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The Great Limestone Valley

by Mary Jasch

Northwest Jersey gardeners and others in the Great Limestone Valley that follows the southern edge of the Appalachian Mountains may wonder why they need a bulldozer to dig holes in their garden soil.

It's all because of the once-mighty mountains and the mile-thick ice that rolled in over 10,000 years ago.

"Outwash is a weird mix of different rock types, It can be made out of anything." Or come from anywhere.

The Kittatinny Valley, the New Jersey section of The Great Limestone Valley, has unique and diverse soils, according to Fred Schoenagel III, soil scientist with the Natural Resources Conservation Service, US Department of Agriculture. It's got shale knolls and limestone outcrops, conglomerate on the ridge tops, glacial till and huge expanses of outwash.

"There are soils that exist here that exist nowhere else in the world," says Schoenagel. "It's the southern terminus of the glacier."

"The limestone in the valley formed from marine life accumulated from ancient seas. The Great Limestone Valley has some of the best soils in the U.S," says Bill Tietjen, Warren County agricultural agent with Rutgers Cooperative Extension.

Limestone soils are good for plants and the gardeners who dig it. The soils are between 10 and 40 inches deep with a pale yellowish/brown color. The texture is loamy and crumbles in the hand. They tend not to have rock or gravel fragments and can hold water without getting water-logged. The average pH is 5.5 from top to bottom.

These soils are associated with a nearby moss-covered outcrop of big rounded upwardly tilting chunks of rock. The exposed rock reveals solution channels where minerals dissolved and ran and pockmarks. In the wild, limestone is covered with plants - mosses, ferns, hardwoods and evergreens.

A shale ridge (knob) is big and rolling - landscape size, rather than the smaller limestone outcrop with its protected soil pockets. The glacier ground the skinny layers of shale so the hills are rounded without outcrops. Bedrock comes close to the surface, but does not dominate the shallow soil like limestone does.

Glacial till on shale knob
A shale knob can have a broad summit with steep, wide slopes. Pieces of broken, platy angular rock that looks like potato chips litter the surface.

Shale soils are more sterile with a pH from 4.5 to 5, and in the wild tend to grow hardwoods with no understory.

Topsoil is thin on shale knobs like the one under Newton, NJ, and digging in it is almost impossible. The thin soil dries quickly. Water doesn't percolate well on this old weathered rock and tends to run off, making dry times difficult for lawns and gardens.

"The answer is to bring in loads of topsoil and do raised beds," says Tietjen. "Add a little fertilizer to compensate, and bring in compost to incorporate into the existing soil and raise it a little."


After the ice rode on top of the mountains and scraped away the limestone and shale, it carved out the valley and dropped some of the rock in meltwater under the glacier.

"Outwash is a weird mix of different rock types," says Schoenagel. "It can be made out of anything." Or come from anywhere.

Outwash areas are relatively flat and smooth on the surface and occur in broad swaths cut out by the glacier.

The soil, a gravelly to sandy loam with rounded stones, is also hard to dig. "One plus is that it has a lot of minerals in it," says Tietjen.

And to make things even more complicated, in areas of outwash that filled limestone valleys, pinnacles of limestone stick out. "There are little groves of trees on rock outcrops," says Schoenagel. "Sometimes you see that in a farm field where a piece was not plowed and it's not wet."

The Kittatinny Ridge is tough conglomerate with quartzy pebbles, sand-size rounded rock fragments cemented in a matrix. In Sandyston, the bedrock is red and the soils are yellow. In Warren County by the Delaware Water Gap, the ridge is high and wide with red sandstone. The soils along the Wallkill River are Black Dirt, the product of a long–ago lake, and near Culver's Lake the soil contains conglomerate till that dropped from the ridge.

Whether you have loamy limestone or slaty shale or any type of soil, Tietjen has the best advice. "The most important thing is to go to the county extension office to have your soil tested," he says, "and see what pH and mineral content (P,K,N) you're starting off with."

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