TOPOGRAPHIC PARADOX IN A BASIC GEOLOGIC SETTING, APPALACHIAN PLATEAU, CENTRAL NYS

Asymmetric cross-sections are common in north-south oriented valleys on the eastern Appalachian Plateau. Steeper west-facing valley walls are underlain by the same shallow dipping siltstone as west-facing walls. Factors contributing to asymmetry are Devonian facies changes, early erosional history and glacial plucking.

Situated on the northern flank of the Catskill Delta, Devonian facies changes include littoral shifts resulting in sandstone and conglomerate interfingering westward with finer siltstone and shale. These changes appear with greater frequency westward from the upper Susquehanna Valley, all within a distance of 20 miles to the Unadilla and 30 miles to the Chenango Valley. Such changes continue westward with diminishing amounts of sandstone and increasing siltstone and shale.

Tertiary drainage off the southern Adirondacks fed rivers across a maturely dissected plateau (deep valleys and broad divides) before headward erosion by the Mohawk River captured and diverted all rivers into the Hudson resulting in valleys with misfit streams.

During the glacial events that followed, deposition favored valleys whereas glacial plucking of cap rock was most effective on uplands. Multiple plucking events initially removed the thinnest cap rock from western-most uplands of the Chenango Valley, thus exposing more erodible finer facies leading to symmetrical valleys. Steeper slopes prevailed only where protected beneath a persistent cap rock.

The thickest cap rock remained on eastern-most divides, as in the Unadilla and Susquehanna Valleys. Here, random plucking stripped cap rock from western valley sides initiating the development of asymmetry.

Eventually, all remaining cap rock was plucked leaving residual asymmetry that is entirely unrelated to the local geologic setting. As such, asymmetry is an artifact of a complex history culminating with glacial plucking.