Our odyssey proceeds west into the heart of the Appalachian fold belt that dominates the landforms of the Appalachians. The U.S. Appalachians extend into northern Georgia and Alabama, south of which they go underground, being buried by sediments comprising the Coastal Plains. Actually, the general orogenic belt that includes the Appalachian Mountains of eastern North America extends westward across the Mississippi River through the Ouachita Mountains of Arkansas/Oklahoma before dying out in Texas. Its northern end passes through Maine into Nova Scotia and Newfoundland and has been traced into the Caledonide mountain systems in the British Isles and Norway. The Atlas Mountains of northwestern Africa also link to the Appalachians. This once continuous belt split at the end of the Paleozoic Age, as the present continents broke apart and their tectonic plates then drifted in several directions. Before then, the Appalachians built up over about 300 million years as subduction zones, back arc basins, and accreted terranes were added to ancient continental nuclei that eventually led to the supercontinent of Pangaea. Three major orogenies mark the history of the Appalachians: Taconic (Ordovician in age), Acadian (late Devonian), and Alleghenian (Permian). Pangaea split apart about 250 million years ago, with one segment (the Appalachians) embedded in the North American Plate and other remnants remaining in the plates that include Europe and Africa (Gondwanaland). The complexity of the geology is evident in this tectonostratigraphic map modified from one made by H. Williams of Memorial University in Newfoundland.
The physiographic subdivisions of the North American Appalachians are indicated in these two maps:
Another illustration that helps to understand the Appalachians as a structural complex is this cross-section of the main geologic components that are now underlying the surface physiography:
This cross-section indicates that the Appalachians are made up of a series of segments each bounded by a major thrust sheet or belt. That is brought out in more detail in the next figure which shows the sheets present in the Appalachian portion of New Jersey:
In central Pennsylvania, the segment known as the Valley and Ridge has very distinctive topography: the ridges are curved, some shaped almost like an arrowhead, and have generally uniform elevations (not pointed or irregular like many western mountains). They are even today forest-covered but the valleys in between are flat to rolling and commonly developed for agriculture. The next two aerial photos, the first vertical and the second oblique shows typical expressions of this land use - forested ridges, valleys almost completely given over to farming:
That second photo is included in this unusually colored Fall Landsat MSS composite. Most of the interior of this image depicts the Valley and Ridge province which runs from southernmost New York state to Alabama. The area shown includes the center of Pennsylvania.
The following sketch map names the principal mountain ridges and other features found in this scene:
Here, the sedimentary rocks that compiled thicknesses up to 15,250 m (50,000 ft) in parts of the geosynclinal deposition basin during the Paleozoic era, were strongly folded into anticlines (uparches) and synclines (downbends). These rocks were also shoved westward along thrust fault sheets. Erosion since the Appalachian Revolution has repeatedly sculpted ridges out of harder rocks (most commonly, sandstones) and valleys out of weaker rocks (such as shales). This erosion results in a zig-zag pattern developed from folds that also curve sharply as they pinch out, as seen in plan or horizontal perspective. The ridges, few of which are higher than 460 m (1,500 ft), remain heavily forested but the valleys are mostly cleared for agriculture. Toward the lower right corner, shales and limestones of early Paleozoic age underlay a broader lowlands, the Great Valley of Pennsylvania (the northern extension of the Shenandoah Valley of Virginia).
The wide isolated ridge to the southeast is South Mountain/Catoctin Mountain, a complex of igneous-sedimentary-metamorphic rocks, which connects with similar rocks in the Blue Ridge in Virginia. The Appalachian Plateau occupies the western (left) part of the image and continues for several hundred kilometers into eastern Ohio. Its rocks are also Paleozoic sedimentary units that were not strongly folded (the compression effects die down in this region) and are still subhorizontal. However, the rock units were lifted en masse during the orogeny and, more recently, to elevations high enough to foster deep local stream cutting. The stream courses here are typical of the dendritic drainage pattern that develops on elevated terrain underlain by rocks of similar resistance to erosion. The topography of the Appalachian Plateau is distinctly mountainous as indicated in the aerial photo below. Some individual mountains have flat tops in accord with the control by underlying horizontal rocks.
The Valley and Ridge topography is visually brought out more dramatically in winter scenes, when the sun angle is lower, so that shadowing is emphasized. This Landsat-1 image illustrates this effect:
Radar has produced many splendid images of the Folded Appalachians, since the ridges stand out sharply because of the radar shadow effect. This X-band image, acquired from an aircraft by Intera Technologies of Ottawa, Canada, shows a swath 50 km (31 miles) wide that is contained in the Landsat image (you can locate it in that image by the distinctive fold pattern near the center of the radar scene). Look direction is from the west (left). The city of Altoona, PA is the bright, stretched out speckled pattern just above left center. The upper left corner shows the topography of the Appalachian Plateau, another subprovince of this mountain system.
