I-68/I-70: A WINDOW TO THE APPALACHIANS by Dr. John J. Renton Dept. of Geology & Geography West Virginia University Morgantown, WV Introduction The Appalachian Mountains are probably the most studied mountains on Earth. Many of our modern ideas as to the origin of major mountain systems evolved from early investigations of the Appalachian region. The Appalachians offer a unique opportunity to experience the various components of an entire mountain system within a relatively short distance and period of time. Compared to the extensive areas occupied by other mountain systems such as the Rockies and the Alps, the Appalachians are relatively narrow and can be easily crossed within a few hours driving time. Following I-68 and I-70 between Morgantown, WV, and Frederick, Maryland, for example, one can visit all of the major structural components within the Appalachians within a distance of about 160 miles. Before I continue, I would like to clarify references to the Allegheny and Appalachian mountains. The Allegheny Mountains were created about 250 million years ago when continents collided during the Alleghenian Orogeny to form the super-continent of Pangea (Figure 1). As the continents collided, a range of mountains were created in much the same fashion that the Himalaya Mountains are now being formed by the collision of India and Asia. About 50 million years after its Figure 1 1 creation, Pangea began to break up with the break occurring parallel to the axis of the original mountains. As the pieces that were to become our present continents moved away from each other, the Indian, Atlantic, and Arctic oceans were created (Figure 2). As North America split away from Europe and North Africa, part of the mountain chain went with North America to eventually become our present Appalachians while the rest of the mountain chain remained with Africa and is now to be found in North Africa as the Atlas Mountains a. Pre-Pangea Figure 2 with some of the rocks exposed in Europe from northern Ireland through Scotland to Scandinavia. Over the next 100 million or so years, the combined efforts of weathering, mass wasting, and erosion wore the topography of the original Allegheny Mountains down to a flat, featureless plain nearly at sealevel. About 60 million years ago, the entire eastern b. 250 MYA - The Formation of Pangea portion of North America from the continental interior to the east coast was uplifted into a broad arch with the apex rising about 5,000 feet (1,525 m) above sea level. As the existing streams, rejuvenated by the uplift, began to carve their channels down to the newly formed baselevel, the present topography of the Appalachian region emerged. A schematic representation of the evolutionary history of the Appalachians is illustrated in Figure 3. c. 100 MYA - After Formation of Pangea It is important to keep in mind as you traverse the Appalachians along the route of I-68 and I-70 that the structures you see, the folds and faults, sea level are very old, dating back to the Alleghenian Orogeny 250 million years ago, while the present topography of the region we now know as the Appalachian Mountains is the result of erosion that has taken place over the past 60 million years. d. After uplift and Erosion Figure 3 2 Basic Geologic Information and Principles In order to help you better understand what you will see along the route of the trip, I have included a review of some basic geologic topics. Geologic Structures: The geologic structures are folds, faults, and joints. The dominant structures you will see during this field trip are folds. Based on their shape, folds are divided into symmetric, asymmetric, overturned, and recumbent (Figure 4). Three criteria are used to describe the shape of folds: 1) the attitude of Figure 4. the limbs, 2) the attitude of the axial plane, and 3) the angle, called the dip, that the limbs make with the horizontal. The axial plane is an imaginary plane drawn parallel to the long dimension of the fold that attempts to divide the cross section into equal halves. A symmetric fold has a vertical axial plane with limbs that dip away from each other at equal angles of dip (refer to Figure 2). The axial plane of an asymmetric fold is inclined with the limbs dipping away from each other, but at different angles of dip. Overturned folds are those whose limbs dip in the same direction. A recumbent fold is defined as one whose axial plane and limbs approach the horizontal. Except for the recumbent style, you will see examples Figure 5. of all of the fold types as the trip progresses. The three major types of faults, thrust (or reverse), normal, and strike- slip are illustrated in Figure 5. Although faults are present throughout the Appalachians where their presence plays a major role in much of topography and structures you will see, their presence is not as evident at the surface as are the folds. Of the three types of faults, thrust faults dominate throughout the Appalachians, reflecting the compressional forces responsible for the formation of the mountains. 