The Everglades is a vast, subtropical wetland ecosystem known globally as the “River of Grass.” This unique environment, covering millions of acres in South Florida, is a massive, slow-moving body of water, functioning as a shallow, seasonal marsh rather than a typical river. The formation of this watery landscape spans millions of years, beginning with its ancient geological foundation. Understanding the Everglades requires examining the deep-time processes that laid the groundwork for this specific, expansive wetland.
The Geological Bedrock: Florida’s Limestone Platform
The foundation of the Everglades is a flat, porous limestone platform created by repeated fluctuations in global sea levels over the last few million years. During warm periods of the Pleistocene epoch, high seas covered much of the Florida peninsula, allowing marine organisms to thrive. Shallow seas saw the accumulation of ancient shells, corals, and tiny, calcium carbonate spheres called ooids.
These deposits eventually hardened into the bedrock known as the Miami Limestone, a significant component of the substrate beneath the eastern Everglades. This formation occurred approximately 130,000 years ago during a high-sea-level period. When sea levels later dropped, rainwater percolated through the exposed marine sediments, cementing the grains together into a hard, highly permeable rock.
This underlying geology provides almost no topographical relief across South Florida. The flatness of the limestone platform prevents the formation of deep river channels, forcing water that moves across it to spread out. The porous nature of the limestone also means that surface water and groundwater are intimately connected, with the bedrock acting as a storage vessel for freshwater in the Biscayne Aquifer.
The Defining Feature: The River of Grass and Sheet Flow
The modern Everglades system begins far to the north, originating in the Kissimmee River Basin, which drains into Lake Okeechobee. Historically, during the wet season, the massive volume of water collected in Lake Okeechobee would spill over its southern, lowest rim. This overflow initiated the unique hydrological process known as “sheet flow.”
Sheet flow describes the extremely slow, gravity-driven movement of a broad, shallow layer of water across the nearly level limestone platform. This layer of water could be up to 60 miles wide and only inches deep. The sheer breadth and slowness of this flow gave the area its famous “River of Grass” identity.
This constant, gentle movement across the flat landscape is responsible for the unique distribution of nutrients and the lack of deep, incised riverbeds. The slow flow rate allows water to saturate the entire landscape, supporting the wetland ecosystem year-round and maintaining freshwater flows well into the dry season. The sheet flow continues to move southward between the Atlantic Coastal Ridge and the Big Cypress Swamp toward the Florida Bay and the Gulf of Mexico.
Ecological Succession and Habitat Creation
Diverse Everglades habitats emerged over thousands of years, driven by the flat, porous limestone and persistent sheet flow. The dominant plant, sawgrass, thrives in the saturated freshwater marsh. Over time, the decomposition of sawgrass and other plant matter has led to the accumulation of thick layers of peat, a highly organic soil.
Peat accumulation slightly raises the elevation of the land in certain areas. The formation of tree islands, or hardwood hammocks, is a direct result of this slight elevation change, often starting over small topographic high points in the underlying limestone. These teardrop-shaped islands are centers of biodiversity, providing seasonally dry refuge for trees and wildlife that cannot tolerate prolonged flooding.
Where the freshwater sheet flow meets the saltwater of the coast, a transition zone forms, allowing for the growth of salt-tolerant mangrove forests. The varying depth of the water and the concentration of nutrients create a mosaic of distinct habitats, including cypress domes and wet prairies.