Former Speaker of the House Tip O’Neill rang true when he said: “All politics is local.” So it is with geology. While celebrities like the Rockies and Blue Ridge Mountains draw our attention, most of us know geology through the struggle to grow a lawn or garden, and in the windblown dirt that litters our garage floors.
So how did my quarter acre on the western border of the Ravensworth landgrant, between Burke and Fairfax, come about? How was it shaped by the machine of time and earth forces that also created the Appalachian Mountains? In other words, what legacy bequeathed the soil of my weekend toil?
The story starts with the rock. And it’s not one but several stories, dating back nearly 600 million years.
Geologist Avery Drake, Jr. of the US Geological Survey (USGS) describes my Northern Virginia neighborhood as a stack of six rock units. The times, conditions and events that created and brought them together in their present positions are muddled. Study of the evidence and comparison to rocks in other parts of the world provide some, but not all, of the answers. One fact is known: like most of us in the Washington, DC metropolitan area the rocks aren’t native; they moved here from somewhere else.
The Rocks Under My Yard
My home sits on a quarter acre of the Sykesville formation, rock which analysis dates from the Cambrian geologic time period, between 500 and 600 million years old. The formation is surrounded by a fault border and differs from its neighbors in age, structure and mineral composition – all clues that it was fractured loose and moved from its site of origin.
An hour’s jog from my front door and along Braddock Road past George Mason University’s Fairfax campus takes me over seven distinct terranes laid down tens of millions of years apart. My route traverses the Sykesville, Indian Run, Annandale, Popes Head, Mather Gorge and Piney Branch formations. I cross four thrust fault borders and a major fold in the earth, which attest to dramatic events that cast these miles-wide slabs of rock atop one another.
Geologic maps plot rock formations in distinctive colors. Northern Virginia’s map is a zigzag pattern of freeform shapes like puddles of spilled paint running parallel and angled generally from southwest to northeast.
Unlike rocks in the Grand Canyon with their clear fossil records and exposed strata extending a mile down into the earth that geologists can read like pages in a book, Northern Virginia strata are crumpled, jumbled and buried from view, except where the ground is cut open by construction or flowing streams. Geologists gather evidence by studying these rock cuts and hammering out samples to analyze back in the laboratory. Important pointers are mineral content, crystal structure and particle size, ranging from barely visible to pebbles.
These are features I don’t see or appreciate jogging by my neighbors’ yards with their dug up trophies of weekend yard work, the weathered cobbles that outline flower beds and the occasional small boulder set proudly by the curb.
The rock cycle, like our breathing, operates ceaselessly: creating, tearing down, moving material from one place to another and creating new rock.
All rock began as igneous, molten material that solidified and formed earth crust about three and a half billion years ago. Once hardened, nature started chipping away with erosion, transporting the particles to low areas, primarily the ocean bottom, and cementing them into new sedimentary rock. Meanwhile, igneous rock continued to form as magma flowed or exploded through cracks in the hardened crust, or welled up near the surface and slowly cooled into granite formations called plutons and batholiths.
Sykesville rock and its neighboring formations, like much of the rock in the Washington, DC area, are of a third type – metamorphic. That is, they are recycled versions of igneous and sedimentary rocks that were buried deep underground and subjected to extreme heat and pressure. The trip through the geologic pressure cooker realigned their atoms into new minerals and crystal structures.
Making Mountains From Rocks
Today, geology makes headlines on the Pacific Rim with volcanoes firing in the Philippines and Indonesia, and earthquakes threatening California’s man-made infrastructure. Between 450 and 250 million years ago the action was here.
During that period, my neighborhood was on the leading edge of orogeny, which means mountain building in plate tectonics theory. Geologists theorize that the continents sit on top of plates which float on the denser, heavier mantle of the earth. The plates move, as they do today along California’s San Andreas Fault.
When two plates collide along a coastline, the heavier ocean floor subducts, or sinks under, the lighter continental crust. The tremendous compressional and uplifting force fuels orogeny. The Blue Ridge and its larger parent, the Appalachian Mountain Range, are the results of at least three orogenies where Washington was caught in the middle.
Making My Neighborhood Rocks
The Sykesville rock under my yard began as a sedimentary melange, a mixture of chunks of seafloor igneous rock and sediment eroded from the land. Melanges form at the collision point between two oceanic plates, as the toe of one plate angles down under and some of its material scrapes off onto the overriding plate. The intersection between plates occurs in a trench three or more miles under the ocean surface at the foot of a steep slope. The material slowly piles up in a wedge against the slope wall as the plates collide at the almost imperceptible rate of an inch or two each year. Meanwhile, sediment washed from the land combines with ocean floor debris and is carried by currents to mix with the grindings from the disappearing plate. The combined weight of water and accumulating sediment compresses this melange into rock.
Many geologists believe that a chain of volcanic islands once stood in an arc off of what became today’s East Coast, much like today’s Aleutian Islands that mark the descent of the North Pacific Plate under Alaska and Asia.
