A Heinrich event is a natural phenomenon in which large armadas of icebergs break off from glaciers and traverse the North Atlantic. First described by marine geologist Hartmut Heinrich, they occurred during the past glacial periods or "ice ages" and are particularly well documented for the last glacial period. The icebergs contained rock mass, which has been eroded by the glaciers, and as they melted, this matter was dropped onto the sea floor as ice rafted debris (abbreviated to "IRD").The icebergs' melting caused prodigious amounts of fresh water to be added to the North Atlantic. Such inputs of cold and fresh water may well have altered the density-driven thermohaline circulation patterns of the ocean, and often coincide with indications of global climate fluctuations.