Reading the Rocks: A Look at the Geology of Long Branch

Text and photos by Hutch Brown.

The Long Branch Nature Center is a popular destination for exhibits, classes, and events. It is also the kickoff point for nature walks of all kinds along Long Branch creek in Glencarlyn Park. As you follow the paved road down to the creek’s confluence with Four Mile Run, you see lots of exposed bedrock—the rock underlying the soil—so it’s the perfect place to study it.

Bedrock

Arlington’s geology is complex, in part because our area straddles the boundary between two physiographic provinces: the Piedmont to the northwest and the Coastal Plain to the southeast. The Piedmont is hilly, with fast-flowing streams and prominent bedrock exposures. The Coastal Plain is flat and made up of sediments, with slow and meandering rivers, such as the Potomac River.

The bedrock near the nature center is called Indian Run sedimentary melange. The term “Indian Run” comes from a Fairfax stream where the rock was first described by geologists. Sedimentary melange (from French mélange, “mixture”) comprises a blend of parent sediments of various types and sizes. Indian Run sedimentary melange ranges in color from grayish-brown (along Long Branch) to light gray (along Four Mile Run). Veins of white quartz are common, as are inclusions of preexisting rocks in the parent sediments.

Indian Run sedimentary melange originated as sedimentary rock on the seafloor half a billion years ago, but now it’s metamorphic rock.

Sedimentary and metamorphic rock, along with igneous rock, are the three basic rock types in the world. Sedimentary rock forms when sediments like clay, silt, and sand pile up and harden into stone. Igneous rock forms from molten magma cooling deep underground into coarse-grained rocks like granite. Metamorphic rock forms because our planet is covered by solid plates of rock floating on the underlying magma. Driven by convection currents in the magma, the plates collide at their margins, putting the bedrock under enormous heat and pressure and changing its structure and chemistry into various kinds of metamorphic rock. (“Metamorphic” is Greek for “change of form.”)

About half a billion years ago, a small oceanic plate collided with the continental plate of proto-North America. Over tens of millions of years, sedimentary rock from the seafloor slid up onto the continental margin to form a mountain range that is long since gone. In the process, the sedimentary rock changed into the metamorphic rock that now dominates the Piedmont in our area.

Overlying Sediments

Near the Long Branch Nature Center, you can see the transition from the bedrock of the Piedmont to the sediments of the Coastal Plain. Overlying the metamorphic bedrock are unconsolidated sediments deposited by ancient rivers about 100-140 million years ago. Forerunners of the Potomac, the rivers carried sand, silt, clay, and cobble (pebbles and rounded river rocks) from far inland and deposited them where the waters slowed as they approached sea level on the Coastal Plain.

The sediments, collectively known as the “Potomac Formation,” start near I-66 and gradually thicken to the southeast. The sediments form a wedge that completely buries the bedrock on the Coastal Plain, becoming thousands of feet thick towards Chesapeake Bay. Exposed by erosion along our creeks, the Potomac Formation looks like packed sandy or silty soil, often mixed with rounded river rocks, and it is dense and hard to pick apart.

On top of the Potomac Formation is a much younger and thicker layer of sediments laid down about 5-10 million years ago in the elevated areas where homes are built today. Geologists call these relatively flat surfaces “terraces.” “Tertiary” is the former name for the geologic period of the deposits, which are still known informally as Tertiary terraces. The Tertiary terrace near the Long Branch Nature Center is higher upslope but otherwise hard to tell apart from the underlying Potomac Formation. It too comprises tightly packed sands and silty clays mixed with pebbles and rounded river rocks.

Fall Line Zone

The Tertiary deposits came from rivers meandering across a flat Piedmont plain. The Piedmont became hilly beginning about 5 million years ago due to gentle regional uplift from the Allegheny Plateau in West Virginia to our own area in the Piedmont. Over millions of years, the uplift has caused our local rivers and streams to cut valleys into the rising Piedmont plateau.

Uplift has highlighted the so-called Fall Line between the Piedmont and Coastal Plain, which is a zone that can stretch for miles. The Fall Line zone for the Potomac River reaches from near Great Falls at least to Little Falls (some 5 miles) and perhaps all the way to the tidewater at Theodore Roosevelt Island (some 15 miles). Long Branch joins Four Mile Run downstream from the nature center in the Fall Line zone for Four Mile Run. The zone contains an abrupt drop called Huffman’s Falls (named for a geologist who studied the bedrock there); it’s the rough (smaller scale) equivalent of Great Falls. At times audible from the nearby W&OD Trail, the falls lie just downstream from the confluence of Long Branch with Four Mile Run, about a 20-minute stroll from the nature center. The Fall Line zone for Four Mile Run extends to about 500 feet downstream from Columbia Pike.

The downcutting force of our rivers and streams due to gentle uplift across our area was magnified by sporadic Ice Ages over the last 2 million years. The glaciers never reached our area, but our local streams were locked in ice for much of each year. Released for only a few months in summer, our streams became raging torrents with far greater downcutting power than today. Nothing else can explain Mather Gorge on the Potomac River or the deep valleys gouged by our local creeks, including our (currently) tiny Long Branch.

Information sources:

Geology walks with Joe Marx, Emeritus Professor of Geology at Northern Virginia Community College.

Texts:

Bentley, C. 2020. Historical geology: A free online textbook for historical geology courses;

Fichter, L.S.; Baedke, J.K. 1999. https://csmgeo.csm.jmu.edu/geollab/vageol/vahist/index.html . Harrisonburg, VA: College of Science and Mathematics, James Madison University;

Frost, W.; Ernest, T. 1999. Simplified geologic map of Arlington County, Virginia, and vicinity. Arlington County, VA. ownstream from its confluence with Long Branch creek. Note the smooth, floodwater-worn Indian Run sedimentary melange.