So far, we have concentrated on the geology and physiography of the Appalachians in Pennsylvania. We will digress for a bit to look at an historical event that took place in Pennsylvania on July 1st to the 3rd, 1863. We focus on the town of Gettysburg, at that time a small hamlet where shoe making was, along with farming, the main means of livelihood. Today the town is larger but is still not visible in this Landsat image (it is near the center bottom):
Even today Gettysburg does not reveal its history when seen in a higher resolution space image:
Yet on those fateful three days in 1863, the Union and Confederate Armies, under Generals Meade and Lee respectively, fought the biggest battle ever waged in the Western Hemisphere. This map summarizes the battle:
The first day's battle took place west of the town near the Lutheran Seminary. It was a standoff. The second day's battle was crucial. Lee sent Longstreets's army to the south in an effort to get around the Union left flank on Cemetery Ridge. Bloody battles took place in the Devils Den, the Wheat Field and the Peach Orchard.
But the seminal moment of the battle, and perhaps the whole Civil War occurred on the 2nd day. This is superbly re-enacted in the extraordinary Ted Turner movie "Gettysburg", which we strongly recommend that you vie (it's on a DVD). It is based on Michael Shaara's Pulitzer Prize winning novel "The Killer Angels". At Little Round Top, Col. Joshua Lawrence Chamberlain positioned his 20th Maine regiment as the southern anchor of the Union lines. If Col. Ames and his Alabamans of the Confederate Army could overrun the 20th Maine, the whole Union line would have been "rolled up", probably leading to a huge Confederate victory, and perhaps a decisive situation that would have ended the Civil War in the South's favor. Here is a clip from the movie showing Chamberlain (played by Jeff Daniels) as he led his soldiers on a charge (after issuing the simple command "Fix Bayonets") that captured Big Round Top:
On the 3rd day, General Lee decided on a direct frontal assault from Seminary Ridge against the center of the Union lines on Cemetery Ridge. This is a map synopsis of this most famous of all American charges, which depicts the convergence at the place they named the Angle, with its (then) lone tree.:
Some 17000 Confederate troops left the woods onto open fields in what is known as Pickett's Charge. Some 'Rebs' broke into Union lines but were overwhelmed. This led a a mass retreat after nearly half of the participating Confederates were wounded or killed. Lee's pathetic comment "It's all my fault" summarized the debacle. Gettysburg was the high water mark of the South's fight against the North, and in the view of historians stands as the beginning of the end of the Confederacy. Here is a photo of a re-enactment of Pickett's Charge (this is done every year by volunteers who make such military events one of their hobbies):
All told, some 51000 casualties (killed, wounded, captured) on both sided attest to this biggest of all battles in the Civil War (and the largest ever in the Western Hemisphere). Today Gettysburg is the most visited site in America's military history. Throughout the battlefield are hundreds of monuments, along with cannon. Typical is this scene by the lone tree at the Angle, the point of farthest penetration during Pickett's Charge.
Resuming our journey, let's zero in on Pittsburgh. This wide-angle photo is a panoramic view of the Golden Triangle and downtown Pittsburgh.
Now, fit that view into this Terra ASTER image of greater Pittsburgh:
Much more detail is evident in this Quickbird 2 meter resolution image; the stadiums used by the (2009 World Champion) Pittsburgh Steelers and the Pittsburgh Pirates are conspicuous near the left edge:
Two rivers, the Monongahela on the south and the Allegheny on the north, meet at the triangle to form the mighty Ohio River as these waters all move towards their meeting with the Mississippi.
Lets leave Pennsylvania. We now take a detour in our flight to look at parts of the northern, central and southern Appalachians. The first scene - an astronaut infrared photo - shows the Piedmont as seen towards the northwest, with the Greater New York area in the bottom center. Below that is a roadcut that displays strongly folded metamorphic rocks - the most prevalent rock types over most of the Piedmont.
This Landsat scene includes part of western Virginia and eastern Kentucky:
This scene shows two distinctly different topographic expressions: 1) ridges and a broad valley (extension of the Shenandoah Valley), and elongate mountains lying mostly in western Virginia, which here includes the Fold Belt of Pennsylvania, now much narrowed, and 2) dissected plateau rocks forming small hills constructed from horizontal rocks which are incised by a network (dendritic) of small streams. The ground photo below was taken in western Virginia and shows the topography typical of the Blue Ridge Mountains.
The next scene from space shows much of the Appalachian Belt from Pennsylvania through North Carolina, as depicted in this Day-Vis (0.5 - 1.1 Ám) image made by the Heat Capacity Mapping Radiometer on HCMM (see Section 9-8 for a description of this 1978 mission).
