.

The original source of the Lakers is from the basalts (several different layers) located in the Midcontinent Rift System (MRS).  This geological rift (think about the great East African Rift Zone) begin to form in the Precambrian (Proterozoic Era) perhaps 1.1 Ga splitting the stable part of the North American “continent” or plate (referred to by geologists as the craton).  The Rift is nearly 1400 miles long extending from northeast Kansas to Lake Superior with an eastern arm curving around and heading toward Ohio.  Hugh amounts of lava erupted along faults while adjacent rivers from the uplands dumped thousands of feet of sediments (later sedimentary sandstones and conglomerates) into the low lands of the Rift.  For some reason the rift “stopped splitting” (a failed rift in geological jargon) and the continent healed.  Most of the rocks in the rift are buried below the surface of the earth and are only known from geophysical studies and drill holes.  For example, the Midcontinent Geophysical Anomaly (MGA) in Kansas delineates the rift since the concentration of magnetite in the Rift rocks creates a magnetic “high” that is picked up by geophysical instrumentation.  However, rocks of the Rift become exposed around Lake Superior and the amygdaloidal agates erode from the basalts.  Since the Rift rocks include substantial amounts of iron, the agates have some sort of a red or orange color---oxidized iron.  Most likely the agates formed post-deposition of the basalt and are the result of percolating silica-rich groundwater filling the many vugs or vesicles in the basalt.

The Wisconsin agates came from a gravel pit in Trempealeau County, a few miles north of my “headquarters” in La Crosse.  They are not large but do have that distinctive Laker Look to them.  Eckert (2000) notes that all of the Wisconsin counties along the river have produced Lakers from sand and gravel quarries—and elsewhere.  I visited with an “ole timer” in La Crosse who found a Laker in a local gravel parking lot.  It pays to keep your eyes on the ground, except when crossing a busy street.

This find prodded me into checking up on the official State Gemstone of Minnesota and found the following proclamation:  Be it enacted by the Legislature of the State of Minnesota: The Lake Superior agate is adopted as the official gemstone of the State of Minnesota.  Approved May 15. 1969. 

A pair of Wisconsin Lakers, both ~2 cm.

I then got to thinking about other state symbols and wondering about official rocks, minerals, etc.  It was only then I realized that not only did several states have some sort of an official gem, several states adopted a variety of agate as their official rock, stone, gemstone, or mineral.  In fact, agates are the most common mineral of the many state symbols.  Consider the following:

Florida Stone: Agatized Coral is so designated.  Of course calling it a “stone” gets out of the “what is it” debate: mineral or rock.  They could have made it the State Gem except that slot is occupied by moonstone.  Yes, Florida has also given us a State Beverage (orange juice).

Kentucky Rock: the official State Rock is the Kentucky Agate but of course agate is really a mineral.  However, the official mineral of Kentucky is occupied by Coal, a sedimentary rock.  I think the “symbol namers” were partaking at the official State Bourbon Festival (Kentucky Bourbon Festival, Inc.).

Louisiana Gemstone: Agate, which is interesting in that the specimens are alluvial and found in gravels and are non-native.  Are these really Lake Superior Agates transported via the Mississippi River from Minnesota?  Maybe Louisianans had tired of drinking their State Drink (milk) and skipped over to Kentucky for some adult refreshments.

Minnesota Gemstone: Lake Superior Agate.  I am particularly partial to the Minnesota State Muffin (blueberry).

Montana Gemstone: Montana is the only state with two official gemstones—sapphire and agate.  The agates are multisource specimens so just about any ole agate from Montana will do.  I actually thought their official motto was “Big Sky Country”.  Nope, The Treasure State.

Nebraska Gemstone: Blue Agate (more of a blue chalcedony) formed in fault and joint planes in wind deposited siltstones and claystones of the Tertiary age Chadron Formation (24-37 my) ---the formation common in the badlands of Nebraska and South Dakota.

Nebraska Rock:  Prairie Agate.  Here we go again—calling a mineral a rock.  I am really uncertain about this one as the official state website notes:  “Agate is variegated quartz noted for its layered varieties. In most specimens, the bands are coarse and differ in color and translucency, as well as in compactness and porosity. The prairie agate, distinguished from most other agates because it seldom has these bands.”  If it does not exhibit bands then why call it an agate? 

Oregon Rock:  Thunder eggs, known to all collectors far and wide.

South Dakota Gemstone: the well known Fairburn Agate collected on the plains surrounding the Black Hills.  These alluvial agates are the same as Tepee Canyon Agates collected from the Minnelusa Formation in situ.   Fairburns are among the more expensive of the agates but are among the more beautiful. 

Tennessee Rock: the state has two official rocks, Paint Rock Agate and limestone.  Of course, they need to change agate to the state gem except that slot is occupied by Tennessee River Pearls.

So there you have, agates are the official “something” of nine states.   In my opinion, Rhode Island wins the prize for “Least Unrecognized Official State Mineral’ which is Bowenite (a variety of serpentine).  Now does anyone recognize that name?  However, they make up for it by having a great piece of barbeque meat as their State Bird, the Rhode Island Red!  Second place goes to Massachusetts with another rather unfamiliar mineral, Babingtonite (calcium iron manganese silicate).  The award for “Lack of Creativity in Selecting a State, Mineral, Rock or Gemstone” goes to Nevada and its State Rock---Sandstone.  Not a specific rock, like Yule Marble or Salem Limestone, but plain old sandstone.  I found this out on the website of Nevada’s First Lady where she also a hot link to:  “Nevada’s Early BIGFOOT Sightings.”  It is also sad to report that my home state of Kansas evidently does not believe in symbols of a geological nature.  I need to get those school kids working!

REFERENCES CITED

Eckert, A. W., 2000, Earth Treasures Volume 1: The Northeastern Quadrant; iUniverse Incorporated.

Pabian, R., B. Jackson, P. Tandy, J. Cromartie, 2006, Agates: Treasures of the Earth:  Firefly Books, Limited.