Fishkill Creek - Geology

Geology

The creek flows through two distinct geological regions within the Highlands: the Mid-Hudson Valley and the Hudson Highlands. Both have different types of bedrock, impacting the nature of the stream and allowing for differences in aquifer development.

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 schist, limestones marble and sandstone quartzite in the eastern regions of Dutchess County. These rocks create better aquifers than their sedimentary counterparts.

In the Hudson Highlands, the stream's bedrock is primarily metamorphic gneiss, with some granite and amphibolite. These are older rocks, formed in high temperatures and pressures more than a billion years ago in the Pre-Cambrian. These generally make poor aquifers, although individual houses can use one if they overlie a fault where groundwater collects.