The Beartooth Mountains of south-central Montana and northwestern Wyoming are a northwesterly trending high rugged range made up mainly of Precambrian metamorphic rocks. The southeastern part of the range is characterized by extensive high-altitude flat or gently rolling plateaus separated by deep glaciated canyons. The plateaus along the crest of the range are at altitudes of more than 3,350 metres (11,000 ft), whereas those on the flanks of the range are much lower. They are almost entirely on Precambrian rocks, and only small patches of Cambrian sedimentary rocks still remain at a few places in the high mountains. Topographic profiles across the southeastern Beartooth Mountains show that: (1) plateaus bearing the sedimentary remnants lie very close to the position of the sub-Cambrian depositional surface as projected from known occurrences of that surface along the south side of the mountains; (2) other plateaus on the crest and southwest flank also lie near this projected position; and (3) high plateaus and other summits on the northeast slope in areas distant from known or projected positions of the sub-Cambrian surface are markedly accordant. We believe the plateaus along the crest and southwest flank of the range to be remnants of an exhumed marine plain of Early Cambrian or Precambrian age, and those on the northeast flank may possibly have a similar origin. The sub-Cambrian depositional surface was protected until early Tertiary time by a cover of Paleozoic sedimentary rocks, and parts of it probably were covered until Pliocene time by Eocene volcanic rocks. The surface subsequently has been profoundly modified by fluvial and glacial erosion and by mass wasting.
