The northern section of Darby Creek is within the Piedmont Uplands physiographic province, while the southern section is part of the Atlantic Coastal Plain province. The Piedmont Uplands section has generally old, hard upland rocks that eroded from the Appalachian Mountains. The rocks from the northern portion of the watershed date to the Precambrian Era and Lower Paleozoic Era. The rocks from the southern portion of the watershed are newer, dating from the Tertiary and Quaternary Periods. Atlantic Coastal Plain rock is generally softer than Piedmont Uplands rock and was deposited in the area about 1.6 million years ago through glacial erosion.