Rounded, polished phosphorite sand grains in Georgia continental shelf, beach, and estuarine sediments average 1 per cent of the total sediment. An amber and a black variety are present everywhere except in estuaries where only the black is present. The black color is due primarily to inclusions of pyrite and carbonaceous matter. The lack of amber grains in estuaries may reflect reducing conditions favoring pyrite formation. The phosphorite and quartz fractions of most samples are similar in size. This plus the high polish on phosphorite grains (which tumbling-barrel experiments show can be due to abrasion) indicate a probable detrital origin of the phosphorite.
Shelf phosphorite must have been derived from Pleistocene river sources or outcropping phosphate-rich ancient sediments on the shelf, for present-day rivers are not carrying this material. The presence of phosphorite in beaches and estuaries indicates that much of these sands are derived by landward transport of sediments from the continental shelf.