The brackish- and fresh-water Beartooth Butte formation of Early Devonian age has been tentatively recognized at 20 outcrops and 2 wells in southern and central Montana and northern Wyoming. It is generally less than 10 ft. thick, but it ranges in short distances from zero to more than 150 ft. in thickness. The lithologic character of the formation is varied, but a basal limestone or dolomite conglomerate characterizes most occurrences. In southern Montana and northern Wyoming, the Beartooth Butte crops out as discontinuous estuarine and fluviatile channel-fill deposits, sinkhole fillings, and regoliths that overlie the Bighorn dolomite of Late Ordovician age and underlie the Jefferson formation of Late Devonian age. The discovery of fish remains of possible Early Devonian age in the top of supposedly Lower Ordovician strata of the Big Snowy Mountains in central Montana may extend the Beartooth Butte formation 100 mi. N. of the other deposits. The Beartooth Butte may also be present between the Maywood formation of Late Devonian age and the top of Upper Cambrian rocks at Logan, in southwestern Montana, and at Beaver Creek, in W.-central Montana. Some sinkhole and channel-fill deposits referable to the Beartooth Butte are found at the top of Silurian rocks in the Williston basin. Correlations between the Beartooth Butte formation, the Water Canyon formation of northern Utah, the lower part of the type Maywood formation of western Montana, and the upper part of the Ghost River formation of W.-central Alberta are suggested. A belt of discontinuous deposits of Early Devonian and probable Early Devonian age is inferred to extend in an almost straight line from northern Utah 900 mi. NNW. to W.-central Alberta. Shallow-water, nearshore marine rocks are probably present in this belt W. of the Beartooth Butte outcrops, whereas Lower Devonian marine rocks are apparently absent E. of those outcrops. It is suggested, therefore, that the Early Devonian sea that produced estuarine conditions in some channel-fill deposits of the Beartooth Butte formation lay at the W. This sea probably occupied a geosynclinal trough in Washington and Oregon and advanced eastward across Montana and Wyoming, whereas earlier and later Paleozoic marine invasions came southwestward from the Williston basin.