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Gaining new appreciation of rocks in the garden

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Stone walls near Brandywine Creek State Park are built of native fieldstone, including amphibolites and Brandywine granite, or gneiss, which gave the Wilmington Blue Rocks baseball team its name. / News Journal file/ROBERT CRAIG
January 26, 2012

Gardeners from northern New Castle County should be grateful that the glaciers stopped their descent at the Delaware Water Gap about 100,000 years ago.

Had they continued on down, they would've scraped off the soil and left us with huge swaths of exposed rock and thin, meager soil as they did in New England.

Instead, we have regolith, a rich combination of weathered rock and acidic soil typical of the Appalachian Piedmont province.

As the vegetation dies back and garden beds become bare, it's easy to notice that rocks are everywhere, with each winter seemingly spawning a new crop.

"In the Piedmont, the rocks you find in your yard are just bits and pieces of weathered rock, and you're most likely to find quartz-rich rocks because quartz is hard and the most common mineral," says William "Sandy" Schenck, a geologist and associate professor of geology at the University of Delaware.

Weathered rock, known as saprolite, yields up fieldstone. "Fieldstone is a common name for rocks farmers found in their fields; [the term] doesn't have anything to do with geology," he says.

The colors of fieldstone reflect the minerals in the rock, with iron creating the signature reddish stain.

"Most of the rock in the Delaware Piedmont has a lot of iron in it, which tends to make our soil red and lightly acidic," says Schenck, adding that plants generally do well in that type of soil.

That winter seems to fling rocks out of the ground is deceiving.

"Rocks don't actually move through the soil because we don't really have those kinds of freeze-thaw cycles. If you remove rocks from the top eight inches, they shouldn't come back. Most likely if you're seeing more rocks, the soil is eroding, unless you're tilling and pulling them up" he says.

Conversely, rocks that seem to disappear, such as those used for edging, are victims of "creep." Because they sit atop the ground, they are directly affected by freeze and thaw cycles. Freezing ground may lift the rock up an inch and a half, but when it thaws the soil beneath it turns to mud, causing the rock to sink.

"Once you place a rock above ground, you expose it to 365 days of weathering, so it'll start to disintegrate, turn into sand, and fall apart, like when a portion of your wall falls down," Schenck says.

While most gardeners are familiar with fieldstone, they will also likely recognize other rocks, like Brandywine blue granite and the black sparkling amphibolites.

The result of a massive collision between ancient North America and a volcanic island chain some 450 million years ago, Amphibolites are metamorphosed lava flows or basalite. They can be seen in the stone walls that surround many of the current and former du Pont family estates, such as Winterthur and Brandywine Creek State Park.

Brandywine granite is technically gneiss, another hard stone used to build walls and countless buildings in Delaware as well. (It is perhaps best known through our minor league baseball team, the Wilmington Blue Rocks.)

Black mica, or biotite, is also prevalent and can found in the sediments along the creeks, particularly in the Red Clay Valley.

While the rocks in a landscape don't necessarily have any effect on soil quality, the minerals they're composed of may influence the local vegetation.

The area around Hoopes Reservoir, for example, contains serpentine, which releases an inordinate amount of magnesium when it weathers. The resulting soil is nutrient-poor and creates a harsh habitat for plants.

According to Schenck, this results in serpentine barrens limited mainly to cedar trees, grasses and greenbriar, a noxious invasive weed.

The use of lime to raise soil alkalinity can trace its Delaware origins to when early colonists mined Cockeysville marble from the Hockessin and Pleasant Hill valleys to convert to quicklime for use on their farm fields.

The marble, named for exposures in Cockeysville, Md., was converted by crushing it and firing it in kilns, where its chemical properties were altered.

The resulting quicklime was loaded onto carts and driven from Valley Road and Paper Mill Road to Limestone Road (so named because it was always covered in white lime), then down to Stanton and put on barges that floated it down the Delaware River to Kent and Sussex counties for the plantation owners there to use on their fields.

Delaware rocks reveal much about the history of our land, the nature of our soil and the creativity of those who mine them. Schenck, who is also an avid gardener, knows this intimately.

"You build a garden, and you hold it up with rocks," he says.

Moira Sheridan is a Wilmington freelance writer and gardener. She is a graduate of the University of Delaware's Master Gardener program. Reach her at masher9@juno.com.

Reference: 
For questions and information, contact DGS at
delgeosurvey@udel.edu, 302-831-2833