The relationship of joints and stream drainage in flat-lying rocks of south-central New York and northern Pennsylvania.

Streams whose courses are oblique to the joint directions (joint-oblique valleys) tend to erode easily owing to increased corrasion and subsequent undercutting at the upstream intersection of joints. Streams whose courses are parallel and perpendicular (joint-parallel valleys) to the nearly orthogonal joint sets erode by waterfall and plunge-pool formation. Most valleys in the Finger Lakes region are joint-oblique, although some well-developed valleys are joint-parallel. These joint-parallel valleys are usually due to 1) a single deep, pervasive joint whose presence acts as a barrier to lateral expansion of the stream, or 2) erosion along joint zones whose intense fracturing produces weak erosional resistance in the rocks. -from Author