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

Examination of the relationship of joints to stream development shows that the oft-cited development of streams parallel to joint directions does not, in general, apply in south-central New York and adjacent 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. The removal of joint-bounded blocks in joint-oblique valleys forms cascades that advance headward by apical erosion. Streams whose courses are parallel and perpendicular (joint-parallel valleys) to the nearly orthogonal joint sets erode by waterfall and plunge-pool formation; bedrock is undercut on the downstream side, and unstable blocks subsequently collapse into the plunge pool. 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.