Town of Bedrock
Between Cold Spring Lane and Harford Road, huge, streaky slabs of gray-green rock jut from the banks of Herring Run, and massive boulders jam the streambed. This stuff was formed from heated, pressurized mineral glop in the planet's bowels at a time when the most advanced life forms on earth were jellyfish. Geologists call it Baltimore gneiss (pronounced "nice"), and, at a billion-plus years of age, it's the oldest material in Maryland.
For this and other local geologic facts, I thank Jim Reger of the Maryland Geological Survey (MGS), who gave me a one-hour introduction to the subject at the agency's headquarters at 23rd and St. Paul streets. He kindly demystified the jawbreakers of his trade (try "medium-grained biotite-quartz-plagioclase-microcline gneiss") by explaining, "It's a lot like vegetable soup. If you put beef in it, it's beef vegetable soup. If you put chicken in it, it's chicken vegetable soup." Rocks are known by what they're made of, and how they were made.
In a reference room with shelves full of specimen rocks and fossils, Reger unfolded two maps that show, in garish colors, what types of bedrock lie just under Baltimore's soil. On the maps, it's easy to pick out relatively recent phenomena, such as river valleys and human-made structures, but the bigger picture seems almost random, a patchwork of irregular shapes bearing little obvious connection to the forms of the land as we know it. The maps' garish color scheme adds to the sense of chaos.
The underlying logic of the patterns on each MGS map is revealed along its bottom margin, where a strip of clashing colors represents a sample cross-section of the rocks below the surface. The cross-section suggests how, over the course of eons, the raw materials of the earth's crust were layered, folded, heaved up, torn asunder and reattached, worn away, filled in with new layers, leaked through, refolded, and worn away again.
Some of the boundaries between rock zones, as shown on the map, are not only clear-cut but straight, as if drawn with a ruler. In Herring Run Park, for example, the billion-year-old Baltimore gneiss suddenly gives over to a brownish, glittery substance called Loch Raven schist, which is perhaps half as old. The boundary between them runs parallel to Harford Road, right past the playground where I take my daughters. Like gneiss, schist is a product of primordial substances that were subjected to extreme heat and pressure. Schist used to be shale, which used to be mud; Baltimore gneiss was apparently a granitelike rock that was cooked, squeezed, and stretched, resulting in its characteristic stripes and streaks.
These durable, available materials became the raw stuff of local vernacular architecture. Quarrying was concentrated in places where rocks naturally came to the surface along the city's three major streams. In Hampden and Woodberry, the old stone mill buildings and cottages take their characteristic color from the half-billion-year-old Carroll gneiss that was cut from the banks of Jones Falls. Old houses along Franklintown Road, in the Gwynns Falls valley, have foundations of Mount Washington amphibolite, a dark stone that abounds on the city's west side. Homely but serviceable, gneiss of various ages and origins was also mined from the present site of Morgan State University, along Herring Run, and beside Gwynns Falls south of Edmondson Avenue.
As steam and gasoline engines made it easier for contractors to haul rock from quarry to construction site, building stone became increasingly mobile. MGS' home, like the nearby Lovely Lane Church, is walled with gneiss (sometimes misidentified as granite) from Port Deposit in Cecil County. Hundreds of Baltimore buildings have walls or foundations faced with Setter's quartzite from quarries in Greenspring Valley and Marriottsville, popular because it cleaves easily into slabs suitable for building purposes. With its pleasing range of colors, from grayish-tan to rust, it may have been the inspiration for Formstone.
The most famous local stone was used for two Washington Monuments, both the column in Baltimore and the obelisk in Washington. Still quarried along the York Road corridor, this glittery Cockeysville marble began some half-billion years ago as limey sediments on the ocean floor. Reger says the national monument's two-tone appearance--the upper half being somewhat grayer than the base--is due to the fact that construction was delayed by the Civil War. During the conflict, the Baltimore County quarries ate their way through the whitest marble and dug into some slightly less pure strata. Relatively cheap and abundant, Cockeysville marble is also the stuff of our town's signature white steps. You can sit on your stoop and contemplate the dizzying depths of time.
812 Park Ave.
Baltimore, MD 21201