Sunday, November 27, 2016

MORE SOUTH DAKOTA AGATES



Memories, pressed between the pages of my mind
Memories, sweetened thru the ages just like wine
Elvis
As many/most readers know, I have a very soft spot in my heart for the State of South Dakota.  I suppose that comes from my attendance, in the mid-1960s, at the University of South Dakota.  During those two years of completing a graduate degree, I became fascinated with the diversity of the state’s geology from the glaciated eastern half to the Missouri River Trench to the “badlands” of the west and finally to the mountains of the Black Hills.  In “those olden days” collecting minerals from outcrops and mine dumps was fairly easy as the land was open or land owners were very accommodating to student collectors.  Today much has changed as land owners are increasingly frightened by liability lawsuits, and unscrupulous collectors (I use that term loosely) have ruined landscapes, left open pits, and swiped vertebrate fossils.  Field collecting is getting difficult. 

As usual, my early fall trip to the Black Hills of South Dakota included a couple of jaunts to the agate beds---locations where collecting is still possible.  There are several postings on this Blog that emphasize the geology of the beds so I will not repeat that information here.  I essentially want to post a few photos to emphasize that the agates are still out there, both in the source beds at Teepee Canyon, and out on the plains where rocks were transported by ancient streams.  Collecting the former requires some large crack hammers, arm strength, gloves and eye protection while collecting on the plains emphasizes “lots of” walking away from the main roads.

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, to spot piles of broken rocks.  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 there are mining claims---I think.  It is best if rockhounds stop in the USFS office in Custer and discuss your plans with one of the friendly employees. 

The agates are encased in chert nodules housed within the lower Minnelusa Formation (Paleozoic: Pennsylvanian).  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”.
Teepee Canyon agate.  Width ~3.1 cm.

Teepee Canyon agate.  Width ~3.9 cm.

Teepee Canyon agate.  Width ~3.2 cm.
Fairburn agates are perhaps the “most famous” agates found in the Great Plains and are valued for their colorful fortification patterns with an abundance of reds (iron oxide), oranges (iron oxide) and blacks (manganese oxides).  There are several localities where agate hunters have collected a variety of stones but the easiest spot for collectors to locate is the “original Fairburn Beds” near the small community of Fairburn, located south along I-90, ~25 miles, of Rapid City near SD 79.  After reaching the community of Fairburn, agate hunters should travel east along French Creek Road (good gravel road) for about 12 miles to a sign locating the original collecting area managed by the Buffalo Gap National Grasslands.  Although known to collectors for decades, these Fairburn beds still yield an occasional agate, and as many colorful specimens of jasper, quartz and chalcedony as can be carried out in your collecting bag.  There are also occasional pieces of petrified wood, and brachiopods replaced by silica.
Fairburn Agate (obverse).  Width bands ~1.1 cm.

Fairburn Agate (reverse).  Width bands ~1.4 cm.
The really interesting story about the Fairburns involves their relationship with the Teepee Canyon Agates described above.  Most geologists now believe that the Minnelusa Formation in the Black Hills is the source of the Fairburn Agates and the siliceous pebbles were transported out to the plains by Tertiary streams draining the Hills.  The agates may be found within conglomerate beds of the Chamberlain Pass Formation and overlying Chadron Formation (both are Eocene in age and part of the White River group). Perhaps agates are most easily observed in the lag gravels covering many outcrops where the finer sediments have eroded away from the Eocene formations leaving behind a veneer of pebbles. 

At any rate, 2016 was again a successful season of locating a few agates, and that was all I asked.  Just hobbling around in the agate fields brought back a flood of pleasant memories from 50 years ago. See below.

I love those random memories that make me smile no matter what is going on in my life right now.  Unknown

I would suggest that if you are interested in Fairburn Agates---pick up a book or three written by Roger Clark, the premier expert on the agates.  Book one, South Dakota's Fairburn Agate, is only available on the used book circuit.  Book number two, Fairburn Agate: Gem of South Dakota, and number three,  Fairburn Agate: South Dakota State Gemstone are in print and published by Silverwind Agates. 

