There are times in every geologist’s life where we start to resent the “cover” that plants and soil represent—when important contacts are obscured. When there are no rocks to be seen for many miles around. On the other hand, sometimes plants are a useful clue. I remember when I was an undergraduate student on a field trip our teacher taught us to recognize his favourite plant, Aspidotis densa, a fern which likes magnesium in its diet, and so tends to grow on serpentine rich soils. He worked in southern Oregon (and northern California), an area which often has serpentine located within fault zones. In fact he said that many geologists he knows working in that area are inclined to put a fault on their map when doing field work if the only clue they see is a single outcrop of serpentine. By extension, in areas with heavy vegetation and no rock outcrop whatsoever, if they see aspidotis densa they make a note of it, because it could mean there is serpentine present, and therefore this could be a fault zone, and they look for other clues (is there a spring that comes to the surface nearby also?).
It has been many years since I was an undergrad, I haven’t done mapping in that part of the world for a very long time, yet I had never forgotten the name of that plant, and, because I learned about it when I was young and impressionable, I always remember to ask local geologists when I come to a new area if there are any important local plants I ought to be able to recognize because they only grow on a specific soil type, and thus give clues to the geology they cover.