[From many - one.]

Simple answer.

Shale splits easily and plants insert roots and other root-like structures into the splits. Also green algae can get in and goo-ify

I'm pretty good on plants, both live and fossil. If you have and can post pictures or id info I can probably help explain.

Generally, many plants of the Carboniferous era do resemble to some degree many modern species. This is especially true of fern-y looking stuff.

[dasboot]

Thanks. Prolly explains what I found. That some split so handily at the impression-bearing layer is probably good evidence it had been infiltrated by moisture and microbes.