From the Mono Lake Tufa State Natural Reserve web page:
"Mono Lake is a majestic body of water covering about 65 square miles. It is an ancient lake at over 1 million years old and one of the oldest lakes in North America. It has no outlet. Throughout the lake's existence, salts and minerals have washed into the lake from Easter Sierra streams. Fresh water evaporating from the lake each year has left the salts and minerals behind so that the lake is now about 2 1/2 times as salty as the ocean and very alkaline. The Mono Lake Tufa State Natural Reserve was established to protect the spectacular "tufa towers", calcium-carbonate spires and knobs formed by interaction of fresh water springs and alkaline lake water."