Boulder Mountain is the easternmost part of Aquarius Plateau. Physiographically it is virtually an independent plateau with an area of about 70 square miles and an altitude which approximates 11,000 feet. The remainder of Aquarius Plateau lies 1000 feet lower and is set off from Boulder Mountain by the westward-facing scarp of the latter. The plateau top is covered with a succession of lava flows from which great landslides have descended to cover the plateau flanks on all sides.

The nearly flat surface of Boulder Mountain is largely covered with a maze of small lakes interspersed with broad shallow depressions or “meadows.” Most of the lakes occupy glacially scoured rock basins, none of which exceeds a quarter of a mile in length. Striations and grooves are abundant, and glacial polish was seen in a few places. The plateau top was covered with an ice cap during Pleistocene time, which grew so large that it spilled over the rim on all sides. Avalanche ice and reconstructed glaciers, as well as sizable ice tongues or outlet glaciers, must have been common. Most of the overflow was stopped when it reached the first sizable landslides below the rim. As a result there are numerous lakes under the rim which are clearly the result of the combination of landslide dams modified by glacial action. A few ice tongues were strong enough to plow their way through the landslides and leave behind broad, very shallow U-shaped valleys which are now occupied in places by morainal lakes. No undoubted glacial deposits were found below 8950 feet.

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