Monday, November 14, 2016

ROCHE MOUTONÈE: EVIDENCE OF GLACIAL MOVEMENT


No stress here.
For the last few years I have taken a late summer trip to a remote lake in Ontario, Canada---to slay the mighty walleye.  Well, not really slay them as most of our fishing is “catch and release.”  We keep a couple of small fish, under16 inches, to coat with Shore Lunch© and throw them in the hot bacon oil for a few minutes.  In another skillet, a can of potatoes is frying away and then come the eggs.  The coffee is hot and black.  It adds up to a day’s worth of fat and cholesterol; however, it is one of those bonding experiences that only comes around once per year.  Included in this experience: catching between 300-400 fish, drinking coffee from a Thermos© jug, plugging along in an aluminum boat, belching without saying excuse me, napping in the sun while waiting for “a bite” and never hearing a vehicle, eating salami sandwiches (and belching some more), having a cool one on the deck, ambling over to the small lodge for a magnificent evening meal including fresh homemade pie (talk about more fat and calories), watching the sun set over the lake with vibrant colors, hearing the loons chatter to each other, and sometimes catching a dazzling display of northern lights.  The quietness of the cool nights is indescribable.

Can't beat the view at sunset.
Nice wildlife including moose.
My bonding partner is a Ph.D. biologist who knows much about plants and animals living in the water.  His favorite trick is to pluck a scale off a large fish, pull out his hand lens (people other than geologists carry these gadgets), count the growth rings, and give me an age for the fish.  Pretty nifty.  I also quiz him about all of the water plants, those things that back in my Kansas home are generally called “moss.”  I have learned much about the aquatic world in my 16 years of bonding.

The geology in this part of the world has two classes of rocks/outcrops: 1) Precambrian igneous and metamorphic rocks; 2) Quaternary glacial crud covering most of the landscape.  However, down in the river valley that holds the natural lake (about 26 miles long) one is able to observe results of glacial scouring.  In fact, the river follows a channel that was “dug out” by the glacier.  It is not a smooth channel as noted by the numerous rocks sticking above the water’s surface and then plunging down 60-80 feet in a short distance.  The best fishing seems to be locating the “humps” where the rocks do not break the surface but where small baitfish congregate and therefore attract large northern pike (40-inch range) and walleye (up to 31 inches). 
Notice the glacial striations or grooves.

The glacier came from "that-a-way" and created this Roche moutonnée.
 At any rate, our conversations in the boat are not always piscatorial but tend towards nature and the environment---what would one expect from a couple of ole academic scientists.  One of my contributions to the discussions has been to explain the formation of a small island in the lake--see photos above. I had been studying the island every time we were near the place since the fishing was exceptional off one end.  And then it hit me—from the back recesses of my mind and my junior-year course in geomorphology I realized the island was a Roche moutonnée.  So, my friend, which way do you think the glacier was moving?  Let me explain how I know the answer (hoping my answer was at least as impressive as the scale aging of fish).  Mosey the boat over by that smooth end of the island and note the smooth but striated surface—but don’t bend the prop in the shallow water.  Now, on the other end where we catch the fish and the water is deeper note the rough surface of the island and the scattered pieces of the bed rock.  The glacier came from “that-a-way” (the shallow end) and smoothed off the knob and left the striations.  However, as it crawled over the knob and begin to descend it plucked off pieces of the bedrock leaving behind the rough surface of the leeward side.  Neat, huh?  Some geologists restrict the term Roche moutonnée to large scale features and term the small features like this island “stoss and lee.”  Personally, I like the French term better.
Cartoon showing formation of a Roche moutonnée or stoss and lee.  Public Domain sketch created by Jasmin Ros.
Cruising up an outlet stream to a kettle lake where a "hump" once produced 42 walleye in one hour.
Unfortunately, that was about the extent of exciting geology around the lake.  We did observe smaller lakes draining into the large lake that were kettles, basins created by large hunks of ice breaking off the main glacier and then melting.  I would have liked him to have observed the Matterhorn, an erosional glacial feature, as I did on a full moon night in Switzerland.  However, I suppose that would have started him on his stories about spending a summer cruising up the Volga River in the USSR studying plankton, or perhaps relaying information about flying (as a pilot) his research crew up to Hudson Bay (and seeing two wolverines as a bonus). 

If readers want to view a large-scale, mountain Roche moutonnée visit one of my favorite Bogs, In the Company of Plants and Rocks, at www.plantsandrocks.blogspot.com (Oct. 5 post).

I cannot even imagine where I would be today were it not for that handful of friends who have given me a heart full of joy. Let's face it, friends make life a lot more fun.          Charles R. Swindoll
I think we have reached the head of the lake!  I am pretty certain that we cannot push the 20 HP motor through those rapids!


