Complete and virtually undisturbed sections of late Neogene and Quaternary sediment were recovered by the hydraulic piston corer from Leg 68 sites in the western Caribbean and eastern equatorial Pacific. Lithologic stratigraphy of Site 502 records the changing paleoceanography of the western Caribbean. Associated with the emergence Of the Isthmus of Panama is the weakening of the west-flowing Caribbean Current. This is reflected in an abrupt lithofacies change, with a marine authigenic-volcanic facies of late Miocene being replaced in the Pliocene by continental detritus of the Rio Magdalena. At Site 503, on the northern flank of the Galapagos Rise, almost the entire sediment section consists of a marine authigenic-hydrothermal facies.