Natural History: Determining the Age of Rocks and Fossils

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Part A Most likely environments for sedimentary formation Limestone containing ammonites Likely from the cretaceous, formed with the transgression and regression of ocean waters, possibly 70-100 million years ago at which time the sea transgressed and regressed a number of times. The presence of the ammonites suggests that the formation was from the upper Cretaceous period. Limestone is a sedimentary rock, composed largely of calcite and aragonite, which are both different forms of calcium carbonate. It makes up about 10% of the total of sedimentary rocks. It is composed of grains of skeletal fragments of marine organisms, pressed together over time with variable amounts of silica. Sandstone Classic sedimentary rock composed mainly of sand-sized mineral or rock grains (quartz or feldspar). Usually porous and are valuable aquifers and petroleum reservoirs. Red color suggests iron oxide staining, which helps cement many of the grains. Likely formed in an area in which there were numerous iron deposits, very common in what is now the American Southwest. Coal Bed Typically formed about 300 million years ago, flooding of dense forests and low-lying wetlands caused vegetation to be buried and over time, compressed. In combination with heat and protection from oxidation by more mud or acidic water the vegetation was converted to coal. Sandstone with grain-size histogram See above on Sandstone; but likely instead of iron oxide coloring the material, these were likely formed as
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