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Tiesta

Layers of rocks of the Twin Cities

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Tiesta

Just to show a unique take from my perception. Let's start at the bottom of the floodplain downriver of St. Anthony falls down to lilydale. This is St. Peter sandstone, one of the most obvious rocks along the Mississippi river. It can in certain ravines look like the badlands because of its unusual properties. Seem like it's highly resistant to flowing water. The samples I took is from battle creek park where all the overlying rocks are gone washed away by the old minnesota River when it is much bigger than the Mississippi River after the last glacial retreats leaving behind only St. Peter sandstone. When physically touched fresher ST. Peter sandstone crumbles rapidly into nearly pure silica sands, which makes me wonder why it's not used in glassworks. Who know? Then after exposure to the elements for a while it tends to chemical react and harden into more resistant form which takes more force like hammer or your feet to break in pieces. The samples I took pic of includes the sand form which are bright white and a harden form.

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Tiesta

I don't have any samples of the next layer, glenwood shale. It is listed because it is very distinct to the rock layers above and below in being a bright blue shale. I hadn't found any fossils in glenwood and St. Peter sandstone and both are rep to be very poor in fossils. It also tend to be hidden by debris from the above layers too. Next is platteville dolomite the hardest exposed bedrock in the twin cities. You can see it exposed in more locations than ST. Peter sandstone includes highway cuts near the international airport. Reports says platteville is loaded in fossils but from experiences I only have found two layers that have fossils 2/3 or more of this massive bed is fossil poor. If anyone have any information that there are more fossil layers in let me know. For now I will just refers to figure 4-14 page 85 of minnesota fossils and fossiliferous rocks by Robert e. Sloan. The first layer is very well known and tend to strongly have negative molds of fossils near the top of magnolia one of the hardest and most bulk of platteville formation. Negative mold is when the original fossil largely disappear leaving behind a cavity that often preserves the inner but fragile structures of gastropods and Cephalopods. From what I see, the fossil layers often start with huge amount of strophomenid brachipods of unknown number of species along with a species of inarticulate Brachipod that also appear in low numbers in decorah limestones. There may be gastropods of one species of guessing lophospira mixed in with the brachipods but often appear in moderate to low numbers beneath the Brachipod bed along with molds of small to medium sized cephalopods, worm burrows and very rarely crinoids and horn corals which can be confused with cephalopods due to their poor preservation. Crystals sometimes appear in the cavities and includes rust, quartz like and some kind of small crystal that have copper like coloring.

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Tiesta

The second layer of fossil I found a few years ago by accident while walking a goat trail next to a cliff of platteville dolomite of miffin section I saw some rhynchonellid Brachipod peeking out which was unusual since they doesn't exist in the first layer of fossils. It's called miffin since it is in some way very different from magnolia. The dolomite blocks is much smaller and the space between blocks of dolomite is filled with a shaly mortar like rock. After extracting the fossils from this layer which can be hit and miss at times since it impossible to tell from other miffin layers from the looks of it the layer vary from half a inch thick to three inches thick and is very rich in a wide variety of fossils. The fossils often consists of small colonies of a particular species. In one spot I can find a good number of heads of one species of trilobite then in another spot grapholites can be found. This layer appear to break the rules that dolomitization destroys fossils but it's possible it have more shales mixed in than magnolia that reduces the chance of fossils being destroyed who knows? The fossils that appears in all spots tend to be strophomenid Brachipod, rhynchonellid brachipod, and rugose bryzoans. Heres two samples of platteville fossil rich layers with the top one magnolia and the second one miffin.

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Tiesta

The top one is unusual diverse and includes strop brachipods and a inarticulate Brachipod along with a few tiny bivalves and a crinoid stem. The bottom one had mostly rhybrachipod along with a few bryzoan.

