Silurian and Devonian rocks are exposed for approximately 8 square kilometers inthe KootenayArc structural province of northeastem Washington. These Middle Paleozoic rocks overlie the Upper Cambrian Metaline Limestone and the Ordovician Ledbetter Formation, which includes slates, siltstones, and limestones. Two Silurian units crop out in this area. Unit A is composed of Lower Silurian graptolitic argillites and siltstones and Upper Silurian siltstones and fossiliferous limestone. These lithologies represent a trend towards shallower water conditions than those operative during deposition of the Ledbetter Formation. Above Unit A lies the quartz granule conglomerate Unit. This unit consists of a silica rich granule conglomerate with associated beds of slate and is interpreted as an inner submarine fan deposit. Devonian rocks are found locally at Limestone Hill. The polymictic Givetian limestone conglomerate crops out at the base of the bill. This conglomerate contains limestone clasts of Lower and lower Middle Devonian rocks, thereby documenting a break in the sedimentary record. The Givetian limestone occurs as float fragments of dark-gray limestone, which preserve a diverse conodont fauna. On the crest of Limestone Hill lies the Frasnian massive and bedded lime stones, which are believed to represent a reef build-up. The post-Paleozoic sedimentary record consists of Quaternary glacial deposits. An olivine trachybasalt of probable Tertiary age crops out for a short distance in the central portion of the map area. The region has undergone several periods of deformation of probable Mesozoic age. The structure is dominated by northeast and northerly trending folds, and northeast and northerly trending steeply dipping normal faults.