These are not fossilized plants; they are made by arborescent structures made of manganese oxide microcrystals (sometimes with a little iron oxide). These arborescent structures are called “dendrites”. The crystalline arborescent structures mainly develop after diagenesis. They are caused by mineralized fluids (H2O) flowing through fractures and along interbed surfaces. The dendrites are generally made up of manganese oxides (MnO2 = pyrolusite, also called psilomelane) combined with some iron oxide.