In gross classification, sediments of the deep seafloor may be of terrigenous or marine origin. Those of marine origin are classified as oozes (generally calcareous) or clays (generally siliceous) depending largely on whether their CaCO3 content is greater or less than 30%. The carbonate contents is, in turn, controlled both by the biological productivity of the surface waters and the circulation patterns of the deep waters of the oceans. Older waters have more dissolved CO2 and attack carbonates more aggressively. There are now a few deep-sea in-situ shear velocity measurements in siliceous and calcareous sediments. The siliceous sediments show velocities that are the lowest, 20–30 m/s at the surface, while calcareous sediments have slightly higher velocities. Both of these have velocities lower than those found in shallow silts and sands. Measurements of the shallow shear velocities show high variability, approaching 50%, over a few km distance in the siliceous sediment (Dorman etal., 1991). These measurements are made using interface waves from seafloor explosions observed using ocean bottom seismographs. These may be correlated with the channeling of sediments but the mechanism is far from clear. [Work supported by ONR.]