Urban geology

Most people are familiar with rocks being brown or gray or tan, or sometimes red or orange. But, when rocks are blue or purple or green, it indicates something more unique. For a tourist, these colors indicate “scenery”. To a geologist, the more exotic colors often indicate a rare or valuable element, and a geologically more interesting environment of formation.

During frequent bike rides along the canal, I have noticed a “purple streak” in a small bedrock outcrop. The outcrop is behind two fences, and on the other side of a drainage ditch from the bike path, and I hadn’t bothered to take a closer look until yesterday. The bedrock is metamorphic rock of the same type that make up the Phoenix Mountain preserve. That rock is many colors, but I had never seen purple before.

Upon closer inspection, the source of the purple turned out not to be some rare or valuable mineral, but instead is something ordinary – spray paint. Yep, taggers. The tag had been nearly pressure washed into oblivion, but the hint of purple remained. Some kid nearly fooled me! I guess that is payback for that time I threw a chunk of asphalt into a rock identification quiz in order to fool my students.

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