During the later Carboniferous some areas of the land alternately sank and rose, giving rise to large lakes and swamps. Gradually layer upon layer of this matted organic matter was compressed in the water and formed that substance we call peat, the first stages in coal formation. From time to time great areas of these peat-filled swamps sank just below the level of nearby seas. Time and again the land subsided, then rose above the sea again. Each time, old forests died and new ones took their place. With each lowering of the land, sea waters flowed over the peat, covering it with new sediment deposits often containing fossil remains. So today we find lawyers of coal alternativing with layers of sedimentary rock. About half of the word's workable coal was formed during the Pennslyvanian, mainly from the giant scale trees.