Neal Immega - What's this rock doing with red and green spots?

This is a fine grained quartz sandstone from a shallow creek that flowed during Permian times at what is now the Craddock Ranch. Notice that the rock is made up of layers that dip to the right. These are called forsets and are the leading edge of a ripple in the sand at the bottom of the creek. Moving water transports sand in the shape of a ripple. If you look closely, you will see that there are several of these angled layers and each layer truncates the previous one because the moving ripples tend to erode some or all of the previous one.

The red and green color comes from iron in different oxidization states. If you could examine the grains with an electron microscope, you would see that the quartz is coated with a layer of iron-rich clay. I think that the original color of the rock was red like all the other sediments in these redbeds. Rivers also transport organic material, like plant debris, along with the sand. Bacterial decomposition of the organic materials will also reduce the iron, changing the color of the iron from red to green. The green spots come from decomposition of isolated bits of organic material.