This fine grained sandstone is surprisingly an arkose, which means that a large percentage of the sand grains are feldspar. It is a bright and clean sandstone, not the coarse grained rusty arkose we visualize as typical. The flat cleavage surfaces of the feldspar grains are clearly visible under 20x magnification. Under a 10x hand lens, these sparkle when the specimens are tilted back and forth. This is a useful example of a fine-grained sandstone, and most students will see it as that. An advanced student of geology might pick it out as an arkose. It is Upper Miocene in age, from the Cajon Valley Formation, terrestrial in origin. The cement is silica, so there is no effervescence with a drop of hydrochloric acid.
The sand grains in these specimens are larger than those of a silt and will feel and look like fine sand to a student. It would be instructive to compare this sandstone with one is larger grained, with one that is cemented with calcium carbonate, and with different colored sandstones, as sandstone has great variety and a student should not see only one example.
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