The Thumb, a few hundred meters north of the Fly Ranch Geyser, Washoe County. This travertine mound continues to be built by water leaking from a well drilled in 1916. The red and green colors are caused by thermophilic algae. (Information from Road Log for Field Trip to San Emidio Desert, Gerlach, Fly Ranch Geyser and Orient Farms, by Dennis T. Trexler and Michael B. Stewart, High Rock Holding LLC.)