How did earths coal deposits form?

Todays coal was formed millions of years ago. Sum of the earliest coal deposits formed only about 1 million years ago whereas the old deposits are from 300 million years ago.

Coal is formed where plant life has died and then eventually sink to the bottom of swampy areas or the water level has risen and covered the dead plant life. Over time as the amount of dead plant life accumulates at the bottom of the swampy area it turns into a dense soggy material known as peat then as the land changed and the pressure upon the peat built due to deeper burial, sandstone and sedimentary rocks that may have formed due to the changing land, the water in the peat is squeezed out and when heat is added from the earths core, the peat is turned into coal.