I look at sidewalks more closely than most people. It has little to do with geology, though—it’s just part of my hobby. But sidewalks do have geological lessons in them. Consider this sidewalk on the north side of upper Broadway near Lake Temescal.
It has a series of light-colored wedges along the fence, on the uphill side. As you might guess, they reflect the pattern of the rainfall and its drainage. Fresh concrete is poured and then caressed with trowels as it begins to cure. This treatment pushes the larger particles of aggregate under the surface and creates a smooth finished surface composed of fine particles and cement. But the surface tends to weather off, partly because the surface is hard to keep moist for proper curing. The imperfectly cured material is prone to attack by rainwater, which is a mild acid. As rain and wind work into the surface, green things both microscopic and visible colonize the rough spots. These turn up the acid attack with exceptionally high CO2 levels similar to those in soils, along with humic acids produced by plant tissues. With time, the crisp white walkway turns rough, variegated—and beautiful, akin to natural stone.
18 October 2011 at 6:28 pm
Very cool. I might have noticed something like that, but never had any idea what had caused it.
19 October 2011 at 7:02 am
Andrew,
Nifty post. Always pleasant to learn something new in the world of urban geology.
David
19 October 2011 at 9:07 am
Interesting. Thank you.