Found in a waterfall in a small perennial stream coming from a nearby spring. Soil not calcareous or serpentine, but is formed from basic igneous and metamorphic rocks (Sobrante series), so more similar to gabbro, which is a serpentine "relative". There is serpentine relatively nearby, and I've observed serpentine endemics in this area.

Jepson eFlora says that this broadly distributed species is an obligate calciphile but that many of the California occurrences are from rapidly flowing and shallow streams on serpentine. It also states that this is the only California Fissidens that has such large and thick leaves that are sometimes almost black (California Moss eFlora page - https://ucjeps.berkeley.edu/cgi-bin/get_moss_treatment.pl?taxon=Fissidens%20grandifrons).