"A very curious structure": the Wangum Falls monocline on Middle Creek, Paupack Township, Wayne County, Pennsylvania
Inners, Jon D., 1915 Columbia Avenue, Camp Hill, PA 17011, email@example.com, Lentz, Leonard J., Pennsylvania Geological Survey, 3240 Schoolhouse Road, Middletown, PA 17057, and Braun, Ruth, Department of Geography and Geosciences, Bloomsburg University, Bloomsburg, PA 17815
In his 1881 report on the geology of Susquehanna and Wayne Counties, I. C. White of the 2nd Pennsylvania Geological Survey described "a very curious structure" at a falls on the "Middle Branch of the Lackawaxen" River in Paupack Township, Wayne County. He wrote, "[T]he bedding plane of the sandstone appears to conform to the shape of the falls, curving over its top and down the sides like a sheet of water passing over the same; it seems to be the result of false bedding combined with a kind of concretionary structure in the rock." Although White failed to recognize it, this structure at today's Wangum Falls on Middle Creek is a textbook monocline.
The Wangum Falls monocline is located on the glaciated Allegheny low plateau about 4 miles northwest of Hawley below the bridge on which Township Rte. 367 crosses Middle Creek. The rocks involved consist of medium- to thick-bedded, crossbedded, gray and red, fine- to medium-grained sandstones of the Lackawaxen Member of the Catskill Formation (Upper Devonian). Height of the monocline is 20 feet with the axis striking approximately east-west. The structure verges to the south, the maximum dip of bedding being 60 degrees south. Beds upstream and downstream of the structure are subhorizontal.
The monocline at Wangum Falls is very unusual, if not unique, in northeastern Pennsylvania. Although its east-west structural orientation appears at first to be somewhat anomalous, the azimuth of its axis is within the standard deviation (10 degrees) of nine 4th-order folds measured by W. Sevon and T. Berg in adjacent Pike County, Pennsylvania (1989), i.e., N81E. Three subvertical, regional joint sets are well developed in the monoclinal strata at Wangum Falls and in the subhorizontal beds upstream and downstream: N5E/88SE, N60E/84SE, and N85E/84SE. These joints, characteristic of Upper Devonian rocks throughout Pike, Wayne, and Susquehanna Counties, combine with the monocline to create the configuration of Wangum Falls. Although the Wangum Falls monocline may be simply just another Alleghanian compressional structure, its southerly vergence, east-west orientation, and near axial-parallelism to a regional joint set hint that it is possibly related to a later tensional stress field, perhaps even Triassic-Jurassic.
Poster presentation presented at the 42nd Annual Meeting (12–14 March 2007) of the Northeastern Geological Society of America, Durham, New Hampshire.
Photo by Jon Inners (retired), Pa. Geological Survey