How do plants affect rock?

Plant material that falls into water that is very low in oxygen, and is covered by additional layers of plant material and silt, will only partially decay. If these mats of partially decayed plant material are subsequently covered by additional layers of sediments, they can eventually be squeezed and compacted by the weight of the overburden, removing most of the water that was present, and be cemented together by minerals that have precipitated out of the disappearing fluids.