The Mid-Hudson Valley region underlying most of Fishkill Creek consists of sedimentary rocks such as shales, sandstones, siltstones and dolomitic limestones that formed during the Cambrian and Ordovician periods of the Paleozoic Era, roughly 450-540 million years ago. These rocks become modified into metamorphic equivalents as a result of the formation of the Appalachian Mountains, which left large thrust faults in the area, the further east one gets from the Hudson. Shales become schists, limestones become marbles, and sandstone become quartzites in the eastern regions of Dutchess County. These rocks create better aquifers than their sedimentary counterparts.