The Upper Cretaceous Frontier Formation in the eastern Pioneer Mountains of southwestern Montana was deposited in nonmarine environments west of the Western Interior Seaway within the Cordilleran foreland basin. These rocks have been assigned to the Frontier because they contain lithologies typical of the Frontier in the region even though they are entirely nonmarine and are thicker than the correlative marine Frontier to the east. The Frontier in the eastern Pioneer Mountains is underlain by the Vaughn Member of the Blackleaf Formation, but the upper part of the Frontier has been eroded and locally is overlain by rocks of Tertiary age. Geologic mapping was conducted and four sections were measured and described to determine facies, thickness variations, and depositional environments. In the eastern Pioneer Mountains study area, the Frontier ranges in thickness from about 1200 ft (366 m) in the south to more than 3400 ft (1036 m) in the north\. Frontier strata in the study area cannot be readily subdivided into mappable units, but two broadly-defined informal lithic units are described. The lower unit contains yellow-brown weathering siltstone, mudstone, and fine-grained quartz-rich sandstone, and is about 250 ft (76 m) thick. The upper unit is composed of yellow-brown to dark-gray siltstone and mudstone, quartz- and chert-rich sandstone, conglomeratic sandstone, and limestone, and is more than 2100 ft (640 m) thick in one measured section. The lower contact of the Frontier is placed at the top of a porcellanite bed that is associated with maroon mudstone and siltstone, limestone, and calcareous dark-gray shale in the underlying Vaughn Member of the Blackleaf Formation. The porcellanite bed directly overlies the highest maroon mudstone-siltstone bed of the upper Vaughn. The Frontier Formation in the eastern Pioneer Mountains was deposited on a broad delta plain dominated by fine-grained sediments. Sandstones exhibit characteristics of anastomosing fluvial channels, average 5% to 10% of the entire formation, and have width to depth ratios of less than 30. The Frontier in the eastern Pioneer Mountains is lithostratigraphically equivalent to the Frontier to the east in the Gravelly, Greenhorn, and Madison Ranges, but may include strata that are younger in age. Frontier strata to the south at Lima Peaks are the thickest in the region (up to 7000 ft; 2100 m) and include facies that are time-equivalent to the marine Frontier, the overlying Cody Shales, and the Telegraph Creek Formation of the Madison Range to the east. The Frontier in the eastern Pioneer Mountains is in part lithostratigraphically equivalent to the Coberly Formation in the Drummond, Montana area (50 mi [80 km] northwest of Butte, Montana) and the Marias River Shales near Great Falls (150 mi [240 km] northeast of Butte) in west-central Montana.