Seismic profiles over the northwestern Great Bahama Bank reveal that it is formed by the coalescence of three smaller platforms and significant lateral progradation. This conclusion is contrary to previous models which assumed that the bank is the result of continuous accumulation on one huge buildup. In Late Cretaceous to early Tertiary time, a north-south-trending depression, the Straits of Andros, separated an eastern platform, Andros bank, from a western platform, Bimini bank. Initially, the Straits of Andros had dimensions similar to the modem Tongue of the Ocean but was progressively filled from east to west In early(?) to middle Tertiary time, a second depression, the Bimini embayment, formed within Bimini bank by folding; it had a maximum depth of about 470 m. This depression was also filled from east to west. In addition, prograding systems built the western margin of Bimini bank more than 25 km westward into the Straits of Florida.