Key Biscayne, although named a "key", is not geologically part of the Florida Keys, but is a barrier island composed of sand eroded from the Appalachian Mountains, carried to the coast by rivers and then moved along the coast from the north by coastal currents.
There is no hard bedrock near the surface of the island, only layers of weak "shelly sandstone" to depths of 100 feet (30 m) or more.
The coastal transport of sand southward ends at Key Biscayne.
In the 1850s Louis Agassiz noted that "[s]outh of Cape Florida no more silicacious sand is to be seen." (
The beaches in the Florida Keys, by contrast, consist primarily of finely pulverized shells.)
Geologists believe that the island emerged around 2000 BCE, soon after the sea level stopped rising, as the sand built up to form new barrier islands on the southern Florida coast.

Key Biscayne is elongated in the north-south direction, tapering to a point at each end.
It is approximately 5 miles (8.0 km) long and 1 to 2 miles (1.6 to 3.2 km) wide.
The northern end of the island is separated from another barrier island, Virginia Key, by Bear Cut.
The southern end of the island is Cape Florida.
The Cape Florida Channel separates the island from the Safety Valve, an expanse of shallow flats cut by tidal channels that extends southward about 9 miles (14 km) to the Ragged Keys, at the northern end of the Florida Keys.
Only Soldier Key, approximately 200 by 100 yards (183 by 91 m) wide, lies between Key Biscayne and the Ragged Keys.
The Cape Florida Channel (ten to eleven feet [three to three-and-a-half meters] deep in 1849) and Bear Cut (four feet [a little more than one meter] deep in 1849) are the deepest natural channels into Biscayne Bay.
They provided the only access for ocean-going vessels to Biscayne Bay until artificial channels were dredged starting early in the 20th century.
In 1849 the island had a fine sandy beach on the east side, and mangroves and lagoons on the west side.
The average elevation of the island is less than five feet (1.5 m) above sea level.

Key Biscayne is located at 25°41′25″N 80°9′54″W (25.690329, -80.165118).
Key Biscayne, although named a "key", is not geologically part of the Florida Keys, but is a barrier island composed of sand eroded from the Appalachian Mountains, carried to the coast by rivers and then moved along the coast from the north by coastal currents. There is no hard bedrock near the surface of the island, only layers of weak "shelly sandstone" to depths of 100 feet (30 m) or more. The coastal transport of sand southward ends at Key Biscayne. In the 1850s Louis Agassiz noted that "[s]outh of Cape Florida no more silicacious sand is to be seen." (The beaches in the Florida Keys, by contrast, consist primarily of finely pulverized shells.) Geologists believe that the island emerged around 2000 BCE, soon after the sea level stopped rising, as the sand built up to form new barrier islands on the southern Florida coast.
Key Biscayne is elongated in the north-south direction, tapering to a point at each end. It is approximately 5 miles (8.0 km) long and 1 to 2 miles (1.6 to 3.2 km) wide. The northern end of the island is separated from another barrier island, Virginia Key, by Bear Cut. The southern end of the island is Cape Florida. The Cape Florida Channel separates the island from the Safety Valve, an expanse of shallow flats cut by tidal channels that extends southward about 9 miles (14 km) to the Ragged Keys, at the northern end of the Florida Keys. Only Soldier Key, approximately 200 by 100 yards (183 by 91 m) wide, lies between Key Biscayne and the Ragged Keys. The Cape Florida Channel (ten to eleven feet [three to three-and-a-half meters] deep in 1849) and Bear Cut (four feet [a little more than one meter] deep in 1849) are the deepest natural channels into Biscayne Bay. They provided the only access for ocean-going vessels to Biscayne Bay until artificial channels were dredged starting early in the 20th century. In 1849 the island had a fine sandy beach on the east side, and mangroves and lagoons on the west side. The average elevation of the island is less than five feet (1.5 m) above sea level.
Key Biscayne is located at 25°41′25″N 80°9′54″W (25.690329, -80.165118).
Key Biscayne