The topography of the floor of the Gulf of Mexico is dominated by the Mississippi Cone, the apex of which lies a few hundred feet below sea level at the Pleistocene mouth of the Mississippi River, and limits of which are formed by the scarps bounding the main basin. On the southwest the Mississippi Cone merges with the remarkably flat Sigsbee abyssal plain.

The sediments of the Cone and the closely related abyssal plain are all remarkably similar. The top 30-50 centimeters of each core from the abyssal plain and lower Mississippi Cone is largely composed of foraminiferal lutite. This bed reaches its maximum thickness of 4 meters on the upper cone and on the upper continental rise. The lower portion of each core is composed of gray silty clay which forms a layer so thick that, with one exception it has never been completely penetrated by a 30-foot coring tube. Micropaleontological correlation and radiocarbon dating have established the abrupt transition at the base of the ooze as the Pleistocene-Recent boundary (11,000 years B.P.).

In sharp contrast cores from three low knolls rising from the abyssal plain contain no gray silts and represent pelagic deposition well back into the Pleistocene.

The deposition of gray silts and clays on the cone and on the floor of the abyssal plain at the same time that pelagic sediment was being deposited on the knolls proves that the gray silts were transported along the sea floor.

Evidence from sediments and topography indicates that the Mississippi Cone was formed by the turbidity current transportation and deposition of silty sediments supplied in quantity by the Pleistocene Mississippi River.