The Cape Florida Channel (ten to eleven feet deep in 1849) and Bear Cut (four feet deep in 1849) are the deepest natural channels into Biscayne Bay. 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 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. The southern end of the island is Cape Florida. The northern end of the island is separated from another barrier island, Virginia Key, by Bear Cut. It is approximately 5 miles (8.0 km) long and 1 to 2 miles (1.6 to 3.2 km) wide.
Key Biscayne is elongated in the north-south direction, tapering to a point at each end. In the 1850s Louis Agassiz noted that "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. The coastal transport of sand southward ends at Key Biscayne. 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. 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.