Chickies Rock, a striking promontory on the Susquehanna River: the early Cambrian type locality of the trace fossil Skolithos and a model site for structural analysis

Abstract

Abstract Chickies Rock rises abruptly from the east bank of the Susquehanna River at Columbia, Pennsylvania, where it breaks through a ridge held up by massively bedded quartzite. Here in the 1830s, Samuel Haldeman recognized long straight tubes that he later described as Skolithos linearis , penetrating the quartzite. Uncertain about the nature of these cylindrical ‘vermiform or linear’, unbranched ‘stems’, Haldeman first treated them as a subgenus of Fucoides , to which root-like traces were then often assigned. By the time James Hall first illustrated the species in 1847, Haldeman was sure they were burrows of a worm-like animal, as the name he had chosen implied. Since 1960, Donald Wise has developed a detailed analysis of structural deformation of the Chickies Anticline, using the Skolithos burrows as ‘plumb-bobs’ against which Taconic rotation of the rock fabric can be measured. Wise showed that later Alleghanian deformation produced prominent folds with external rotation up to twice as great as that of the rock fabric itself. Now conserved as a county park, Chickies Rock attracts many visitors. Its cliff-top is a superb viewpoint from which to contemplate progressive westward migration of the Appalachian drainage divide, ongoing since the late Jurassic opening of the Atlantic Ocean.