Abstract The Appalachian Piedmont in south-central Pennsylvania and north-central Maryland contains metasedimentary siliciclastic rocks (phyllites to quartzites) that were deposited largely offshore of Laurentia, prior to and during the early history of the Iapetan Ocean. The Peach Bottom area is centered on the belt of Peach Bottom Slate and overlying Cardiff Quartzite, which is surrounded by the late Neoproterozoic and early Paleozoic rocks of the Peters Creek and Scott Creek (new name) Formations. Their provenance was the Brandywine and Baltimore microcontinents that lay farther offshore of the Laurentian coast. This area also includes an ophiolitic mélange that formed in front of an advancing island arc in Iapetus. All these rocks lay largely undisturbed throughout much of the Paleozoic, experiencing only chlorite-grade greenschist facies metamorphism through deep burial. Alleghanian thrusting associated with the growth of the Tucquan anticline imparted their present widespread, monocline, steep southeast dip of the bed-parallel foliation.

Abstract The Appalachian mountain system of eastern North America comprises a complex of deformed rocks which has stimulated geologic thinking since the area was first settled in the 17th century. Little attention was paid to it in the early days of European settlement, but in the early nineteenth century interest grew rapidly, leading to the creation of numerous state surveys and research that has continued unabated to this day. The earliest tectonic concepts were understandably rather primitive and mostly concerned the simpler foreland portion of the orogen. Upheavals, explosions, and other vertical motions were the main ingredients of the theory of the Rogers brothers and the geosyncline of Hall and Dana. Awareness of the importance of horizontal movements increased as continued detailed mapping revealed the presence of large thrust faults, culminating in the middle of this century with the elucidation of a décollement tectonics that pervades the entire Appalachian foreland. The Appalachian crystallines were originally considered to be part of the crust, and they formed a large part of Appalachia, Dana’s geanticline that supplied sediment to the adjoining geosyncline. Studies early in the 1900s began to reveal the complexities of this terrane and by the middle of this century, the crystallines had been divided into a number of distinctive belts. With the application of the plate tectonics scheme in the 1970s, geologists realized that these belts probably are micro-continents, either broken off from the Laurentian craton, or having come from other plates. With the accession of deep seismic data, the crystallines are no longer thought of as being rooted in the crust, but are believed to have undergone an allochthony at least as extensive as the foreland’s.

The Middle Devonian Acadian orogeny affected the entire Appalachian orogen from Newfoundland to Alabama with varying intensities of deformation and metamorphism. Part of the erosional debris from this uplifted area of tectonism was shed westward into the adjacent Appalachian basin. The basin subsidence and filling varied along its length, but not in concert with the adjacent tectonism. In the Early Devonian, the basin was stable with very little subsidence. Beginning in the Middle Devonian and continuing throughout the Late Devonian, the eastern part of the basin (now Pennsylvania and central New York) underwent rapid subsidence, where the largest volume and coarsest sediments were deposited as the Catskill Delta. To the southwest, grain size and thickness progressively decrease to the thin sequence of black shales in eastern Tennessee and Kentucky. To the north (northern New York, eastern Ontario and southern Quebec), no real evidence exists that a Devonian basin developed—the sediment from the orogenic zone probably passed over the Laurentian shield to the Michigan basin and perhaps elsewhere. The Acadian orogeny did not impinge upon the basin and its contained sediments, in distinct contrast with the Taconian and Alleghanian orogenies, both of which directly affected the basin, albeit in different ways. Reported Devonian structures within the basin are insignificant (growth folds), questionable (radiometric dating of faults), or probably incorrect (folding and angular unconformity). Acadian structures that occur between the basin and the main tectonic belt include open, upright folds and steep faults; cooling of Taconian metamorphic terrane during the Devonian and Early Carboniferous is also indicated for these rocks.