Converging plates spawn volcanoes as subducting plate material reaches 60 or more miles into the earth’s mantle, melts and flows upward as magma. A shallow back-arc basin forms between the island chain and the coast. The fore-arc side is a deep trench slope where the colliding plates grind against each other and build a melange wedge.
Traces of volcanic ash and other evidence suggest that the Sykesville rock under my yard originated in such a fore-arc trench. So did the Indian Run Formation. Mather Gorge rocks also formed as a melange but probably not in a trench. The Piney Branch formation is an intact piece of ocean crust, basalt magma that welled up out of a mid-ocean ridge and rapidly solidified in dense layers, that was ripped loose in a single chunk. The Annandale Formation had very different origins. It lacks sea floor plate debris and began as a simple sedimentary sandstone.
The rock layers that form my Northern Virginia neighborhood are like a hand of cards dealt one by one, then picked up and reshuffled to new positions. Called thrust sheets when they move, rock layers are propelled by compressional forces of plate techtonics. They slide over, grind and deform the underlying rock as glaciers do. Though glacial speed is supersonic compared to rock movement.
Before it consolidated into hard rock, the Sykesville Formation was overrun by the Mather Gorge Formation. Similarly, the Annandale slid over the Indian Run melange. Then the Sykesville-Mather Gorge pair moved atop the Indian Run-Annandale pair. Later movements added the Piney Branch and, finally, the Popes Head Formation to the stack.
These movements occurred underwater and brought together rocks of different ages and origins. Then magma surged up under and intruded into cracks throughout all but the Piney Branch rock. The magma did not penetrate all the way to the surface and so it slowly cooled and solidified into granite, forming the Falls Church Pluton, which appears near the Ravensworth landgrant’s northern border, and the Occoquan Batholith that underlies the central and southwestern areas (see geologic map above).
Making the Appalachian Mountains and My Neighborhood
These events occurred over tens of millions of years. They set the stage for the three successive orogenies that followed, starting about 450 and ending about 250 million years ago: the Taconic, Acadian and Alleghanian orogenies.
Each orogeny marked the arrival of colliding land masses that eventually joined North and South America, Europe, Asia, Africa, Australia and Antarctica into one super continent called Pangaea. Advancing plate movement collapsed the island chain and welded it to the edge of today’s North America. The stack of formations that became my neighborhood buckled and folded like a hand closing into a fist. Tremendous heat and pressure recrystallized minerals into new metamorphic rock, as the layers bent back on themselves, twisted and intertwined.
The final collision with Africa in the Alleghanian Orogeny shoved my neighborhood along with the entire Washington area westward, as it crumpled the land and completed the multi-orogeny process of uplifting the Appalachian Mountains. My neighborhood was literally in the center of the geographic world, until about 200 million years ago when the continents began the journey to their current positions on the globe.
Then and Now
Volcanic eruptions and earthquakes certainly marked many of these events on the earth’s surface. However, today’s landscape, including my neighborhood, was buried thousands of feet deep in the basement under peaks lifted up by orogeny as high as today’s Himalayan Mountains, and has emerged as erosion scraped away the overlying layers. Coastal Plain sand and gravel units in the eastern and southern parts of the Ravensworth landgrant are evidence of relatively recent erosion, which occurred in just that last several million years (see geologic map above). And, of course, the process continues with wind and rain chipping away at rocks and streams bearing away soil from my yard to become the material of future rocks and orogenies.
Want to add some interest to your daily jog or commute? Get a geologic map of your neighborhood from the US Geological Survey. As your route traces over millions of years of earth history, reflect that despite man’s changes the earth is about as William Fitzhugh (the Immigrant) and George Washington saw it and the Atlantic Ocean has widened a mere 20 feet since they trod this same turf.
- Avery Ala Drake and A. J. Froelich, Geologic map of the Annandale quadrangle, Fairfax and Arlington Counties, and Alexandria City, Virginia, 1986, USGS Geologic Quadrangle: 1601; Avery Ala Drake, Geologic map of the Fairfax quadrangle, Fairfax County, Virginia, 1986, USGS Geologic Quadrangle: 1600; Avery Ala Drake and Peter T. Lyttle, The Accotink Schist, Lake Barcroft Metasandstone, and Popes Head Formation: Keys to an Understanding of the Tectonic Evolution of the Northern Virginia Piedmont, 1981, USGS professional paper 1205; Avery Ala Drake, Tectonic implications of the Indian Run Formation, a newly recognized sedimentary melange in the Northern Virginia Piedmont, 1985, USGS professional paper 1324; Avery Ala Drake, Metamorphic Rocks of the Potomac Terrane in the Potomac Valley of Virginia and Maryland: The Piedmont of Fairfax County, Virginia, July 13 And 18, 1989, American Geophysical Union; J. Wright Horton, Jr., Avery Ala Drake, Jr., and Douglas W. Rankin, “Tectonostratigraphic terranes and their Paleozoic boundaries in the central and southern Appalachians,” Geological Society of America Special Papers 230, p. 213-246, 1989 ↩
- See especially This Dynamic Planet: The Story Of Plate Tectonics, Online edition ↩