The Lemhi arch was a northwest-trending landmass in central Idaho during late Proterozoic and early to middle Paleozoic time. The arch first formed in late middle Proterozoic or early late Proterozoic time, when middle Proterozoic miogeosynclinal sedimentary rocks were arched into an elongate dome. It was deeply eroded in late Proterozoic time, and as much as 4500 m of clastic rocks were stripped away. The eroded edges of the middle Proterozoic rocks on the west flank of the arch were partly covered in late Proterozoic(?) and Cambrian time by the onlapping Wilbert Formation, but sedimentation apparently did not continue into later Cambrian time. On the east flank of the arch, westward-thinning marine sedimentation began with deposition of the Middle Cambrian Flathead Form tion and continued through Late Cambrian time.

During Ordovician and Silurian time the east flank of the arch was dry. The west flank was partly submerged in Early Ordovician time, and the onlapping nearshore clastic and carbonate rocks of the Summerhouse Formation were deposited. The Wilbert and Summerhouse formations are successively overlain by the eastward-thinning marine rocks of the Kinnikinic Quartzite (Middle Ordovician) and the Saturday Mountain Formation (Late Ordovician and younger). The west flank of the arch was briefly exposed to erosion after deposition of the Saturday Mountain Formation, but it was again partly submerged in Middle and Late Silurian time when the eastward-thinning Laketown Dolomite was deposited.

Both flanks of the arch were exposed in Early Devonian time, but in Middle Devonian time deposition was renewed on the west flank, as fresh and brackish water sandstone was deposited in channels cut deeply into the Ordovician rocks. Later Middle and Late Devonian age rocks of the Jefferson Formation on the west flank of the arch indicate eastward transgression, followed by Late Devonian marine sedimentation. The east flank of the arch was exposed until late in the Devonian, when a thin sequence of marine carbonate rocks of the Jefferson and Three Forks formations was deposited across the top of the arch and marine sedimentation in this region was continuous from the miogeosyncline far onto the craton. The Lemhi arch continued to influence marine deposition even after it was submerged, separating the region of shelf deposition in southwestern Montana and east-central Idaho from the region of miogeosynclinal deposition in central Idaho. The arch was a landmass again through much of Mesozoic time and was overridden by the Medicine Lodge thrust plate in late Early and Late Cretaceous time.