The Eocene Green River Formation in the Sand Wash Basin, Colorado (United States), contains the largest known lacustrine columnar stromatolites. Spectacular columns, as much as 5.5 m high with diameters of 7 m, occur over a broad area. The stromatolites are composed of laterally continuous centimeter-thick layers that can be traced from the base to the top of the column (synoptic relief), thus these stromatolites stood as much as 5.5 m above the lake floor. The layers consist of one to several different kinds of microbialites, which makes these large columns even more unusual. The giant columns were the result of a combination of factors including the lake transgressing a flooded woodland, in situ tree stumps providing elevated substrates above the lake floor, an abundant supply of calcium-rich spring and surface water that mixed with saline-alkaline lake water, and the subsidence rate of the nearshore lake environment not exceeding the rate at which the stromatolites grew.