Big brick is a metaphor. Not compressed, but solid. Essentially enough plant matter that you can't walk through it, plant matter that is "solid" from ground to the tree branches.
I've seen unburned forests where that undergrowth is more dense and taller. The more time, the more that underbrush thickens and grows. Eventually there is an old growth forest and that stuff is shaded out. AFAIK that process plays out o er centuries. We're typically in the first century of growth for west coast forests (few old growth forests remain)
as an avid outdoorsman, please explain this. I’ve never seen a “brick of plant matter”.
I have seen overgrown shrubs and grass, but I’ve never seen the forest form a compressed brick of plant matter.
Also if plants are living they have water in them.