Topography of the Appalachians and Piedmont

The Great Valley

The Great Valley is adjacent to the Valley and Ridge region to the west (Inland Basin), extending from New York as far south as Georgia.

Precambrian Ridges

Extending up and down the Appalachian/Piedmont region is a rigid spine of Precambrian rock.

The crystalline, metamorphic rock has allowed the spine to resist erosion to some extent over the last several hundred million years, while the overlying younger sedimentary rocks have eroded away. The resistant nature of the Precambrian rock is responsible for the mountainous topography of the Green Mountains, the Berkshires, Hudson and Housatonic Highlands, Reading Prong, Ramapo Mountains, South Mountain and the Catoctin Mountains.

At the southern end of the Precambrian Ridges region, Pennsylvania’s South Mountain marks the northern extent of the Blue Ridge physiographic province. The Blue Ridge refers to the Precambrian rock making up the spine of the southern Appalachian Mountains from Pennsylvania to Georgia. The rocks of the Blue Ridge are bent into a large upward fold. The upward fold has many smaller folds superimposed upon it. The wrinkles cause the rolling topography of much of the mountainous Precambrian Ridge region. In Maryland, the Catoctin Mountains are part of the Blue Ridge region as well.

Taconic Mountains

The Taconic mountain-building event during the Ordovician created the modern Taconic Mountains of the Appalachian/Piedmont region, located between New York, Vermont, Massachusetts and Connecticut.

The Taconic volcanic islands, formed over the subduction zone of the North America and Baltica plates, were on a collision course with North America. As the volcanic islands drew nearer to the continent, they pushed ahead of them like a bulldozer the Cambrian and Ordovician sedimentary rocks of the seafloor. The crust continued to compress until the volcanic islands were sutured to the side of North America. The compression stacked slices of the seafloor on top of one another, like a collapsed telescope, and pushed the slices a good distance to the west. The Cambrian sedimentary rock resisted erosion, protecting the less-resistant underlying layers of rock. Today’s Taconic Mountains are a section of the stacked slices that have been isolated by erosion.

Piedmont

The Piedmont region abuts the Triassic Rift Basins of Pennsylvania and extends south through Maryland to the Coastal Plain boundary.

The topography of the Piedmont is primarily rolling hills, composed mostly of metamorphic rock that is uniform in its resistance to erosion. Therefore no ridges stand out in particular from differential weathering. There are a few notable exceptions, however, due to the presence of highly resistant rocks such as the quartzite of Sugarloaf Mountain. Near Baltimore, there are a series of ‘domes’ that have Precambrian gneiss in the middle, surrounded by rings of quartzite and marble. The Piedmont rocks have been squeezed so tightly and are so complexly deformed, that the folds have been overturned and folded, and later eroded to expose the resistant Precambrian gneiss that stand out in relief as domes.

Rift Basins

Two connected rift basins, the Gettysburg and Newark Basins, form lowlands in the Appalachian/Piedmont region.

The basins begin at the southeastern tip of New York and continue through New Jersey, Pennsylvania, and Maryland. The basins exist because of the rifting of Pangea during the Triassic and Jurassic. As the continents tore apart, cracks in the crust acted as fault planes on which blocks of crust slipped downward to form basins. The basins were filled with layers of less-resistant sedimentary rock as well layers of cooling lava on the surface, which formed basalt. Occasionally, the magma did not make it to the surface. Instead, it squeezed its way between the rock layers and cooled to form diabase. Over time, the basins were tilted and eroded, exposing the alternating layers of sedimentary and igneous rock. The layers of basalt and diabase are far more resistant to erosion than the sedimentary rock, so they stick out in relief as ridges while the surrounding sedimentary rock is eroded away.

The Palisades, along the west side of the Hudson River in New York and New Jersey, are resistant exposures of diabase. The Wachtung Mountains of New Jersey are tilted remnants of three basaltic lava flows. The basin remains a topographic low today, bounded on the west by the up-faulted Precambrian Ramapo Mountains.