The swath width covered by the image is 715 km (444 miles); spatial resolution is 500 m (1,640 ft). To get your bearings at this scale, locate Washington, D.C. and Pittsburgh, PA in an Atlas. This overview scene is shows the entire central part of the Appalachian Mountain system in a single view. The fold belt of the Valley and Ridge Province is distinctive. At the northern end, in Pennsylvania, the zig-zag pattern described above is determined by the closed or pitching anticlines and synclines. This type of folding largely disappears to the south, being replaced by subparallel ridges separated by valleys. Note the sharp bend or kink in the belt near Roanoke, VA. This feature is, in part, fault-controlled. We can see the Blue Ridge because it is a thin dark line (plus South Mountain) that lies to the east of the Shenandoah Valley in Virginia which continues as the Great Valley through Pennsylvania (both in light tones).
The Piedmont to the southeast is not particularly obvious in this image. Its terrain does not express well because of generally low relief. Its boundary with the Coastal Plains is the Fall Line, so-called because waterfalls form along some streams as they descend from harder Piedmont rocks to softer rocks toward the coast. Its boundary is also indistinct (diffuse), but generally the Piedmont terrain has a darker tone is darker than that of the Coastal Plains (more farming and less trees). The Cumberland Plateau, west of the belt, is much darker because of heavy forestation. As that province grades into the Allegheny Plateau, the dissected morphology remains, but forest cover diminishes.
The Piedmont and Coastal Plains extend from New Jersey on the North to Alabama on the South. This next image (made from MODIS data?) of eastern Virginia and Maryland shows how differences in both soil and land use can be discerned by color differences. Inspect this:
The dark blue in the upper left establishes the Blue Ridge. To its east, in orange tones (such tones also mark the Shenandoah Valley), are the Catoctin Mountains and other hills that are the higher part of the Piedmont. The Piedmont to the east is roughly the area that contains considerable darker olive colors. The Coastal Plains further east and south have wine-purples and reds as prevailing colors. Note the area around Washington, D.C. (itself in blue), where the Fall Line runs right through the city.
Lets now venture south along the Appalachians. A section of the Great Smoky Mountains, north of Ashville, occupies the lower right corner of the image. This is an assemblage of relatively homogeneous crystalline rocks that includes the Blue Ridge on their eastern side. Running diagonally through the image center is part of the Tennessee Valley, which is made up of rolling hills underlain by more easily eroded limestones that tie into the Shenandoah Valley to the north. The Valley and Ridge is represented here by continuous folds of steeply dipping rocks that are also involved in the Pine Mountain thrust. The upper left is the beginning of the Cumberland Plateau, a southern extension of the Allegheny Plateau of Pennsylvania. Note the similarities and differences of this subscene and the earlier one. Here the South Mountain counterparts are much wider.
Chattanooga, Tennessee lies within the Appalachian Fold Belt. It was the site of a major battle (Chickamauga) in the Civil War. Here is a Landsat subscene:
What is normally thought of as the southern tip of the Appalachians occurs where the mountains seem to end abruptly in northern Alabama. The city of Birmingham lies right at the tip, located in part in inland Coastal Plains. Look carefully first at this Landsat mosaic of all of Alabama. Birmingham is the red blotch in the upper center.
If you didn't find it, use this enlargement from a different Landsat scene to set the context.
Here is a more detailed look at the city.
Birmingham has an impressive skyline:
North of Birmingham is the city of Huntsville, AL (near the center). There, the Marshall Space Flight Center, where the Saturn-V rocket was designed, is located. Here is a Landsat-1 view - again, parts of the Appalachian Ridge system and the Appalachian Plateau are evident; the Tennesee River and Bull Shoals also appear.:
Although it lies on the coastal plains of Alabama, this seems a proper place to show an image of Mobile. This city of a half million is situated around Mobile Bay where, in August of 1864, Admiral David Farragut and a Union flotilla defeated the Confederate ships guarding the Bay, thus sealing off the last major seaport through which supplies to the southern states could be landed from overseas sources:
For the writer (NMS), this next image remains an especially fond memory. On the first day that imagery from ERTS-1 was expected to become available to NASA personnel, about 100 program management people and Goddard scientists were gathered at the ERTS Data Receiving Center (in Building 22 at Goddard) to see the results. The initial RBV image was a disappointing polaroid of the Dallas-Fort Worth area (see page 4-5) that showed almost no details. Then, an hour later the first cannister containing a continuous strip of 70mm black and white transparencies representing the first MSS images was brought into the room. Because none of the guests knew how to use the image projection unit, I was delegated to load it for viewing. As I scrolled downward from imagery taken over Minnesota down to southern Missouri, all views were completely cloud-covered. Disappointing! Then, hallelujah, the next image - the Ouachita Mountains in Arkansas - was completely free of clouds. It was stunning! Much beyond my expectations. Thus did I convert to ERTS/Landsat and to remote sensing in general.