Physiography Topics

1. Introduction
2. Physiography v. Geology
3. Importance of Roanoke
• Coastal Plain
• Piedmont
• Mesozoic Basins
• Blue Ridge
• Valley and Ridge
• Appalachian Plateaus
• Virginia's Rivers


Physiographic Provinces v. Geologic Provinces

• Physiography describes the earth's surface. This includes the landforms (mountains and valleys, their shape and steepness), the way rivers flow across the land, and the way in which the land erodes. Geographers recognize physiographic provinces based on the shape of the land.

Geologic provinces are recognized by geologists as regions of similar rocks, structures and geologic history.

• There is a close relationship between Virginia's physiograpic and geologic provinces in that the geology affects the way the land erodes.

• Virginia has had an active geologic past. During the Paleozoic era, Virginia was the site of a plate collision resulting in the formation of the Ancestral Appalachian Mountains and the supercontinent of Pangaea.

• During the Mesozic era, Virginia was the site of rifting, that led to the formation of the Atlantic Ocean. At the same time, the Ancestral Appalachian mountains were eroding down. The Ancestral Appalachian Mountains were largely eroded away by the end of the Mesozoic.

• During the late Cenozoic, there has been uplift of the crust. This has caused rivers and streams to erode through the bedrock, revealing some of the deeper rocks and structures (tilted rocks, folds, and faults) that had formed during the building of the Ancestral Appalachians.

• The Modern Appalachians that we see today is a result of differential erosion - the different rocks weather at different rates. Hard, resistant rocks, such as sandstone or quartzite weather more slowly than soft rocks such as shale, limestone, slate, or schist. The harder rocks form the mountains while the softer rocks form the valleys.

Short Summary of Virginia's Geologic Provinces

• Virginia's geologic provinces are similar to (but not exactly the same) as the physiographic provinces. Virginia has 5 major geologic provinces, although some geologists consider the Mesozoic Basins to be a sixth province.

Geologic Provinces of Virginia. The heavier black lines are the province boundaries. Notice that the Mesozoic Basins (blue colored areas) are found mostly in the Piedmont but can overlap into the geologic Blue Ridge. Also notice that north of Roanoke, the geologic Blue Ridge is wider than the physiographic Blue Ridge.

Coastal Plain Geology

• The Coastal Plain is composed of sedimentary rocks of Jurassic to Cenozoic age and contain the youngest rocks in Virginia.

• The sedimentary rock layers are near horizontal with a slight dip to the east. The sediments are sandy and muddy and contain abundant fossils. This results in the older rocks (Cretaceous) exposed along the Fall Line and the younger rocks (Quaternary) exposed next to the ocean. Jurassic rocks are completely buried.

• The Coastal Plain is the landward extension of the continental shelf. Many of the rocks were deposited during times of higher sea level and lower crustal levels. In the past, the position of the shoreline was west of present-day Richmond.

• The nearly flat orientation of the sedimentary layers, and their lowland location near the ocean is the cause of the flat landscape that characterizes the physiographic Coastal Plain.

Piedmont Geology

• The Piedmont is composed of primarily igneous and metamorphic rocks of Proterozoic to Paleozoic age.

• The Piedmont is geologically complex and is divided by geologists into several "terranes" or subprovinces.

• Some of the terranes are composed of belts of volcanic rocks that were active in the Iapetus Ocean. Other belts contain metamorphosed sedimentary and igneous intrusive rocks. Large faults separate the terranes from each other.

• All of the rocks were strongly folded, faulted, and metamorphosed during the time of the plate collisions that led to the building of the Ancestral Appalachians and the Pangaea supercontinent during the Paleozoic.

• The rocks are deeply weathered to form a thick soil that covers the bedrock.

• Many different types of rocks are found in the Piedmont, and the differences in how they weather cause much of the rolling landscape we see in the physiographic Piedmont.

Mesozoic Basins Geology

• The Mesozoic Basins consist of Triassic to Jurassic age sedimentary and igenous rocks. These rocks are distinctively younger than the surrounding Piedmont and Blue Ridge.

• Because they are younger than the formation of the Ancestral Appalachians, they are not strongly metamorphosed or deformed.

• They formed during the break-up of Pangaea and opening of the Atlantic Ocean.

• Dinosaur footprint fossils are found near Culpeper.

• Many of the sedimentary rocks are easily eroded, causing the Mesozoic Basins areas to form regions of lower and flatter landscapes than the surrounding Piedmont or Blue Ridge. This is what characterizes physiographic Mesozoic Basins.

Blue Ridge Geology

• The central part of the Blue Ridge contains one-billion year old intrusions that are the remains of a mountain range (the Grenville Mountains) that predated the Appalachian Mountains. These are the oldest rocks in Virginia.

• Other parts of the Blue Ridge contain younger volcanic and sedimentary rock (Neoproterozoic to Cambrian).

• Volcanic rocks from 750 million ago years exist in the Mount Rogers area.

• Comparing the physiographic province boundary between the Blue Ridge and Piedmont with the geologic province boundary, geologists find rocks of similar age and type on both sides of the physiographic boundary. Geologists prefer to place the Blue Ridge-Piedmont boundary approximately 15-30 miles to the east at a fault zone where the rocks change in age and type.

• As a result, Charlottesville and Lynchburg are in the physiographic Piedmont according to geographers, but are in the geologic Blue Ridge according to geologists.

• North of Roanoke, part of the geologic Blue Ridge features a band of hard quartzite and resistant igneous rock that creates the distinctive narrow ridge that characterizes the physiographic Blue Ridge.

• South of Roanoke, the land has weathered to a broad upland with an eastern escarpment that characterizes the physiographic Blue Ridge.

Valley and Ridge Geology

• The Valley and Ridge consists of Cambrian to Mississippian sedimentary rocks that include carbonates, shales and sandstones that were faulted, tilted, and folded during the Pennsylavanian and Permian periods.

• Thrust faults have repeated the rock layers. We see this now as repeated parallel sandstone ridges and shale and carbonate valleys.

• A prominent belt of Cambrian and Ordovician carbonate rocks are exposed in the "Great Valley" of Virginia. This includes the Shenandoah Valley, the Roanoke Valley, and the New River Valley.

• Interstate 81 follows the Great Valley and connects the cities of Winchester, Harrisonburg, Staunton, Roanoke, Christiansburg, Radford, Wytheville, and Bristol.

• The tilted orientation of the various sedimentary rocks creates differential erosion, where the harder sandstone erodes more slowly forming distinctive ridges, while softer and easily eroded shale and carbonate forms the valleys. The parallel ridge and valley landscape is typical of the physiographic Valley and Ridge.

Appalachian Plateaus Geology

• The Appalachian Plateaus consists of much of the same Paleozoic sedimentary rocks as found in the Valley and Ridge, but are horizontally lying or broadly and gently folded.

• This forms a stack of rocks with older (Cambrian) rocks on the bottom and younger (Pennsylvanian) rocks on the top.

• Present-day erosion exposes only the youngest rocks at the top of the stack. This includes the Pennsylvanian rocks that contain coal.

• The boundary between the Valley and Ridge and the Appalachian Plateaus is where the rocks change from being steeply dipping in the Valley and Ridge to being nearly horizontal in the Appalachian Plateaus.

• Commonly, the top-most Pennsylvanian rocks are composed of hard sandstone, forming mountain tops. Rivers have through the sandstone to expose softer rocks below, forming narrow deep valleys. This type of landscape is typical of the physiographic Appalachian Plateaus.