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. Ground sloths and other mammals are also known to create middens. Like dung fossils, middens are trace fossils.
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.
Because every bit of the packrat midden is derived from plant matter, isotopic analyses of urine crystals can read the record of ancient rainwaters. In particular, the isotope chlorine-36 in rain and snow is produced in the upper atmosphere by cosmic radiation; thus packrat urine reveals conditions far above the weather.
