Digging into the History and Mystery of an Arlington Streambank

Text and photos by Hutch Brown.

As county volunteers, my son Alex and I routinely monitor Arlington’s streams for water-polluting bacteria (E. coli). Our current site is upper Lubber Run in Woodlawn Park, a small urban park in a relatively flat part of Arlington. While taking water samples in spring 2024, Alex noticed an anomaly in the streambank in what scientists call the “soil horizons” (soil layers): a grayish layer of soil underneath the usual horizons.

Surprising Soil Layer

The site’s streambanks are lightly wooded and deeply incised, forming a channel more than six feet deep. We expected to see the usual soil horizons, which are classified by soil scientists, from top to bottom, as O, A, B, and C. The O horizon (decomposing leaf litter) was negligible, but we found a thick A horizon (topsoil) rich in organic matter. Organic matter largely decomposes before reaching the B horizon, which is typically light in color. The B and C horizons have rising proportions of iron and aluminum oxides, giving them shades of yellowish-white ranging into reddish-orange.

Below that, we found what appeared to be a second (buried) A horizon—a grayish soil relatively rich in organic matter. And just above it was a thin band of whitish gravel.

Soil Analysis

A friend who is a professor of soil science came to town, and we took him to the site. Intrigued, he took soil samples back to the State College of New York in Syracuse and had them analyzed for us.

The analysis found a 14-inch A horizon and a 24-inch B horizon in three parts—but no C horizon at all. Instead, it found a 10-inch layer of “redoximorphic features” (redox, for short) in the lowest (B3) part of the B horizon. Redox refers to a zone alternately enriched and depleted in iron and manganese oxides after periods of soil saturation and desaturation, creating soils with shades of grayish- and reddish-brown. The lab analysis also confirmed an inch-thick band of gravel over a buried A (Ab) horizon 10 inches thick.

What could have caused all this?

Wetlands and Milldams

Before European settlement, beavers were ubiquitous in our area. Research has shown that many pre-settlement streams in the mid-Atlantic region formed small interlocking channels within extensive floodplain wetlands maintained for thousands of years by beaver dams. The wetlands accumulated few sediments but stored vast amounts of organic carbon.

North American beaver pelts were in high demand in Europe in the 1600s–1800s. The thriving fur trade almost wiped out the beavers, thereby indirectly draining the wetlands and releasing streams to flow unimpeded by beaver dams. European colonists cleared the land for farming and built dams across the region to mill the grains they grew. The millponds gradually filled with sediments from widespread soil disturbances caused by upland logging and farming.

Mystery Solved?

Eroded streambanks can reveal traces of this history. The soil profile from Lubber Run seems to show a gray wetland soil in the buried A (Ab) horizon, possibly from a pre-settlement marsh. The marsh would have looked much like Huntley Meadows in Alexandria does today. With beaver removal and stream release, Lubber Run would have deposited cobble on gravel bars and left a thin band of gravel overlying the wetland soils.

The B horizon likely came from sediments accumulated from erosion in upland areas during Arlington’s logging and farming days, possibly at the bottom of a millpond. Or, instead of a millpond, fluctuations in the water table might have saturated and desaturated the site’s lower soils, creating redox features. The organic-rich A horizon at the top of our soil profile would have formed in later years of plant regrowth, death, and decay following the period of widespread soil disturbances by European settlers.

So, when you’re strolling along a creek with eroded streambanks, take a good look at the soil layers you might see. They can offer clues to our local history—and what you find might surprise you, just as it surprised us on upper Lubber Run.

The article is based in part on research by Professor Dorothy Merritts of Franklin and Marshall College in Lancaster, PA, described in the spring 2025 issue of Notes on Nature (“Rethinking Stream History in Our Region”).