Is Lower Cambrian formation named for Tyler Peak at head of South Creek; type section on "southwest-facing slope north of an intermittent tributary of South Creek on the west side of the Lemhi Range" in NE 1/4, Sec 20, T7N, R29E, Butte Co, ID (Snake River basin). Unconformably overlain by Lower Ordovician Summerhouse Formation (revised); conformable to gradational contact with Lower Cambrian and Late Proterozoic(?) Wilbert Formation (below). Sandstone dominates upper part; yellowish brown to dusky purple, calcareous, locally limonitic, very fine- to fine-grained, commonly medium bedded, locally flaggy, common bioturbations and local cross-laminations; locally 1.3-m-thick moderate to dark-reddish-brown dolostone bed at base of sandstone, above quartzite. Quartzite, pure, vitreous, light-gray to grayish pink, fine- to coarse-grained, medium to thick bedded, cross-laminated. Fissile shale at base is greenish gray to light gray, iridescent sheen, interbedded locally with sandy quartzite and siltstone. Is 267 m thick at type; is truncated northwestward from top down by pre-Ordovician erosion; is absent in southern Beaverhead Mountains. Shallow regressive marine deposit. Strata were earlier part of Swauger Quartzite (Ross, 1961); later in lower part of Summerhouse Formation (Ruppel and others, 1975). Early Cambrian age from trilobite fragments.

Source: GNU records (USGS DDS-6; Denver GNULEX).