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Published May 03, 2010, 03:39 PM

Just how permanent rock picking is depends on several variables

FARGO, N.D. — Just about every farmer in Agweek country might have had the chance to pick field rock in their lifetimes. Kenneth Lepper, a geologist at North Dakota State University in Fargo, notes that much of the Upper Great Plains and Midwest were glaciated in the so-called Quatemary period, 10,000 years to 2.6 million years ago. The most abundant sediment is called “till,” which is fine-grained but contains cobbles and boulders — field rock.

By: Mikkel Pates, Agweek

FARGO, N.D. — Just about every farmer in Agweek country might have had the chance to pick field rock in their lifetimes.

Kenneth Lepper, a geologist at North Dakota State University in Fargo, notes that much of the Upper Great Plains and Midwest were glaciated in the so-called Quatemary period, 10,000 years to 2.6 million years ago. The most abundant sediment is called “till,” which is fine-grained but contains cobbles and boulders — field rock.

“Till generally hosts fertile soils as the glaciers pulverized fresh rock into small grains with large surface areas,” Lepper says. “This provides plants with relatively easy access to nutrients that are found in the rocks, like potassium, for example.”

In the Red River Valley, sediments — clays and silts — near the surface are much less rocky, but rocks and boulders still are encountered. These are mostly stones that were frozen into icebergs that floated out into ancient Lake Agassiz. As the icebergs melted, they dropped the boulders into the lake.

Lepper says there are two schools of “common knowledge” about how permanent rock picking is.

If rocks are picked out of the top 10 inches, are they gone? Or will they be replaced by more stones being heaved up by the earth? Lepper says that’s hard to generalize.

Variables include the nature of the soil, its moisture content, the freezing depth of the soil and other things. In Fargo, for example, fence post footings are placed 48 inches deep to get safely below the freeze/thaw layer.

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