Mississippi River at Winona

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A trip through a road cut on County Road 15 near Homer from the river valley to the top of the bluffs was led by Dr. Summa from the Geology Department at Winona State University. Check out the WSU website for additional geology information, especially some of their own field trips. She described the different rock formations emphasizing the age process that says layers of sedimentary rock should be deposited in a nearly horizontal layer with each layer closer to the surface being younger than the layer below (since it was deposited later).

The lowest exposed rock is the Franconia Formation from the Upper (later) Cambrian Age. The Franconia Formation is made of very fine grained sandstone that is interbedded with green-gray shale. The fine sand with the shale indicates a far shore water environment for the formation because the larger particles settle out near the shore where the water velocity is higher. The smaller particles settle out further from shore where the water velocity is lower. Above the Franconia Formation (younger) is the St. Lawerence Formation, still part of the Upper Cambrian Age rocks. The St. Lawerence Formation is siltstone with large concentrations of dolomite. Both of these formations have variations in composition form sand to silt mixed with carbonate (limestone and dolostone) particles. The variations in thickness of the layers along with the variations in composition indicate an environmental change that caused the variations in the deposition. The best explanation we have for these variations is a change in sea level.

Here we have a ledge of more weather resistant carbonate rocks with the less weather resistant sandstone and siltstone below it. The softer sandstone and siltstone are easily moved by rain water and wind where the harder carbonate rocks don't break apart. This causes a ledge overhang. If a river was running across a formation like this a waterfall would be the result.

The upward arching pattern of the rock layers (especially below right) tells us that the deposition was taking place on a shallow shelf where wave action could affect the sediment at the bottom of the sea. The wave action would move the sediment back and forth causing the large ripple effect just like waves on smaller lakes cause small ripples in the sand and mud at the shore.



	Above the St. Lawerence Formation is the Jordan Sandstone (still part of the Upper Cambrian age). The sandstone contains quartz and feldspar, the result of weathering of granite. The small sand particles are well rounded indicating a long transport causing the smooth particles. The bedding of the sandstone is highly variable from troughs cause by wave action as in the above right photo to to planar (basically level sheets) in the above left photos and those below. In these photos (left and below) Dr. Summa is showing us the cross stratification of the rock that is characteristic of currents in one direction that would be the result of a river delta or possibly tides. In the photos below notice that the bedding forms a kind of sideways V shape where the bottom is sloping upward to the left but the top is nearly level. This is an unconformity and is evidence of erosion. Layers at an angle were shaved (bulldozed some would say) off before the next layer was deposited.
	As sediments are first deposited on the beach the bottom of the rock formation is put into place. As more sediments are placed on top over millions of years the sediments are packed and water carrying minerals slowly trickles down through the sand. Minerals, in this case carbonates, are deposited and help to hold the sand particles together. In order for the minerals to become sedimentary rock it needs to be buried very deep, at least a kilometer (about 3300 feet). Then, in order for us to see it at or near the surface something has to change to lift it up and take off the sediment at the top. Plate tectonics causing an uplifting generally causes things to tilt as well because not everything gets lifted the same amount. Imagine picking up a heavy board, how likely is it that the board that was level stays level as you lift it? As it lifts the rock will bend and fracture. These fractures may form joints that allow water to flow through. As we saw before these joints can be the start of caves or sinkholes.

Joints (like in the photo below right) allow water to sink to lower levels where it will saturate the rock. When the rock is saturated with water and the water stays for a long time the color changes (notice the color change in the layers of the rock below and above) because minerals in the water precipitate out. This is called Liesegang banding. This relates directly to oil fields and the identification of good location for oil wells as well as locations for water wells in different geological conditions. These are places where fluids fall along the joints and then pool in layers, saturating the rock.

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