Pressing the peat down further in the earth gradually changed the material to coal. The pressure from sedimentary rocks squeezed the water from the peat. Sandstone and other rocks called sedimentary ( SAID-EH-MEN-TARY) rocks were formed, and the pressure caused by their weight squeezed water from the peat just like your hand might press on a wet sponge and force the water out. Seas and great rivers caused sand, clay and other minerals to land on top of the peat, burying it further and further into the ground. Over long periods of time, the makeup of the earth's surface kept changing for many reasons. As plants and trees died, their remains sank to the bottom of the swampy areas, making layers and layers of plant material and eventually forming a soggy, thick material called PEAT. Most of the coal we use now was formed about 300 million years ago, when much of the earth was covered by steamy swamps.
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