|In winter and
spring, San Francisco's sidewalks can become green, but not with envy.
Depending on the mineral content of the concrete and nearby pavement and
even buildings, sidewalks can be a breeding ground for algae and moss.
During rainy months, long stretches of sidewalk are rife with the stuff.
This phenomenon often occurs on bricks with high iron content and around
brick buildings. Nobody seems too troubled by the presence of Emerald
Sidewalks. To some, in fact, the are a familiar detail and a reminder
of the changing seasons in the heart of the City.