Sunday, November 13, 2016

COLLECTING STRIPED ROCKS IN UTAH

Google Earth© image of area around Milford in southwestern Utah.
Several years ago, I had the opportunity to consult on a project involving a pipeline bringing natural gas from southwestern Wyoming to southern California.  My job was to examine outcrops along the Wyoming and Utah portions and make decisions about the need for mitigation to protect fossils.  That distance is a long way to walk; however, I and my crew examined every rock outcrop along the route and consumed many cans of tuna, sardines, and Vienna sausage.  As we traveled along trying to unravel the geology we had the opportunity to notice some great scenery.  One place that still stands out in my mind was exploring the Mineral Mountains near Milford in southwestern Utah.  I was quietly leaning against a tree perched along a very large canyon, eating my can of sardines, and sort of daydreaming about what a great spot I had chosen.  Furthermore, I imagined that Native Americans also had enjoyed the view since it was easy to spot large mammals moving up or down the canyon.  In addition, a geologist exploring a mountain range named Mineral seemed like some sort of a nirvana had been reached.

There are certain half-dreaming moods of mind in which we naturally steal away from noise and glare, and seek some quiet haunt where we may indulge our reveries and build our air castles undisturbed.             Washington Irving

But, times marches on and we needed to get the pipeline route cleared so a call to duty was sounded.  But then we had a serendipitous moment for just on the other side of the tree was a large area covered by black obsidian flakes.  A spectacular site that confirmed our idea that Native Americans also had enjoyed our view! 

Now finding black obsidian in southwestern Utah is not an unusual occurrence; however, these flakes had been worked and were the “leftovers” from the making of projectile points.  We found the flakes since loose sand was been removed from the area by shifting winds.  We examined a few of the flakes and then left them undisturbed in their natural state.  No need to disturb the Gods.

Another event, perhaps as exciting as the chipping area, was noting the power station near Milford generating electricity from geothermal resources.  Neither I, nor my crew, had seen such a creature.  The Roosevelt Hot Springs Geothermal Area was commercially developed in the early 1900s, mostly as bathhouses, hot pools and adjacent hotels.  In 1984 the power plant was brought on-line and uses steam powered turbines created by bringing up hot brines via wells.  The area is perched on top of a fractured and faulted subsurface intrusion allowing circulation of the hot brines. At times, hot water followed a fracture and created a surficial hot spring (and filled a swimming pool).  At other places, the hot water deposited amorphous silica and opal.  One such place we observed was Opal Mound along the Opal Mound Fault.  Opal Mound is essentially a venting area for silica-rich hot water; however, the deposit is rather common opal and non-precious.

I had sort of forgotten about Milford and the opal and moved on to various other projects and interests.  However, a few years ago while touring the Electric Park venue, one of the Tucson shows, I ran into a couple who were selling Utah Lace Opal (ULO).  I am always interested in Utah minerals and therefore had a nice conversation Larry and Joyce Wright, the owners of Aspen Rock and Gem.  It seems the couple had been rockhounding and looking for Opal Mound but found it claimed and so headed out to hunt for obsidian.  What they discovered on their hike was not obsidian, but a banded “hard foamy rock” that could be stabilized and then cut, polished and used as jewelry in cabs or wire wrapped.  The trade name for this rock is Utah Lace Opal. 
Utah Lace Opal, non-stabilized.  Width ~5 cm.
It appears that the ULO was deposited via silica-rich mineralized thermal water migrating along a fracture system that Aspen Rock and Gem calls a silica splinter seam.  This seam goes down about 70 feet and all mining of the ULO is done by hand.  The Company also states the ULO was formed 2000 years ago over a 300-year period. I was unable to verify those dates but presume they have a strong basis for the results.

I purchased a small sample of ULO that was not stabilized but simply cut.  Without stabilization, the ULO has numerous vugs that create the “foamy” appearance.  So, Utah Lace Opal is a proprietary name but what about the description of the opal?

Much opal in the world can trace their origin of silica to marine or fresh water single-celled organisms called diatoms and radiolarians (both have siliceous skeletons).  However, the ULO traces its origins to silica-rich mineralized hot water where the solution captured the silica from rocks in the subsurface.  As these high-temperature fluids reached the surface they experienced rapid cooling and an amorphous silica precipitated as opal (sometimes called silica sinter or geyserite).

Although opal is commonly called a mineral, it is a mineraloid since it varies in chemical composition and is microcrystalline and/or amorphous since the atoms do not occur in a regular structure; however, there is an orderly arrangement of closely packed spheres of silica.  One distinguishing feature of opal is the amount of water in the atomic structure, perhaps up to 20%:  SiO2-nH2O where n represents a variable amount of water. 

