Laminated calcium carbonate crust, "calcrete," from Key Largo, Florida was cut into five separate laminae groups which were radiocarbon dated to determine the crust's origin and rate of deposition. Laminae groups ranged uniformly in age from 5680 years B.P. at the bottom to 400 years B.P. at the top, indicating that the crust formed during the last 5,000 to 6,000 years. Dating of crust from Big Pine Key, Florida, 108 kilometers from the Key Largo site, showed a similar progression of ages from top to bottom.