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Geology/Interpretation of geological strata

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I have a fascination with what rock formations in our area tell us about what was actually happening all those ages ago. I have attached photos of typical formations and wonder if you can interpret them to say what happened and in what sequence e.g. sediment was laid down by a river, then much later the sea level rose (we are close to the ocean), followed by a distortion and tilting of the surface etc. Thank you.

Answer
Vernon:

It is kind of difficult looking at the pictures, to determine exactly what the lithology type the rocks are.  Not being able to walk the outcrop also limits what I can surmise, but I will make a stab at it. Not having a scale ini the picture also causes some limitations.

The first photo looks like a somewhat thick bed of sand stone or silt stone..  The upper portion of the picture looks to be more silty than the lower based on color, but it is hard to say from the picture. It might be that they are the same rock, but I can't tell unless I saw a freshly cracked surface and matched it to the lower freshly cut section.  The lighter inner portion of the rock looks like sandstone and the red to pink staining is iron that has leached into the porosity of the rock, which is another tell tale that it is sandstone.  The rock is highly jointed and fractured.  I see a hint of deformation in the bedding, by that it looks as if the rock is bent a bit.  Without taking a strike and dip measurement it is difficult to tell if the road cut is perpendicular to a fold, an anticlinal fold, or the fold in plunging at an angle to the exposure cut.  That is about all I can tell about the first picture.

You comment that it was on a hill does tell me one thing.  In the Appalachians, sandstones form the ridges.  Think of it this way.  You have a square layer cake made up of thick layers of cake and thinner layers of icing.  You cut it in half and turn one half on its side and begin to eat the cake from between the icing.  That is what the Shenendoah Valley and Blue Ridge mountains represent.  The valleys were the areas of cake which were limestone, eaten away by erosion by solution forming wide valleys separated by narrow ridges, the icing, which when hardened represents the sandstone layers that were more resistant to erosion and formed th ridges.  Originally the layers of horizontal lying sandstone and limestones were layed down offshore. Limestones in deep water, sandstones in shallower water.  The layers formed overtop of each other as sealevel rose and deep water moved inland, and then receeded, allowing sands and silts to be deposited over rocks that had formed in deeper water that had moved farther off shore. This rise and fall cycles are called transgressive and regressive sequences when seen in the stacking of the rocks itself.

Now the second picture.  This one is more interesting.  The float material obscures a bit of the detail, but the lower secton looks to be dipping to the left.  This could indicate one of several things.  Either a fault, or we have the road cut cutting the dipping beds at an angle taht makes the exposure look lke a fault.  We could be looking at an angular slice of a dipping limb of a fold which gives the impression of a fault.  Another thing that the center of the photo looks like is an unconformity.  An unconformity is where previously dipping beds are eroded and then overlain by horizontal beds during another depositional event.  What you end up with are one section of dipping or vertical beds overlain by horizontal beds in a "T" like arrangement but at different angles.  Tha is what it looks like here, but due to the similarity of the rocks, I am leaning to my first impression, that of the road cut giving the illusion of a fault or unconfomity when actually the beds are still in their original although somewhat deformed orientation.  Taking dip measurements along the exposure would allow the puzzle to be unraveled, but you need a brunton compass and a little knowledge of field mapping and working out graphical geometry problems.

If you are really interested in knowing more about the exposure, you might be able to view the geologic map for the area and see exactly how the original structure was mapped by the surveynig field geologists.

If you live in the US you can go on line and determine the name of the map quadrangle in which the outcrop lies.  Then go to your state geologic survey website and query the quad and see what survey publication report covers that area.  These reports might be available on line or for a small fee.  They will have associated detailed maps.  Another source is the "Roadside Geology Guidebooks" for some states.  They discuss the geology you can see in roadcuts along the roads in the states and tell you where you can go to see features or "type" sections that define rock sequences laid down during the different geologic periods i your area.

My wife is from Acton, Calif.  And I went to see a roadcut exposure of the San Andreas Fault outside of Palmdale.  The traffic was pretty busy but it was worth it.

I hope this was informational. Feel free to ask followups if some points were not clear.  

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Keith Patton

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I can answer questions concerning physical and historical geology, environmental geology/hydrology, environmental consulting, remote sensing/aerial photo interpretation, G&G computer applications, petroleum exploration, drilling, geochemistry, geochemical and microbiological prospecting, 3D reservoir modeling, computer mapping and drilling.I am not a geophysicist.

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I have 24 years experience split between the petroleum and environmental industries. I have served as an expert witness in remote sensing, developmental geologist, exploration geologist, enviromental project manager, and subject matter expert in geology and geophysical software development.

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American Association of Petroleum Geologists
American Association of Photogrammetrists and Remote Sensing

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Bachelor and Master of Science
Registered Geologist in State of Texas

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