The Quartz Page (http://www.quartzpage.de/gen_mod.html) noted that opal is classified as:
  • Opal-C - microcrystalline opals made of cristobalite [ high temperature polymorph of quartz].  Transitional state in the formation of diatomites [rock formed mostly of diatom tests] and radiolarites [rock formed mostly of radiolarian tests] from opaline skeletons. Found in nodular concretions in sediments,
  • Opal-CT - microcrystalline opals made of intergrown cristobalite and tridymite [another high temperature polymorph of quartz].  Found in common opal, as well as a transitional state in the formation of diatomites and radiolarites from opaline skeletons. Found in opaline concretions in sediments
  • Opal-AG - amorphous opals with a gel-like structure. This is the structure of potch [common] opal, precious opal and fire opal.
  • Opal-AN - amorphous opals with a network-like structure.  A common example is hyalite or glass opal.
As best that I can determine, the geothermal solutions rising along faults zones near Milford deposited both common opal and hyalite; however, Aspen Rock and Gem noted “it [ULO] went from one silica phase to another forming Opal-C to Opal-CT to quartz and is seen in various stages and is still maturing.  Vugs with Botryoidal Hyalite Opal clusters form stalactites and stalagmites giving lacey layers to the formation.”   Since neither cristobalite nor tridymite may be identified with ordinary microscopy, I was unable to note Opal-C or Opal-CT in my specimen.  It appears that I have common opal (Opal-AG) and glassy hyalite (Opal-AN).
The vibrant, and distinguishing, bands of ULO are created by common and hyalite opal with various impurities of aluminum (blue-green), magenta (manganese), orange (iron), gray (magnesium), yellow (titanium) (Aspen Rock and Gem).  
Photomicrograph common opal.  Width FOV ~1.2 cm.

Photomicrograph glassy and colorful hyalite opal.  Width FOV ~1.2 cm.
The Mineral Mountains, those of leaning trees great for a nap, are a complex and rugged range.  Structurally the range is a large horst (upthrown block bounded by faults) located in the transition zone between the Basin and Range Physiographic Province to the west and the Colorado Plateau Physiographic Province to the east.  Most of the exposed rocks consist of a intrusive pluton of mid-to late Tertiary age (Mineral Mountain Pluton).  The range is well known for producing gold, silver, copper and lead from contact metamorphic zones associated with Paleozoic rocks and the intrusive rocks.  In addition, the metamorphic heat has cooked a Late Paleozoic limestone and fluids have filled some of the small fractures creating a beautiful rock marketed as Picasso Limestone.  Crosby (2014 in MinDat.com) stated that the carbonate is the Permian age Toroweap Limestone while Stibbett and Nielson (1980) believed the small outcrops need additional study and the carbonate could be as old as the Mississippian Redwall Limestone.
Polished specimen of Picasso Marble so named due to the abstract nature of the veins.  Width ~ 7 cm.
It is tough to locate much confirmable information on the geology and mineralogy of the Picasso Limestone.  A stray BLM report noted that a few tons of the material was blasted and hard sorted by Penny’s Gemstones LLC from the Silver 1-2 and Silver 3-4 mines in the southern part of the Range.  Crosby (2014 in MinDat.com) stated that all outcrops of the Marble are under claim by David L. Penny of Beaver, Utah.  I presume David Penny and Penny’s Gemstone are synonymous.  Some banter on a chat room type of site indicated the Marble is mined out and now only available from the stock at Penny’s shop in Beaver, Utah (http://www.ltagallery.com).  At any rate, Picasso Marble is a beautiful rock and is valued for carving (especially bear fetishes), slabbing, and cabochons since it easily takes a very high polish (note light reflections on photo).  I don’t have the slightest idea about the mineral composition of the cross-cutting veins in the Marble but have seen undocumented chatter about silver sulfide, serpentine and various copper minerals. 

Another popular Utah rock with even less provenance information (at least that I can locate) is Zebra Stone or White Tiger Stone---or you can substitute rock or marble in place of stone!  There seem to be several localities in the world that produce a “zebra stone”; however, most pieces quarried from Utah are labeled as such.  It seems that any rock with black and white stripes from limestone to quartzite to sandstone to granite to gneiss etc. may be termed zebra rock.  I have noticed zebra rock for sale at numerous rock shows and usually it is simply identified as being collected in Utah.
Photomicrograph dolomite "stripe" in Zebra Rock.  Width FOV ~1.6 cm.
Zebra Stone composed of crystalline dolomite.  Width ~10 cm.

