Layers of gravel, sand, and clay were laid down by streams. These layers cover much older marine sedimentary rocks rich in petroleum oil. Movements in the earth's crust have folded and cracked the older rocks. Heat and pressure force the oil up slowly through the cracks and onto the surface of the ground. Shallow pools of sticky asphalt formed in low-lying areas such as stream beds.
Oil continues to seep up to the surface as it has for hundreds of thousands of years.