This seepage has been happening for tens of thousands of years, during which the asphalt sometimes formed a deposit thick enough to trap animals. The deposit would become covered over with water, dust, or leaves. Animals would wander in, become trapped, and die. Predators would enter to eat the trapped animals and would also become stuck. As the bones of a dead animal sink, the asphalt soaks into them, turning them dark-brown or black in color. Lighter fractions of petroleum evaporate from the asphalt, leaving a more solid substance, which then encases the bones. Dramatic fossils of large mammals have been extricated from the tar, but the asphalt also preserves microfossils: wood and plant remnants, rodent bones, insects, mollusks, dust, seeds, leaves, and even pollen grains. Examples of some of these are on display in the George C. Page Museum. Radiometric dating of preserved wood and bones has given an age of 38,000 years for the oldest known material from the La Brea seeps. The pits still ensnare organisms today, so most of the pits are fenced to protect humans and animals.