The northwest-trending Lemhi arch of central Idaho first formed in late middle Proterozoic time, and as much as 4,500 m (14,760 ft) of middle Proterozoic clastic rocks were eroded in later Proterozoic time. The west flank of the arch was partly covered in late Proterozoic(?) and Early Cambrian time by the Wilbert Formation. On the east flank, westward-thinning marine sedimentation began with deposition of the Middle Cambrian Flathead Formation, and continued through the Late Cambrian. During Ordovician and Silurian times, the east flank of the arch was dry. The west flank was submerged in the Ordovician, and the Summerhouse Formation, Kinnikinnic Quartzite, and Saturday Mountain Formation were deposited. The west flank of the arch was briefly exposed after deposition of t e Saturday Mountain Formation, but was partly submerged later in the Silurian, when the Laketown Dolomite was deposited. During the Middle and Late Devonian, deposition was renewed on the west flank of the arch, where the Jefferson Formation indicates eastward transgression. The east flank was exposed until the Late Devonian, when a thin sequence of the Jefferson and Three Forks Formations was deposited across the top of the arch, and marine sedimentation was continuous from the miogeocline far onto the craton.

The Lemhi arch continued to influence marine deposition even after it was submerged, separating shelf deposits in southwest Montana and east-central Idaho from miogeoclinal deposits in central Idaho. The arch was overridden by the Medicine Lodge thrust in late Early and Late Cretaceous times.

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