By which process are fossil fuels formed?

There are three major forms of fossil fuels and these are coal, oil and natural gas. All three were formed many millions of years ago before the time of dinosaurs. Plants and trees died and sank to the bottom of swamps which filled the land. The dead matter formed layers of a spongy material called peat. Over time the peat was covered by sand, clay and other minerals which turned into sedimentary rock. As the layers of rock piled up, their weight squeezed the peat until water came out of it and it eventually turned into coal, oil and natural gas.

The answer is observations and interpreting.