A Natural History of the Central Appalachians

Chapter 01: Introduction to the Central Appalachians

CHAPTER 01 THE REGION DEFINED HEREIN as the Central Appalachians consists of a system of linear ridges with intervening valleys, deeply dissected plateaus, and other landforms that produce a generally rugged terrain in western and southwestern Virginia, eastern and central West Virginia, western Maryland, and a portion of south central and southwestern Pennsylvania (fig. 1). It encompasses portions of the Blue Ridge, Ridge and Valley, and Appalachian Plateau physiographic provinces as delimited by Fenneman (1938). For the purposes of this book, the southern limit of the Central Appalachians is considered to correspond to the state boundary between Virginia and North Carolina (which occurs at 36° 30' N). The northern limit in south central and southwestern Pennsylvania is not as clearly defined but is represented by the approximate southernmost boundary of the area of eastern North America covered by ice during the last glaciation, something that will be considered in more detail later in this chapter. The eastern limit coincides with the eastern edge of the Blue Ridge Mountains, the western limit with a line drawn from the valley of the Monongahela River in north central West Virginia and southwestern Pennsylvania (at approximately 79° 51' W) to the point in southern West Virginia where the boundaries of West Virginia, Kentucky, and Virginia intersect (about 37° 32' N, 81° 58' W). Although this line was arbitrarily drawn simply to delimit the general extent of the region actually being considered in this book, most areas to the east of the line occur at elevations greater than two thousand feet. The Central Appalachians are only one part of the Appalachian system, which extends more than fifteen hundred miles from central Alabama to the island of Newfoundland in Canada. At its widest point, the entire system (including mountain ridges along with associated hills, plateaus, and other elevated portions of the landscape) is as much as three hundred miles across. Although not considered among the world’s great mountain ranges, the Appalachians are the single most important topographic feature in all of eastern North America. INTRODUCTION TO THE CENTRAL APPALACHIANS All the rocks associated with the plant fossils were sedimentary rocks, rocks formed from the mineral materials (or sediments) that settle and accumulate on the bottom of a body of water. The sediments that ultimately accounted for the formation of coal were produced by a range of mountains in what is now eastern Virginia. Vast amounts of sediments must have been involved, since there are places where sequences of rock layers containing coal seams are found over a vertical distance of more than one thousand feet, and the period of the earth’s history during which the coal seams were formed lasted for many millions of years. The most commonly encountered type of sedimentary rock in the Central Appalachians is sandstone, which forms from sand-sized mineral particles. Two other fundamentally different types of rocks marine origin. The predominant vegetation type, based on the evidence provided by the fossils themselves, was swamplike. Trees were present, but they were primitive forms quite unlike anything currently found in Central Appalachian forests. The fossils we collected that day were in a layer of shale (a type of rock formed largely from clay-sized mineral particles) just above what is usually referred to as a coal seam. The exact origin of coal was a disputed subject until the eighteenth century. Coal is essentially organic material (mostly from plants) that has been chemically and physically altered to the extent that its botanical origin is no longer readily apparent. As will be discussed later in this book, these coal seams have had a profound impact on parts of the Central Appalachians. FIGURE 2 Fossil of a seed fern that lived 300 million years ago 16 are metamorphic rocks and igneous rocks. The latter are formed directly from the mineral material that makes up the deeper, molten layers of the earth’s crust, while metamorphic rocks are formed from sedimentary rocks or igneous rocks whose basic structure is altered after being subjected to intense heat and pressure. Just what does the age of the fossils (approximately 300 million years) collected from the locality near Fairfax Stone mean in the context of the history of the earth? First of all, evidence from radiometric dating indicates that the earth itself is about 4.55 billion years old. Geologists and other scientists...