This map illustrates the spatial variability of the thickness of the loose, unconsolidated glacial sediments (called drift) that typically cover the bedrock of Michigan. The black areas lacked sufficient data when the original map was compiled (1981). The dark brown region in the northwestern part of the Lower Peninsula contains the thickest drift in the state (over 600 feet thick). The green areas represent zones of relatively thin drift (usually less than 100 feet thick and in some places less than 25 feet thick). Many of the thin drift areas of the state are underlain by carbonate-rich bedrock types which make the bedrock aquifers in these areas especially vulnerable to surface contaminants.
