The geologic formations that have the most influence on the Everglades are the Miami Limestone and the Fort Thompson Formation. The Miami Limestone has two facies. The Miami Oolite facies, which underlies the Atlantic Coastal Ridge from southern Palm Beach County to southern Miami-Dade County, is made up of ooids: tiny formations of egg-shaped concentric shells and calcium carbonate, formed around a single grain of sand or shell fragment. The other facies, which underlies the eastern lower Everglades (in Miami-Dade County and part of Monroe County) consists of fossilized bryozoan organisms. The unique structure was some of the first material used in housing in early 20th-century South Florida. The composition of this sedimentary formation affects the hydrology, plant life, and wildlife above it: the rock is especially porous and stores water during the dry season in the Everglades, and its chemical composition determines the vegetation prevalent in the region. The Miami Oolite facies also acts to impede flow of water from the Everglades to the ocean between Fort Lauderdale and Coot Bay (near Cape Sable).