Various species of packrats live in the world's deserts, relying on plant matter for their entire intake of water as well as food. They gather vegetation in their dens, sprinkling the stack with their thick, concentrated urine. Over the centuries these packrat middens accumulate into rock-hard blocks, and when the climate changes the site is abandoned.

Packrat middens are found in the Great Basin, of Nevada and adjoining states, that are tens of thousands of years old. They are examples of pristine preservation, precious records of everything that local packrats found interesting in the late Pleistocene, which in turn tells us much about the climate and ecosystem in places where little else remains from those times.