The Crater Line consists of a series of rock exposures outcropping in an arcuate pattern around the southwestern flank of the Rum Jungle granite. The exposed rocks are believed to represent part of the Brocks Creek group of Lower Proterozoic age. The Crater formation, the major mappable unit in the line of exposures, consists of metamorphosed clastic rocks totalling approximately 1500 feet in thickness. Significant radioactivity is restricted to three stratigraphic zones within the Crater formation. These have been mapped and are designate Crater Pebble Beds, Number One Pebble Bed, and Number Two Pebble Bed. Number One Pebble Bed appears to contain the most significant anomalies. The radioactivity is restricted to conglomerate beds. There may be a genetic relationship to the greater permeability formerly localized in the conglomeratic zones. The radioactivity is not localized by tectonic structures such as folds, faults, or changes in dip. No source of the radioactivity has been identified. The radioactivity probably emanates from members of the uranium disintegration series. Four areas containing significant anomalies and deserving further investigation were found along the Crater Line.