 The piece in my collection was purchased several years ago at a rock shop in southern Utah.  When asked about locality data all the proprietor would part with was a claim and quarry west of Sevier Lake in Millard County.  Per the geologic map of Utah, the rocks exposed along the west side of the Lake are Cambrian and Ordovician in age.  However, a stop at the BLM office in Fillmore indicated that zebra stone can be found in Lawson Cove, also in Ordovician rocks but southwest of Sevier Lake in the Wah Wah Mountains. My specimen is composed of both white and dark (blackish) crystalline dolomite. Zebra Stone or White Tiger Stone is a rock with an interesting pattern and certainly catches the attention of collectors and rockhounds. 

My last striped rock from Utah has very good locality data since it collected several decades ago near Vernon, Utah, in the West Desert (Tooele County).   In fact, Vernon Hills Wonderstone is one of Utah’s best known collectable rocks.  It is a welded, glassy, volcanic tuff (air fall ash, rhyolitic in composition) in which the glassy particles were “welded” together by heat and compaction.  In fact, mid-Tertiary welded tuffs are very common in western Utah; however, the Vernon Wonderstone is different from most in that it displays a variety of colorful concentric or layered patterns.  These vibrant colors are due to post-depositional, circulating hydrothermal waters depositing pyrite (mostly) that was later oxidized by rainwater creating hematite, goethite and other iron oxides.  Lapidaries love Vernon Hills rocks since the non-porous nature of the rock, along with a high percentage of silica, creates a suitable hardness for polishing.
Polished Vernon Hills Wonderstone.  Width of polished surface ~5 cm.
Wonderstone is sort of an overused term.  Besides the rhyolitic wonderstone, many types of “picture sandstone” and “picture jasper” have been stuck with the same moniker.  However, I suppose the rhyolitic wonderstones of Utah and western Nevada are the best known.
As far as I know, the Vernon Hills location is on BLM land and is still available for collecting.  In fact, during my last trip the area was covered by literally tons of pieces.  There may be mining claims; however, they are well marked.

I am not a big collector of striped rocks or any of the “picture types;” however, since they were collected in Utah, home of most of my professional work, I picked them up at shows or on the ground. 

REFERENCES CITED 

Stibbett, B.S. and D.L. Nielson, 1980. Geology of the Central Mineral Mountains, Beaver County, Utah: Department of Energy Division of Geothermal Energy Contract DE-AC07-78ET28392, DOE/ET/28392-40.

 As for reaching into the back recesses of my mind and remembering the Mineral Mountains, I am always reminded of our recent Nobel Laureate in Literature:  Take care of all your memories. For you cannot relive them.

Tuesday, November 1, 2016

STROMATOLITES,CYANOBACTERIA AND WORMS



One of my favorite Blog sites, written by master storyteller and botanist Hollis, operates out of Laramie, Wyoming, home of the University of Wyoming.  Although I did not attend graduate school in Laramie, the University employed a fantastic “old-time” vertebrate paleontologist by the name of Paul McGrew.  Dr. McGrew sort of took me under his wing and mentored me through the dissertation process involving Eocene mammals.  I spent several days with him exploring for mammals out in southwestern Wyoming and later returning to Laramie seeking his guidance and help with identification of my collection.  I have always held a special place in my heart for his kindness in accepting and helping a neophyte paleontologist.

But the Blog that I mentioned, In the Company of Plants and Rocks (www.plantsandrocks.blogspot.com), had a September 1, 2016 posting about stromatolites found in the Medicine Bow Mountains of southeastern Wyoming. Cyanobacteria, formally known as blue-green algae, were some of the few organisms inhabiting warm marine waters along the coast line of a proto North America in the Precambrian ~ 2 Ga.  These warm waters were depositing calcium carbonate but also receiving an influx of fine-grained clastic sediments.  As the living Cyanobacteria mats, often growing in the inter-tidal zone, were covered with sediments the organisms reacted by growing upwards and these mats then developed into domelike structures.  Today these structures, known as stromatolites, are preserved in a dolomite (originally limestone) and beds of phyllite and argillite (the original fine-grained sediments subjected to low grade metamorphism).  Readers should check out the Blog posting for some great photographs.
 
