Add the "-t" option to geom(8) utility, to display geoms hierarchy.
Sample output:
% geom -t Geom Class Provider da0 DISK da0 da0 PART da0s1 da0s1 PART da0s1a ffs.da0s1a VFS da0s1a DEV da0s1 DEV da0 DEV da1 DISK da1 swap SWAP da1 DEV cd0 DISK cd0 cd0 DEV