3 Joints are fractures in rocks along which there has been no appreciable movement. All rocks Figure 6: Stratigraphic Column everywhere contain joints as will be evident in every rock outcrop exposed along the roadway. Pennsylvanian System Dunkard Group Stratigraphy: Stratigraphy is the study of sedimentary rocks. A characteristic of all sedimentary Monongahela Formation rocks is that they are bedded. Initially, all sediments are laid down horizontally with the oldest layer Conemaugh Formation Allegheny Formation being on the bottom with the overlying layers becoming progressively younger. Whether they now Pottsville Formation appear horizontal at the surface depends upon the kind of deformation they have been subjected to Mississippian System Mauch Chunk Formation subsequent to their being converted into rock. If they are simply uplifted, the bedding of the rocks Greenbrier Formation Pocono Group exposed at Earth’s surface will still be horizontal. An excellent example are the rocks seen in the Purslane sandstone Grand Canyon that are the result of the vertical uplift of the Colorado Plateau over the past 20 million Rockwell Formation Devonian System years. If, on the other hand, the rocks were subjected to compressive forces, as in the case of the Hampshire Formation Chemung Formation Appalachians, the rocks will undergo folding. Foreknobs Formation Whether flat-lying or folded, the sedimentary rocks found in any area are summarized in a Brallier Formation Hamilton Group stratigraphic column which shows the vertical sequence of the rocks and their relative ages. The Mahatango Formation stratigraphic column for this trip is found in Figure 6. One of the most difficult aspects of any Harrell/Marcellus shale Needmore shale geology field trip is to keep track of the various rock units you will be seeing. I would suggest Oriskany sandstone Helderberg Formation that you keep Figure 6 conveniently available so that you can readily refer to it to identify the Keyser formation stratigraphic location of the rocks you will see. Silurian System Tonoloway limestone Wills Creek Formation Bedding Attitude: Throughout most of this trip, the sedimentary rocks were uplifted with Bloomsburg Formation McKenzie Formation compressional deformation that resulted in the beds being folded and faulted. In the area of Clinton Group Rochester shale Morgantown and to the west, in a region called the Appalachian Low Plateau. The structures Keefer sandstone underlying the Low Plateau are symmetrical folds with amplitudes so small that the rocks appear Rose Hill Formation Tuscarora sandstone horizontal. A few miles east of Morgantown, you will leave the Low Plateau and enter The Ordovician System Appalachian High Plateau. While the structures underlying the High Plateau are mostly symmetrical Juniata Formation Martinsburg Formation folds, the amplitudes of the folds are high enough that the structures form ridges. As you drive Chambersburg Formation Chambersburg limestone eastward across the High Plateau, the fold amplitudes and the angle of the bedding observed in New Market limestone the outcrops along the roadway changes. In one exposure, the bedding may be horizontal while in Row Park limestone Beekmantown Group others, it may dip toward you (to the west) or away from you (to the east) Get into the habit of paying Pinesburg Station dolomite attention to the attitude of the bedding of the rocks along the roadway. In areas of folded rocks, the Rockdale Run Formation Stonehenge limestone bedding will be horizontal as you cross the axis of an anticline or a syncline. As you approach the Cambrian System Conococheague limestone Elbrook limestone Waynesboro Formation Weverton Formation pre-Cambrian System Catoctin Formation 4 axis of an anticline, the bedding will dip toward you (Figure 7). Because all of the folds in the Appalachians trend NE-SW, this will be to the west. Note also that as you approach the axis of an anticline, the rocks become progressively older, that is you are going down stratigraphically. In any area of folded rocks, the oldest rocks will be exposed in the axis of a breached anticline or in the center of a water gap cutting through an anticlinal fold. Conversely, as you drive toward the axis of a syncline, the bedding will dip away from you (in this case to the east) and the rocks will become progressively younger; you will be going up stratigraphically. In an area of folded rocks, the youngest rocks will be found in the axial region of a syncline which, most often, will be a valley floor. Monitoring the attitude of the bedding as you drive is the best way for you to keep track of the folds that you will be crossing. Figure 7. Physiographic Provinces Continents are subdivided into physiographic provinces. A physiographic province is defined as a region of which all parts are similar in geologic Figure 8. structure and climate and which has had a unified geomorphic history; a region whose pattern of landforms differ significantly from that of adjacent regions. The Appalachian region is divided into four physiographic provinces, which are from west to east: the Appalachian Plateaus, the Valley and Ridge, the Blue Ridge, and the Piedmont (Figure 8). While the trip will terminate in the Piedmont, many geologists do not consider the Piedmont Province as being part of the Appalachians. 