Geology Walk in Holmes Run Gorge

Text and photos by Hutch Brown.

On August 30, 2025, several ARMN members joined local residents for a geology walk in Holmes Run Gorge hosted by the Northern Virginia Mineral Club and Friends of Holmes Run. Holmes Run, part of Alexandria’s Cameron Run watershed, cuts a gorge through the Piedmont bedrock (the foundational rock underlying our local soils) before crossing under I-395 and becoming Cameron Run on the flat Coastal Plain. The northern part of the gorge now lies under Lake Barcroft behind its dam at Columbia Pike. The bedrock is hundreds of millions of years old, as described below.

ARMN member Hutch Brown led roughly two dozen participants in exploring the geology of Holmes Run Gorge. We started at the Jerome “Buddie” Ford Nature Center in Alexandria and walked a mile or so upstream to just below the Lake Barcroft dam in Fairfax County. Along Holmes Run just below the visitor center parking lot, we saw outcrops of bedrock known as Indian Run sedimentary mélange, one of several metamorphic bedrock types in the northern Virginia Piedmont, which formed from seafloor rock during a mountain-building event about 474-450 million years ago.

Our first stop was to examine one of the two main bedrock types in the gorge, beginning with Occoquan granite, a coarse-grained rock with visible crystals. Most granite has a classic salt-and-pepper appearance caused by gray and white grains of quartz, feldspar, and muscovite mica mixing with black flecks of biotite mica. Occoquan granite dominates much of the bedrock from Alexandria through Fairfax to the Occoquan Reservoir (hence the name) and beyond.

The next stop was at an outcrop of Indian Run sedimentary mélange. This is a fine-grained rock, medium gray in color until broken with a rock hammer, when it appears light gray from sparkling muscovite mica. Like Occoquan granite, Indian Run sedimentary mélange is rich in the most common minerals on Earth: quartz and the feldspar group of minerals. The Indian Run rock originated from mixed sediments on the floor of an ancient ocean.

Moving upstream along Holmes Run, we found an unusual bedrock type called muscovite monzogranite. We learned the importance of using a rock hammer to break off pieces of bedrock to see the interior, which can look very different from the weathered external appearance of the rock. Muscovite monzogranite has the same mineral makeup as Occoquan granite but with mostly light-colored quartz, feldspars, and muscovite mica with almost no dark-colored minerals like biotite mica. This chalky-looking rock is also much finer-grained than Occoquan granite.

Just below the Lake Barcroft dam, we saw the final bedrock type in Holmes Run Gorge. Closely related to Occoquan granite, Falls Church tonalite (pronounced TOE-nuh-lite) has the same salt-and-pepper appearance due to the same blend of quartz, feldspars, and micas, but it also contains dark grains of hornblende and is much finer grained than Occoquan granite. Another difference is in its mix of feldspar types: it has very little orthoclase feldspar, which gives some Occoquan granite a pinkish hue.

All four bedrock types in Holmes Run Gorge originated about 450-500 million years ago, when Earth’s continents were slowly joining together into a single supercontinent called Pangaea (pronounced pan-JEE-uh). As the continental plates collided over hundreds of millions of years, huge mountain ranges formed and then eroded away. The mountain-building events generated our area’s bedrock types and emplaced them where they are in the Piedmont today.

Through erosion, Holmes Run has exposed the igneous and metamorphic roots of an ancient mountain range called the Taconian. Once as high as the Alps are today, the Taconian Mountains rose over a 25-million-year period beginning about 474 million years ago. We also learned about the rich compendium of sediments hundreds of feet thick laid down on our area’s bedrock by ancient rivers up to 140 million years ago. The rounded river rocks on Holmes Run gravel bars came from those deposits; they include quartz and quartzite as well as sandstones, siltstones, mudstones, and other sedimentary rocks from far inland. We even found trace fossils of ancient marine animals preserved in quartzite.

The next time you walk along Holmes Run and look at the rocks along the creek, you might stop to appreciate how old the bedrock is—and how unusually varied. All four bedrock types, both igneous and metamorphic types, predate Pangaea. Few other places in our area have four distinct bedrock types within such a small space—and with such a great variety of other ancient rocks and fossils on gravel bars.