Deposits of several glaciations from oldest to youngest, pre-Bull Lake, Bull Lake, and Pinedale are in and near the Henrys Lake basin, a northwest-trending hourglass-shaped trough in southeastern Idaho. Glacial deposits of pre-Bull Lake age are represented by till remnants confined mostly to the higher levels of the surrounding mountains. The extent of Bull Lake ice into the basin is uncertain. Glacial deposits and ice-marginal features suggest that a lobe of Bull Lake ice the Henrys Lake ice lobe moved into the basin and advanced as far north as the north edge of Henrys Lake. After this lobe melted back far enough to expose the north end of the basin, melt waters, chiefly from alpine glaciers in Targhee Creek and its tributaries, formed the broad low Howard Creek fan at the mouth of the valley of Howard Creek. Shortly thereafter, a complex of channels was cut into the fan by periodic melt-water floods which flowed westward through Targhee Pass during the waste of another Bull Lake ice lobe which lay in the West Yellowstone Basin. Melt waters from various sources ponded to form a shallow lake in front of the wasting Henrys Lake ice lobe. Waters from this lake drained southwestward around the southwest flank of the lobe via an ice-marginal channel. Upon total melt of the ice lobe, the lake was drained, the ice-marginal channel was abandoned, and the waters resumed their pre-ice course to the southeast. During the interglaciation, prior to the onset of the Pinedale Glaciation, northward-flowing streams deposited a veneer of obsidian sand and gravel along the south edge of the basin. In Pinedale time, alpine glaciers formed in the high mountain valleys near the basin, but none advanced into the basin.
