How did it become so square?
Gilled fungi growing in crevice between large conifer logs. Gills are dark grayish-brown.
Hemistropharia albocrenulata is a member of Common Gilled Mushrooms and Allies (Agaricales) order.
Mushrooms of the Redwood Coast: Comprehensive Guide to the Fungi of Coastal Northern California, Siegel and Schwarz, 2016 (species not listed).
California Mushrooms: The Comprehensive Identification Guide, D. Desjardin, M. Wood, and F. Stevens, 2nd printing 2019 (species not listed).
Fungi of California: Home page https://www.mykoweb.com/index.html and https://www.mykoweb.com/CAF/ and https://mykoweb.com/CAF/skey.html and https://www.mykoweb.com/CAF/keys/index.html (species not listed).
Growing on a decorticated, well-decayed hardwood log, with a number of other species of fungi. The only 'occurrence' noted. Habitat: secondary hardwood forest (ca. 80-100 years old), with a ± closed-canopy. Observed with GNA Student Intern Ariana Rivellini.
A few days elapsed between the first picture and the third picture.