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Tiesta

Next up are decorah shale, the most well known but also the most misinformed rock layers. From my impression, there are a lot of fossil layers in the decorah but it's hard to Id any specific layers. For example one location may have a ton of net bryzoan, another have a lot of clathrospira, trochonema mixed in. But from what I have seen at lilydale there's one layer tens of feet thick that have nothing but decorah shale and is fossil poor. The other layers are fossil rich and vary in the amount of decorah shale, which is very brittle, often breaks in thin sheets. Fresh exposures tend to be like that with the longer it is exposed to air it decompose into blue clay. The clay can resists erosion as it takes a lot of moisture to penetrates it but when it does it become very sticky and can remain that way for several days after a good rain event. When dry the clay tend to be almost literally a rock and peel from fossil easily while wet clay tend to stick.

It also vary in decorah limestone,deposits which are not constant and can vary in thickness like crazy and includes shale in varying amounts. Colors of the limestone can vary include bright red from rust coating to yellow tan for pure limestone to shiny blue from crystallization limestone that often forms the core of the limestone deposits. The good fossils tend to be in the thinner outer limestone which are easier to work with than the much harder crystalization limestone that makes up most of the cores. The crystallized limestone often ruins most fossils and often only brachipods and a few bivalves survive the crystalization process.

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Tiesta

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First one show classic limestone yellow tan but gray blue ones can exists. Have rust coated it from till layer which can contains a lot of iron or from steel pipes. Second picture shows blue clay which is unique in the Twin Cities if you see blue clay from a dig near the river then fossils are not that much further in the ground. The rock in the second picture showed the limestone part chipped away and the shiny very hard crystallized limestone are exposed.

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Tiesta

Next is galena formation. To me the lines between this formation and decorah are more fuzzy. The rocks which can vary between being as soft as decorah or harder have a few characters. In the harder parts, they lose all yellow and become gray blue. Fossils tends to be in scattered layers with some thick layers having no fossils at all. Fisherites often occurs in a certain layer of galena along with more complex trace fossils. Many animals from decorah often cross over into galena. After a certain layer I can't explain what it is like sudden the community shifts. Bryzoans stays the same but brachipods changes with many interesting and beautiful species which wasn't present or are rare in decorah become more common. That includes more inarticulate brachipods like hesperorthis tricaria, petrocrania halli, with unusual strop brachipods like sowerbyella species. Gastropods and crinoids become rarer with gastropods more than two inches long appearing a bit more in frequency than decorah ones. Overall the huge diversity of animals that appear in decorah become greatly reduced in galena formation. Oh yeah there's also one more distinct layer in the galena formation. A thick layer of microfossils that appear to the naked eyes often half a feet thick or less appears as little shiny reddish pebbles are also presents. I believe they are numerous fossils of a species of giant ostracodes which must for a certain period of times was the most successful copepod that feed on plankton. I get the impression that galena was a period of upheaval with certain species becoming very dominant for a short amount of time then disappears.

Edited by Tiesta

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Here's a galena shell hash I found a few days ago. All the classic strophomenid genus that characteristic older rock layers are gone. In this piece I can see majority are sowerbyella species. A few of others includes bryzoan, rhynchonellid brachipods, crinoid, broken trilobite pieces.

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Now I'm thinking someone said something about galena being part of decorah. Is that true? The other thing that I'm been thinking is im now convicted the thick limestone stacks the ones with plenty of holes in them that sit on top of platteville near the east spring waterfall and also the entire east area past the big landslide area. The fossils I found there vary a lot from Fisherites to cephalopods and that's weird. Now I think those rocks are leftover from mining when they remove decorah clay for brick making which means decorah and galena are mixed in together. That means I will have to do some more research which mean galena may be more diverse than I thought.

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Tiesta

Cambrian rock beds are poorly exposed in the twin cities and I hadn't personally found any Cambrian bedrocks. At Stillwater at a restaurant quarried rocks used for soil retainance have Dolomites that contains stromalite fossils from what look like Cambrian origins.

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