Seems like an odd name for somewhere in a National Park. Especially as there has never been any coal mining in the valley.
The answer is simpler: “Forest Coal” is another name for what we now call charcoal. Before the discovery that coal could be roasted to make coke for iron-making, this area was heavily forested with Alders, which make excellent charcoal. The valley – and our farm – still has lots of charcoal platforms. These are about the size of a full-sized snooker table, and are just flat bits of land created on slopes, where the charcoal could be made, near to where the trees were felled.
We have a couple of them which still have a layer of charcoal beneath the soil, even though the practice died out in the 1700’s. See if you can spot them.