The geologic column of the nearby Taylorsville region shows the Hosselkus Limestone as 140 ft thick and of Late Triassic (early Karnian) age.

It has been recognized at numerous outcrops between Spanish Ranch and Prattville and northwestward beyond Pit River in the Klamath Mountains, and is considered to be younger than the Swearinger slates and older than the Trail beds.

It contains numerous Arcestes and abundant pentagonal crinoid stems which indicate it is of Late Triassic age.

[2] Over 208 described species of invertebrates have been found in the Hosselkus Limestone and nearby Brock Mountain.

The upper subzone carries a few survivors of the Tropites group, Juvavites, Gonionotites, Metasibrites, and Arcestes.