The Upper Mississippian to Permian Nahlin ultramafic body of the Cache Creek Complex, is about 100 kilometres long and up to 8 kilometres wide and is the largest alpine-type ultramafic in the Canadian Cordillera. On the southwest it is typically thrust faulted against Lower Jurassic sedimentary rocks of the Inklin Formation, Laberge Group and Upper Triassic Stuhini Group volcanic rocks, and on the northeast, it is in fault contact against Upper Paleozoic rocks of the Cache Creek Complex. This Upper Paleozoic stratigraphy includes a package of limestone, marble, calcareous sedimentary rocks which are part of the Upper Mississippian to Permian Horsefeed Formation of the Cache Creek Complex.

The best exposures for this limestone package are in the northeast corner of the Tulsequah map area where fine-grained, thick-bedded limestone outcrops in a broad west-northwest trending belt parallel with the southern edge of the Atlin Horst. This belt is an extension of a major limestone unit mapped in the Atlin map area to the north (GSC Memoir 307). It occupies the core of a greatly attenuated antiform that plunges gently toUards the southeast and is overturned slightly toward the southwest. The base of the limestone is not exposed, but a partial section measured on a ridge south of Victoria Lake give a minimum thickness of about 760 metres.

Well preserved specimens of the Late Permian fusulinid "Yabeina" are abundant within the Upper sections of the limestone. This suggests that the top of the Permian section coincides approximately with the top of the limestone, and that the overlying chert and slates are Permo-Triassic in age.

In 1975, these limestones were mapped as Permo-Pennsylvanian limestones and dolomitic limestones belonging to the Horsefeed Formation (GSC Paper 47-74), and the overlying chert and pelites belong to the Permo-Pennsylvanian Kedahda Formation (Cache Creek Complex).