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Conditions conducive to the formation of fossils include quick burial in moist sediment or other material that tends to prevent weathering and to exclude oxygen and bacteria, thereby preventing decay. Shells and bones embedded in sediment in past geologic time, under conditions suitable for preservation, left exact reproductions of both external and internal structures. Skeletal remains have been preserved as a result of the engulfment of an animal's body in ancient asphalt pits, bogs, and quicksand. At Rancho La Brea, near Los Angeles, Calif., asphalt deposits have yielded a rich variety of skeletons of birds and mammals. Some fossils have been found buried in volcanic ash; such fossil deposits exist in the Cenozoic rocks of the W United States.
The Creation of Natural Molds
Sometimes, after specimens were enclosed in the rock formed from the hardened sediments, water percolating through the ground dissolved out the remains, leaving a cavity within which only the form was preserved. This is known as a natural mold. When such molds are discovered by fossil hunters, casts can be made from them by filling them with plastic materials. If molds have been filled with mineral matter by subsurface water, natural casts are formed. Molds of insects that lived many millions of years ago are sometimes found preserved in amber. These were formed by the enveloping and permeation of an insect by sticky pine tree resin which hardened to become amber. So perfectly formed are these molds that detailed minsect's minute structure. Molds of thin objects such as leaves are usually known as imprints.
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