NORTH-SIDE TRIBUTARY ASYMMETRY (NSTA) IN THE APPALACHIAN FORELAND

Available digital-elevation images, the Geologic Shaded-Relief Map of Pennsylvania (GSRM) (Miles, C. E., 2003, 1:500,000 scale) and the poster for the 2006 NEGSA meeting (GSAP) (1,8,000,000 scale) (www.nced.umn.edu/maps) present spectacular views of a striking but apparently heretofore unheralded physiographic phenomenon in the Appalachian foreland, what we here term "north-side tributary asymmetry" (NSTA). In NSTA, streams with some degree of E-W orientation have tributaries entering primarily from the north with few or no tributaries entering from the south. North-side tributaries are long; south-side, short.

NSTA in NY and PA occurs on relatively undeformed, sedimentary rocks that have a very low, south, regional dip. NSTA is most striking in the Catskill Mtns. In NY (GSAP), is pronounced throughout the Appalachian Plateaus Province of PA (GSRM), is most delicately developed in southwesternmost PA (GSRM) and adjacent WVA (GSAP), and is locally present in parts of OH, KY, TN, VA, NC, and SC (GSAP).

Most NSTA in NY and PA is interpreted to result from a natural tendency for streams to headward erode up dip (Lattman, 1954). During tributary headward erosion, the E-W master stream incises only and does not exclude development of south-side tributaries. Less common are E-W master streams that migrate down dip significantly as they incise. Trailing, up-dip tributaries develop as the master stream migrates. The lateral migration perpetuates a down-dip, stream-adjacent, steep slope that prohibits south-side tributary development. Where there's space, parallel master streams occur, the migrating, steep slope of the master stream eventually beheads tributaries to the next down-dip master stream, thus creating hanging valley forms.

We suggest that NSTA, as it occurs today, is not inherited from the long-term erosion network that stripped several kms of strata from the Appalachian Plateaus Province. Instead, NSTA results from a more recent change in pattern that may be a long-term response to slight, regional tilting caused by Late Cretaceous uplift of the Adirondack Mtns. We further suggest that the prominent NSTA incision present today is post Middle Miocene (<16 Ma) and pre-Pleistocene (>2 Ma). A now largely eroded predecessor NSTA evident in the Deep Valleys Section of PA may be older.