Seven dredge hauls on seamounts, and nine cores of the sea floor show that glacial marine drift was deposited over several hundred thousand square miles of the northeastern Pacific during Pleistocene time. Some ice-rafted pebbles are covered by as much as 50 mm of manganese dioxide, suggesting a minimum age of 50,000 years.
Since glacial marine sedimentation ceased, blue mud has spread out from shore. The drift is covered by at least 127 cm of blue mud at a distance of 250 miles from shore. Southeast of Kodiak, Alaska, blue mud is covered by a thin layer of ash from the 1912 eruption of Katmai Volcano.
Ice-rafted pebbles surrounded by envelopes of well-sorted coarse sand are incased in pieces of a thick manganese crust from Gilbert Seamount. The pebbles settled on the relatively smooth surface of the manganese crust and were incased by the slow upward growth of the crust. Finer sediment, which settled from icebergs, was swept off the smooth crust except for well-sorted coarse sand which collected in the pockets in the crust surrounding the larger pebbles. The formation of the pockets and the envelopes of coarse sand is ascribed to the action of weak currents although the specimens were dredged from a depth of at least 3600 feet.