Cluster of small stromatolites (top view) collected from Green River Formation, Wyoming. Width FOV ~9 cm.
The Plants and Rocks post reminded me of a stromatolite collected long ago from rocks of the Green River Formation (probably the Laney Member) exposed in southwestern Wyoming.  The interesting “thing” about these small stromatolites is that the Green River Formation was deposited by processes in a large, intermontane basin occupied by a fresh water lake, Lake Gosiute. The stromatolite “maker” was probably a lacustrine alga of uncertain taxonomic affinity but named Chlorellopsis coloniata (Khattak, 2016). Like the Medicine Bow stromatolites, the mats of the Cynobacteria/alga trapped sediments from the water and domal structures were created.  Recent work by Frantz and others (2014) has shown that Green River stromatolites may be used to determine changes in lake volume, water temperature, water depth, and location of shorelines.  Although the Green River stromatolites were originally composed of calcareous minerals, today the calcite has been completely replaced by silica (chert).

Last summer (and in previous summers) I had the opportunity to visit the “Driftless Area” around La Crosse, Wisconsin.  This section of southern Wisconsin along the Mississippi River seemly escaped glaciation by the widespread Quaternary glaciers that otherwise affected the northern quarter of the “lower 48”.  Without the cover of glacial drift (debris left behind), rocks of Cambrian and Ordovician age are well exposed in the Driftless Area.  There are minor outcrops of Precambrian rocks near Black River Falls, but otherwise the basement rocks are in the subsurface.  Wherever Cambrian rocks are found in the stable heartland of North America, geologists call it the Craton, the initial rocks are always sandstone.  Sometime around 600 Ma marine waters begin transgressing onto the Craton leaving behind beach and near-shore sands that later became sandstones.  These were overlain by offshore sands, and in some localities, by deeper water muds.  By around ~500 Ma the shallow sea had reached Wisconsin and a nice section of Cambrian sandstones are well exposed, especially along I-90 leaving La Crosse and heading west across Minnesota.  But, it is interesting to note that the latest Cambrian sandstones indicate the marine waters were regressing from the Craton near La Crosse.  In fact, the shallow marine waters did leave the Craton and an unconformity (missing time in the rock record) exists between the Cambrian sandstones and the overlying Ordovician carbonate rocks.
 
Cambrian sandstone exposed along I-90 near La Crosse, Wisconsin.  Note the numerous vertical burrows of a worm-like animal named Skolithos.
Ordovician marine waters came in with a vengeance, or at least with a rush, and covered much of the North American continent with warm, shallow, carbonate-depositing seas full of soft-bodied marine life.  Evidently the composition of the early Ordovician marine water did not encourage construction of animal shells.  However, the limestones (now turned to dolomite) are full of trace fossils such as tracks and borrows of these soft-bodied animals.  In addition, stromatolites are common in these dolomites and probably represent Cyanobacteria trapping particles of sediments.
 
Quarry in Prairie du Chien dolomite near La Crescent, Minnesota.

Cyanobacteria/alga mats and stromatolites preserved in sandy dolomite, Prairie du Chien group.
 
The Worms Crawl In,
The Worms Crawl Out,
Into your stomach,
And out your mouth.
Building stone quarried from Prairie du Chien Group now used as a door step in La Crosse.  Not that slab is covered by burrows and tracks of soft-bodied animals.

The earliest Ordovician rocks in the La Crosse area are termed the Prairie du Chien Group.  Rocks of this group are commonly very hard dolomite and “hold up” the bluffs so common along the upper stretches of the Mississippi River.  Quarries are common along the River and therefore building stones often expose numerous geological features, especially trace fossils.  Other common features in the Prairie du Chien are concretions, or partial concretions, of silica (chert), and vugs containing calcite crystals.  As I noted, most of the Prairied du Chien is composed of dolomite (magnesium carbonate); however, primary dolomite rarely forms today and so most carbonate petrologists believe the original limestone “turned into’ dolomite by a process known as dolomitization.  Fully understanding this process where some of the calcium ions are replaced by magnesium ions is somewhat above my pay grade.         
Bluffs along the Mississippi River near La Crosse with Prairie du Chien caprock. The bluffs at La Crosse are 600-700 feet above the River.

Small calcite crystals exposed in a vug of dolomitic limestone of the Prairie du Chien group.

Chert nodule in dolomitic limestone.

REFERENCES CITED

Frantz, C.M., F.A. Corsetti, V.A. Petryshyn, M. Wagner and A. Tripati, 2014, Stromatolites as fine records of terrestrial environmental conditions; examples from the Eocene Green River Formation (Wyoming) (abst. PP34A-03): American Geophysical Union December 2014 Abstracts.

Khattak, O., 2016, Calcimicrobes/Cyanobacteria (Blue-green Algae): www.geologylearn.blogspot.com.