What are we seeing here? The band of rocks is called "tufa," a porous sedimentary rock (a type of limestone made of calcium carbonate), originally formed within the lake, but exposed in the early 20th century when water was diverted to Los Angeles. Atop the tallest tufa tower is an osprey nest. Ospreys are large birds of prey (raptors) with wingspans up to six feet. One parent (on the right) had been sitting on the nest when the other returned with dinner (it had just landed on the rim of the nest and was steadying itself when I made this photo). Because the lake has a very high salt content, it supports little life other than brine shrimp, so the ospreys must fly to another lake, several miles away, for food. But they choose to nest here on Mono Lake because of the lack of predators.