"Schlieren" is a geological term for thin layers of dark minerals found within a granite. Geologists can't agree on how they form. This side of Enchanted Rock is a great place to see them. The schlieren in this picture are the dark lines that run from the back right to the front left. Individual schlieren can be traced for tens of meters. The more diffuse dark lines that run from the back left to the front right are lichen streaks that result from differential flow of water off the rock when it rains. Thin exfoliation sheets are present along the left side of the image.
Dr. Joe Reese for scale.

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