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Approximately 105,000 yr ago (based on uranium-series dating), waves in a giant wave train swept up to an elevation of about 375 m on the island of Lanai. The waves deposited the Hulopoe Gravel, which near the present shoreline consists of basalt boulders, coral fragments, and calcareous beachrock slabs, and near the upper limit of the deposit consists of sand and shell fragments. The maximum heights of similar but lower deposits on nearby islands, when adjusted for their estimated subsidence due to volcanic loading during the past 105,000 yr, indicate that the source of the wave was about 50 km southwest of Lanai. We hypothesize that failure and downward movement of the huge Lanai submarine landslide created an ocean disturbance, which produced waves that rushed across the Lanai reef and beach, picked up limestone and lava fragments, and deposited them high on the island as the Hulopoe Gravel. Backwash from the waves stripped soil and rock from the islands and carried much of it to the sea.

The Hulopoe Gravel is 8 m thick in a gulch 200 m inland from the Lanai shoreline, where it consists of three beds, successively, 2, 4, and 2 m thick. These beds are considered to have been laid down by successive waves in the wave train. Each bed consists of two units: a lower unit of basalt and limestone boulders, cobbles, and sand, and an upper bimodal unit of large basalt boulders with a pebbly sand matrix. These subunits are inferred to be deposited from the runup and backwash of each wave.

At the upper surface of the Hulopoe Gravel, basalt boulders averaging 0.5 m in diameter are arranged in dunelike ridges about 1 m high and 10 m apart. Nearby, where young streams have cut into and exposed the lower beds of the Hulopoe, clasts at the boundaries between the beds are locally imbricated and dip landward. We interpret these features as aspects of torrential flow and crossbedding created during the high-speed backwash of the great waves.

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