The black, thick deposits that we often see on the beach and floating on the ocean is a natural seepage from the ocean bottom called "tar". Tar, or "rock oil," is a type of petroleum. It has been around for thousands of years and has many uses. According to petroleum geologists, natural seeps can benefit the environment since oil is a natural organic substance and is food to some types of organisms.
Tar is only found in certain types of rocks, like shale, sandstone and limestone. Most of these rocks are located on the bottom of the ocean. These rocks are called porous rocks because they have many small holes in them. Porous rocks will "seep out" petroleum when there is movement in the earth or shifting of ocean floor sediments. Since petroleum is lighter than water, it rises to the top of the ocean where currents, tides and wind carry it to shore. Geologists believe tar seeps have been leaking natural tar into the environment for thousands of years.