The North-South Segment of the Eno River
Why does the course of the Eno River change direction and begin flowing in a north-south direction just east of Cox Mountain?
The short answer:
The Eno River flows into an extinct geologic fault that formed more than 200 million years ago during the break-up of Pangea. The faulted rock is weaker and easier to erode causing that north-south segment to be the preferred flow path of the river.
The long answer:
Rivers and streams typically follow the geologic structure or grain (like the grain in wood) of the rocks that they flow over. Some of these geologic structures include joints, layering, faults, fractures and foliations. Geologic structures commonly control the orientations, patterns and local gradients of streams and rivers. Zones where pre-existing fractures, joints and faults are present within rock may be more easily eroded. Running water likes to take the “path of least resistance” and will flow into zones that are easier to erode. When geologic structures like joints and faults intersect, the flow path of rivers and streams may form kinks or abrupt bends at sharp angles. A close examination of the course of the Eno River reveals many kinks and abrupt bends.
In the Eno River area, most geologic structures (such as layering, joints and foliation) are oriented in a northeast-southwest trend – parallel to the trend of the entire Appalachian Mountains. The section of the Eno River from the town of Hillsborough to just west of the Few’s Mill area flows in a general northeast direction with many small-scale kinks and bends. This section of the Eno appears to follow the dominant structure of the underlying rock parallel to the major trend (northeast-southwest trend) of the rocks, with minor deflections as the river follows small-scale fractures and joints. However, at Few’s Mill in Eno River State Park, the Eno River abruptly bends toward the south at a sharp angle. This sharp angle bend coincides with the Eno encountering a prominent linear feature (a lineament) visible on the LiDAR map (pl. 2). The same prominent north-south lineament extends toward the north and is occupied by Buckquarter Creek.
Lineaments may reflect the trace of faults and fractures or the orientation of rock features such as layering and foliations. Several prominent lineaments (several miles long) are present on the LiDAR map (pl. 2) of the Eno River area. Many of these lineaments are parallel to the known direction of faulting associated with the split-up of Pangea and the formation of the Triassic basins. At many locations, portions of the Eno River and its tributaries occupy the trace of these lineaments.
Outcrops of fractured rock, off-set map patterns of geologic units and the presence of other features consistent with faulting have been observed within the trace of the prominent north-south lineament. These observations have led to the interpretation that the flow of the Eno River in this section is controlled by faults that formed during the breakup of Pangea and the formation of the Triassic basin more than 200 million years ago (pl. 1).
