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Seams of coal are sometimes fifty or more feet thick.
No forest could make such a layer of coal; it is estimated
that it would take a twelve-foot layer of peat deposit to
make a layer of coal one foot thick; and twelve feet of
peat deposit would require plant remains a hundred and
twenty feet high. How tall and thick must a forest be,
then, in order to create a seam of coal not one foot thick
but fifty? The plant remains must be six thousand feet
thick. In some places there must have been fifty to a
hundred successive huge forests, one replacing the other,
since so many seams of coal are formed. But it is further
questionable whether the forests grew one on top of the
other, because a coal bed, undivided on one side, some-
times splits on the other side into numerous beds, with
layers of limestone or other formations between.

http://www.archive.org/stream/earthupheaval010880mbp/earthupheaval010880mbp_djvu.txt

The low sulfur/low BTU sub-bituminous coal beds around Gillette, WY can be 100 feet thick with 100 feet of cover. An examination of the coal layers shows the imprints of leaves and ferns. The text books say the beds were created in a fresh water swamp with succeeding generations of woody plants being deposited, then covered by a variety of geologic forces. The WY coal is soft and flaky and a piece of it left outside will slump and degrade in a month. Not far away, the Hanna Basin in Montana has thinner seams that are harder, higher in BTU, yet also low in sulfur. The theory is the Montana coal was subjected to more compression but both were created in fresh water seas.
The Illinois Basin has a half dozen different varieties and sulfur/BTU levels, some laying over each other with 150 feet separating the two seams. The sulfur content is said to be associated with a brackish sea environment.
In the east, coal can be found deep, close to the surface and close to mountain tops, with as many as five seams stacked on top of one another. Each layer represents a period of time when an inland sea and warm temperatures provided the right environment for the build up of peat. Many seams dip to the southwest or east and some seams roll. obviously affected by tectonics.
In Pennsylvania, anthracite can be found in U shaped beds which are quarried, then mined in drifts from an open cut.
I gave 260 million years as an average age but some coals are younger and some could be very old.