Fossils Along Our Local Creeks 

Text and photos by Hutch Brown.

I was walking along Four Mile Run in Arlington’s Barcroft Park when I found the rock at the right (5” X 5”, with loupe for scale) on a gravel bar. I’ve seen others like it near other local streams, including Long Branch and Lubber Run in Arlington and Holmes Run in Alexandria. The rocks are easy to find if you know what they are and where to look.

 So what makes these rocks interesting? They contain lines that represent traces of wormlike marine animals that lived more than half a billion years ago! Most of the lines (except for the leftmost one in the upper photo, which appears to hold an actual wormlike fossil) are tracks—signatures left by ancient animals, like dinosaur footprints. They are known as trace fossils. 

The trace fossils are embedded in a kind of rock called Antietam quartzite. Typically buff-colored with tinges of orange, the rock is named for a Maryland creek of Civil War fame, where the Battle of Antietam was fought in September 1862. Antietam Creek joins the Potomac River upstream from Harpers Ferry in West Virginia. 

Quartzite is a kind of metamorphic rock (the word “metamorphic” stems from the ancient Greek terms meta-, “changed,” and morphe, “form”). Sandstone metamorphoses (changes form) into quartzite under intense heat and pressure. Antietam quartzite is so poorly consolidated that it still resembles sandstone, and some geologists call it that.

But the trace fossils are a dead giveaway—Antietam quartzite is known for them. The fossil was first described in 1840 by the geologist Samuel S. Haldeman. He found it in quartzite at Chickies Rock, a landmark in southern Pennsylvania. Haldeman named his find Skolithos linearis (perhaps coined from Greek skolex, “worm,” and lithos, “rock,” combined with Latin linearis, “consisting of lines”).

The lines in the skolithos “wormrock” are from burrows in the sand left by ancient creatures similar to marine animals alive today—wormlike phoronids that live in shallow marine environments just offshore. They burrow into the sands and use tiny tentacles to filter food from currents flowing overhead. 

The fossiliferous sandstone metamorphosed into Antietam quartzite under intense heat and pressure as Earth’s continents collided about 300 million years ago, forming a supercontinent called Pangaea (pronounced pan-JEE-uh). Antietam quartzite formed as a layer of rock overlying ancient continental granite—the same granite we find today in Blue Ridge outcrops like Old Rag. So the fossil-bearing Antietam quartzite I found in Arlington must have originated in the Blue Ridge, more than 50 miles away. 

How did it get from there to here? 

By about 140 million years ago, erosion had abraded our area into a flat and featureless plain drained by meandering rivers. The rivers brought sediments (sands, silts, clays, pebbles, and rounded river rocks) from far inland and deposited them on our local bedrock (the foundational rock underlying our soils) in layers that became hundreds of feet thick. The sediments are still there, and they are dense and hard to pick apart, but—as you can tell from the photo at right—they are not solid rock. 

As our local streams, including Four Mile Run and Holmes Run, cut channels through the sediments over the last several million years, they washed ancient river rocks into streambeds and onto gravel bars, where they make up most of the alluvium today. So, the alluvium in our local streams comprises rocks from far inland, including quartzes and quartzites as well as siltstones, sandstones, and other sedimentary rocks from sources far to the west. 

The sediments include billions of quartzite specimens with skolithos trace fossils, making them easy to find in and along streams that cut through the sediment deposits in the southeastern part of our area. On geologic maps, the sediments are usually shown in shades of green (for the Potomac Formation) and yellow/orange (for the overlying Tertiary terraces). A geologic map of Arlington or Alexandria shows green-lined streams like Four Mile Run and Holmes Run, and these are great places to look for skolithos fossils on gravel bars. So, the next time you visit a park along one of these streams, look beneath your feet; you too might find rocks with evidence of creatures that lived over half a billion years ago!