5 As the name implies, there are several Appalachian plateaus (Figure 9). The most northerly plateau, the Mohawk Plateau, extends northward from the Mohawk and Hudson rivers into New England. Next, the Glaciated Allegheny Plateau extends from the Mohawk Plateau southward to the maximum extent of the Pleistocene ice advance just south of the New York-Pennsylvanian border. The largest plateau, the Unglaciated Plateau, extending southward to Kentucky, is divided into an eastern Allegheny Mountain Section or High Plateau, a western Low Plateau, and the southernmost Cumberlsand Plateau. The terms, High and Low Plateau are my own means to designate the two basic regions within the Unglaciated Plateau. The Valley and Ridge Province is subdivided into the eastern Great Valley Section and the western Appalachian Mountain Section. Appalachian Low Plateau: The Appalachian Low Plateau is the westernmost of the Appalachian Plateaus Physiographic provinces. The Low Plateau is largely underlain by rocks of Pennsylvanian age (refer to stratigraphic column in Figure 6). Morgantown is located near the eastern edge of the Low Plateau. As previously mentioned, the rocks exposed in road cuts from Morgantown are deformed into very low amplitude symmetrical folds that appear as though they are perfectly horizontal. The reason why the bedding appears to be Figure 9. horizontal is because the dips on the limbs are so low (generally less than 2o or 3o) that the human eye cannot discern any dip on the beds. Although faults are probably present in the area, they are of very low displacement and are rarely seen at the surface. Typical of areas underlain by essentially horizontal rocks, the dominant stream pattern throughout the Low Plateau is dendritic (Figure 10). In general, the streams have progressed to the mature stage of geomorphic development with most of the major Figure 10. streams having developed floodplains and meanders. Other characteristics of a mature 6 topography include average distances from hilltops to valley floors, the relief, of a few hundred feet, with adjacent streams separated by rounded hills with relatively shallow slopes. The topography in the vicinity of Morgantown is typical of the province. Most geologists consider the western limit for the Low Plateau to be located in central Ohio where glacial deposits and older Paleozoic rocks crop out. The easternmost limit of the Low Plateau and the beginning of the High Plateau is located just east of Morgantown at the western base of Chestnut Ridge. Chestnut Ridge is a dominant topographic feature that extends from just north of Uniontown, Pennsylvania, southwestward to Weston, West Virginia. The structure responsibly for the ridge, Chestnut Ridge Anticline, is a symmetrical anticline. The NE-SW trend of Chestnut Ridge, and of all of the other major Appalachian structures, is the result of the great pressures that were applied from the southeast when the Allegheny Mountains were created about 250 million years ago. One can compare and contrast the topography of the high and low plateaus by referring to the Morgantown 7.5' topographic map available from the West Virginia Geologic and Economic Survey. The Alleghenian Mountain Section or Appalachian High Plateau: The Allegheny Mountain Section of the Unglaciated Plateau, or Appalachian High Plateau, extends from Chestnut Ridge on the west to the Allegheny Structural Front on the east (refer to Figure 8). Rocks within the province range in age from Pennsylvanian to Devonian with the oldest rocks being exposed in the axial regions of breached anticlines (Figure 11). The dominant structures within the High Plateau are relatively high amplitude symmetrical folds with the increased amplitudes of the folds being due to the presence Figure 11. of thrust faults within the anticlinal cores that provided a vertical displacement of the rocks within the fold (Figure 12). The effect of the increased fold amplitudes is an increase in regional relief that results in a more rugged terrain than that seen Figure 12. throughout the Low Plateau to the west. Because of the increased relief of the High Plateau over that of the Low Plateau to the west, the Appalachian High Plateau has also been referred to as the Allegheny Mountains or the open-fold section of the Appalachian Plateaus. From central Pennsylvania to central West Virginia, the eastern-most edge of the Appalachian High Plateau is called the Allegheny Strucutral Front (Figure 13). It is important to note that the highest elevations in the Appalachians are Figure 13. found just west of the Allegheny Structural Front and not, as one might expect, in 7 the Appalachian Mountains to the east. The reason for this rather anomalous situation is due to the fact that the apex of the broad arch that was uplifted about 60 million years ago from the Mississippi River Valley to the Atlantic Ocean is located along the eastern edge