Cover page need figure & title by Proposed Table of Contents

Historical Geology

In some areas, tectonic plates will move away from one another, or diverge, and in the void space created by this divergence, magma wells up from the mantle to create new plate material^iv. In other areas, tectonic plates will crash into one another, or converge, and create mountain systems, like the Himalayas or the Alps^v. Continents are portions of tectonic plates that consist of lighter, older rocks that ride passively on the moving plates^vi. Figure of diverging and converging plate margins w/ continent on top

Volcanoes, earthquakes, and developing mountains can be found on the leading edge of a continent, which is the site of tectonic plate convergence^vii. The trailing edge of a continent lies within a plate and is typically a low-relief, gentle coastal plain where thick sequences of sedimentary rocks can be found^viii.

Currently, the pattern of plate movement is away from the center of the Atlantic Ocean and towards the Pacific Ocean^ix. Around the margin of the Pacific Ocean (where plates are converging) the continents are mountainous, and volcanoes and earthquakes are prevalent^x. Near the Atlantic Ocean, the continents are much flatter with little geologic activity other than the deposition of sediments^xi.

Every half billion years or so, the flow direction in the mantle changes, and tectonic plate motion reverses^xii. When plate movement changes, margins of continents undergo a transformation; diverging plates now become converging plates, and vice versa^xiii. The history of plate movement show that those parts of continents that border ocean basins alternate repeatedly between two states: (1) the trailing edge margin of a continent with little tectonic activity and (2) the leading edge margin of a continent that has much tectonic activity^xiv. These half billion year cycles are called Wilson Cycles, named after J. T. Wilson who discovered them^xv.

The mountain systems that are created by convergent margins are called orogens, and the creation of an orogen is called an orogeny^xvi. Orogens form when previously passive margins become active margins due to the reversal of plate motion^xvii. This causes the sedimentary deposits laid down during the passive margin stage to be compressed, deformed, and intruded by magma from the mantle^xviii.

For more in depth information, please visit (http://pubs.usgs.gov/publications/text/dynamic.html)
The first orogeny to shape the Raystown Lake area was the Taconic Orogeny, which occurred from the mid to late Ordovician, or 450 to 435 million years ago^xix. The main event that caused most of the deformation in this orogeny was an early Ordovician reversal of plate motion between the volcanic island arc and the North American plate that caused the Iapetus Sea from spreading^xx. As a result of this reversal, the island arc and the North American plate collided, and the North American plate was subducted beneath the island arc^xxi. This collision caused an extensive amount of uplift and fracturing along the edges of the continents, and a basin and range were formed^xxii. Figure of the collision and the subsequent uplift

The next Orogenic event to deform the rocks around Raystown was the Acadian Orogeny, which occurred from the Silurian to the Devonian, or 380 to 350 million years ago^xxiii. The cause of the Acadian Orogeny was the late Silurian/mid Devonian collision of the northern part of Avalon with the North American plate^xxiv. The oceanic Avalon plate was subducted under the North American plate^xxv. Deltas soon formed on the North American plate, and as the mountains rose, the deltas were pushed farther inland, creating cyclic sequences of rock units^xxvi. Figure of the collision and uplift

The final Orogenic event that deformed rocks seen on our trips was the Alleghenian Orogeny, which occurred from the Mississippian to the Permian, or 320 to 250 million years ago^xxvii. The cause of the Alleghenian Orogeny is the continent-continent collision between the North American Plate and the Laurasian (African) Plate^xxviii. The Appalachian Mountains were formed on the North American plate, and were originally similar in size to the Himalayas^xxix. There were many faults formed due to this collision, including low-angle thrust faults and many anticlines were formed in the foreland^xxx. Figure of the collision and uplift

The subsequent erosion of the rock units around Raystown has changed the Himalaya-sized Mountains of 250 million years ago to the smaller scale Appalachian mountains and the Ridge and Valley province we see today. This erosion began at the end of the Alleghenian Orogeny and is still continuing today. The less resistant rocks of the area, like carbonates and shales, are more easily eroded than the more resistant rocks of the area, like the sandstones. This imbalance in erosional rates created the valleys, where the less resistant rocks have eroded, and ridges, where the more resistant rocks are located.