Asphalt Rock – Asphalt is a solid or nearly solid organic substance composed of carbon and hydrogen, which is formed when the lighter parts of petroleum evaporate and the heavy tarry residue remains behind. Natural asphalt, that is made by nature and not in an oil refinery, is found in the pores of some limestone and sandstone. It occurs in rocks of the Pennsylvanian Age and in small amounts in cretaceous sandstone. Since many porous sandstones and limestones do not have asphalt in them, particularly where the rocks outcrop and weathering has a chance to act on them, much exploration must be done in order to find a good deposit of asphalt rock. This is usually done by digging small pits of quarries, or by drilling shallow test holes.