Huffman’s Falls: A Geologic Gem in Glencarlyn Park

Text and photos by Hutch Brown.

Glencarlyn Park (shown at right) straddles the spectacular gorge cut by Four Mile Run and its tributaries in west-central Arlington. About a hundred acres in size, the park includes areas of forest that are relatively free of invasive plants, partly because they have never been logged. The Old-Growth Forest Network has estimated the age of the forest community there at a hundred years, but some trees (judging from tree rings) are considerably older. The native oak/hickory, oak/heath, and mesic mixed-hardwood forest types contain several county-designated champion trees, along with a hidden geologic gem: a bonafide waterfall on Four Mile Run called Huffman’s Falls.

The origin of “Huffman” in the name of the falls is obscure. But the “falls” part is obvious: Four Mile Run drops about eight feet over the two parts of Huffman’s Falls—far enough, following rainstorms, to create a roar that you can hear from the nearby Washington and Old Dominion Trail, a popular biking/walking trail along Four Mile Run.

Arlington’s streams have eroded down to the bedrock—the ancient rock underlying the soils in our area. The bedrock originated almost half a billion years ago when Earth’s plates collided over tens of millions of years to form mountains as high as the Alps are today. What geologists call Earth’s “tectonic plates” are like eggshell pieces floating on a viscous egg white, with convection currents in the underlying magma driving the plates into each other. An example is the Himalayas, formed by the Indian Plate colliding head-on with the Eurasian Plate.

Mountains from the collision of tectonic plates are long since gone, but their roots are now the bedrock along our local streams. Most bedrock is what geologists call “metamorphic” (from Greek words for changing form), meaning rock that changed under extreme heat and pressure during the mountain-building event from one rock type into another. The metamorphic bedrock in Glencarlyn Park is called Indian Run sedimentary melange, named for the Fairfax County creek where a geologist first described it. Despite its name, Indian Run sedimentary melange is not the type of bedrock known as “sedimentary” (made up of clays, silts, sands, pebbles, and rock fragments that piled up and hardened into rock). “Sedimentary” in the name refers to the sedimentary parent rock, and “melange” comes from French mélange (mixture), indicating an exceptionally rich blend of sediments in the parent rock. Metamorphism altered the finer sediments in the Indian Run bedrock into mica and other minerals, but the rock still contains visible quartz cobble as well as fragments of older rocks.

Huffman’s Falls (shown at right) formed from a completely different bedrock type called “igneous” (from the Latin word for fire), meaning rock derived from fiery magma. When magma erupts as lava, it cools into “extrusive” igneous rocks like basalt; when it cools instead deep underground, it crystallizes into “intrusive” igneous rocks like granite. During the collision of tectonic plates that shaped our local bedrock, rising magma penetrated what became the metamorphic bedrock and crystallized into granite. Huffman’s Falls is an isolated outcrop of granite in a sea of Indian Run sedimentary melange.

You can see the difference in pieces of both bedrock types broken off with a rock hammer. The photo at right shows both pieces lying on weathered Indian Run sedimentary melange (which also contains embedded white quartz cobble). The Indian Run fragment (next to the loupe, shown for scale) is dark gray in color and very fine-grained. It contains gray quartz, white muscovite mica, and light-colored plagioclase feldspars, with enough green chlorite and black biotite mica mixed in to give the rock its relatively dark color.

The granite fragment looks quite different. The crystals are larger and the rock is lighter in color, with a pinkish cast from orthoclase feldspar, which is typically pink. Granite is made up mainly of quartz and feldspars, with less muscovite and biotite mica than in the neighboring Indian Run bedrock, which contains no orthoclase. There’s no confusing the two.

Huffman’s Falls is large enough to share with Great Falls on the Potomac River the signature signs of waterfall wear and tear during floods. As the photos at right show, the rock is worn shiny smooth in places, and “potholes” have formed where whirlpools have ground away the rock under swirling sands and gravels.

Arlington County straddles the boundary between the Piedmont plateau region to the northwest and the Coastal Plain to the southeast. In school, I learned that Piedmont rivers and streams typically have waterfalls at the so-called Fall Line, where the water plunges from metamorphic rock onto the softer sedimentary rock of the Coastal Plain. But Arlington County has no sedimentary bedrock on the Coastal Plain or anywhere else. At Huffman’s Falls, Four Mile Run spills from metamorphic Indian Run sedimentary melange upstream over granite at the falls onto more Indian Run bedrock downstream. Similarly, the Potomac River at Great Falls pours from Mather Gorge metagraywacke upstream onto the same Mather Gorge metagraywacke downstream. (Metagraywacke, as the prefix “meta-” suggests, is another kind of metamorphic rock.)

In fact, the drop in rivers and streams from the Piedmont plateau region onto the Coastal Plain is never abrupt: it happens gradually across a zone many miles long. The Potomac River’s Fall Line zone reaches all the way from Great Falls through Little Falls to Theodore Roosevelt Island. Similarly, Huffman’s Falls is part of a Fall Line zone on Four Mile Run that reaches from well upstream of the falls to well downstream of Columbia Pike.

Quartz veins (shown at right) swirl through the granite at Huffman’s Falls. During collisions of tectonic plates, superheated fluids circulate through what geologists call the “country rock” (the preexisting bedrock), where it is intruded and altered by contact with tremendous heat from rising magma. Because quartz has a relatively low melting point, it remains dissolved in the fluids long after other minerals precipitate out. Quartz-rich waters fill fissures in the cooling and contracting bedrock, whether igneous or metamorphic, leaving veins and nodes of white quartz, some of them huge.

Quartz is not only one of the most common minerals on Earth but also one of the hardest, with a value of 7 of 10 on the Mohs hardness scale used by geologists. Rocks like granite and Indian Run sedimentary melange are far more erodible than quartz, which drops out of the weathering bedrock as loose pieces on the ground, from pebbles to boulders in size, and as alluvium on gravel bars. More erosion-resistant than other rounded river rocks, quartz is ubiquitous on gravel bars in Four Mile Run Gorge.

Managed by Arlington County largely as a nature park, Glencarlyn Park features notable natural, historical, and recreational treasures in Four Mile Run Gorge, including Huffman’s Falls, with its distinctive geology. A 15-minute stroll from the Long Branch Nature Center, Huffman’s Falls is also a pleasant sit spot for enjoying the sights and sounds of a lovely burbling stream in a natural setting on a nice day.

This article and photos are based on a geology walk that I led in May 2026. The main sources are Historical Geology: A Free Online Textbook for Historical Geology Courses by Callan Bentley (2020); Tectonic Implications of the Indian Run Formation by A.A. Drake, Jr. (1985); and Simplified Geologic Map of Arlington County by W. Frost and T. Ernest (1999).

See also earlier blog article: “Reading the Rocks: A Look at the Geology of Long Branch.”