of the Appalachian Plateaus. An important economic aspect of the Appalachian Plateau are the mineable coal deposits that are located within its boundaries with the coal beds being contained within the Pennsylvanian rocks. One particular coal bed, the Pittsburgh coal, often referred to as the most valuable rock layer in the world, accounts for 25% of all the coal mined in West Virginia. Appalachian Mountain Section: The Appalachian Mountain Figure 14 Section of the Valley and Ridge Physiographic Province extends from the Allegheny Structural Front to the Great Valley and constitutes what is commonly referred to as the Appalachian Mountains. The name reflects the dominance of northeast-southwest-trending parallel valleys and ridges within the province. For the most part, the ridges are high amplitude asymmetric to overturned folds, commonly broken on the western limb by high-angle thrust faults (Figure 14). The oversteepening of the folds to the west indicates an east to west direction of rock transport during the original mountain building episode. The wavelength of the folds in the Valley and Ridge (the distance between fold axes) is significantly less than the wavelength of the more open folds in the High Plateau to the west. Many of the anticlinal structures have been breached by erosion, exposing the oldest rocks of the region, the Ordovician, within anticlinal valleys. An excellent example of a valley that formed as the result of the breaching of an anticline that you may be familiar with is Germany Valley located in Pendleton County, West Virginia (Figure 15). There are occasional synclinal ridges that you Figure 15 will observe along your route, the most well known being 8 Sideling Hill, Maryland. Synclinal ridges are formed as Figure 16a adjacent anticlines are breached with the subsequently valleys eventually being eroded below the elevation of the adjacent synclinal valleys, creating what geologists call Ainverted topography@(Figure 16a and 16b). Eventually, the resistant rock layer becomes a tough caprock that remains high above the surrounding terrain as the rocks within the adjacent anticlinal structures are removed. It is important to emphasize that the summits of all of the ridges within the Appalachian Mountains are lower in elevation that of the easternmost edge of the Appalachian Plateau with the elevations of each more easterly ridge being generally lower than that to the west. This relationship results in the rather unique situation of one going down into the Appalachian Mountains when approached from the west. Figure 17 Because of the dominant northeast-southwest trending ridges and valleys, the stream pattern within the Valley and Ridge is trellis with the major streams cutting across the structures and tributaries draining the valleys (Figure 17). Figure 16b There is evidence that many, if not most, of the water gaps that cut across anticlinal structures follow vertical fault zones with the streams taking advantage of the zone of weakness. 9 The Great Valley Section: The Great Valley Section of the Valley and Ridge Province, commonly referred to as the Shenandoah Valley, extends eastward from the easternmost ridge of the Appalachian Mountain Section of the Valley and Ridge to the base of the Blue Ridge Mountains. Rocks within the valley are nearly all Cambrian and Ordovician limestones. Because of the water soluble nature of calcium carbonate, the rocks have been dissolved down to the mean level of the streams, resulting in a broad, flat valley. The few low ridges observed within the valley are in large part due to the occasional non-carbonate rock layer that is a bit more resistant to erosion. Although natural exposures of limestones are limited because of their solubility, numerous limestone rock outcrops can be seen in the fields. Typical of Figure 18 regions underlain by limestones, the soils are thin and are composed almost entirely of the insoluble materials, clay minerals and quartz, that were originally contained within the limestones. Also typical of areas underlain by limestones, the Great Valley shows extensive development of karst topography readily identified by the extensive number of sinkholes throughout the region (Figure 18). Because of their alkaline character, the soils are ideal for the growth of calcium-loving grasses which explains the widespread use of the land for the grazing of cattle. The red to orange color of the soil is typical of areas underlain by limestone and is due to the fact that the Table 1 insoluble materials released by the dissolution of the limestones are coated by combinations of red iron oxides, Fe O , and yellow iron oxy- 2 3 hydroxides, FeO(OH). Structurally, the valley is underlain by many northeast-southwest trending high- displacement thrust faults and highly deformed asymmetric and overturned folds which, because of the limited exposures, are not always easy to observe. As one approaches the easternmost portion of the valley, the rocks show evidence of low-level metamorphism, the result of being located closer to the original zone of deformation (Table 1). 10