 The Florida peninsula appeared above sea level between 100,000 and 150,000 years ago. As sea levels at the end of the Wisconsin ice age rose, the water table appeared closer to land. Lake Okeechobee began to flood and convection thunderstorms were created. Vast peat deposits south of Lake Okeechobee indicate that regular flooding had occurred about 5,000 years ago. Plants began to migrate, subtropical ones from the northern part of Florida, and tropicals carried as seeds by birds from islands in the Caribbean. Although the limestone shelf appears to be flat, there are slight rises — called pinnacles — and depressions caused by the erosion of limestone by the acidic properties of the water. The amount of time throughout the year that water is present in a location in the Everglades determines the type of soil, of which there only two in the Everglades: peat, created by many years of decomposing plant matter, and marl, the result of dried periphyton, or chunks of algae and microorganisms that create a grayish mud. Portions of the Everglades that remain flooded for more than nine months out of the year are usually covered by peat. Areas that are flooded six months or less are covered by marl. Plant communities are determined by the type of soil and amount of water present.
