Sediments range from sandy to clayey medium grayish-green silts. The upper inch or two is black, rich in organics, and odorous. Quartz, the most abundant mineral, forms 50 to 90 percent of the sediment. Typically, less than 12 percent of the particles are clay size. Clay minerals are illite and chlorite. Water content ranges from 35 percent at a depth of 6 feet to 70 percent near the surface.