By this time of year, most local farmers and gardeners have finished pawing through the seed catalogs and planning this summer’s harvest. What most have tried not to think about is the other crop that’s likely to turn up in their fields this spring: stones.
As a kid growing up in New England, I just assumed that spring stones were a fact of life worldwide. Little did I know then that New England has it bad - about as bad as it gets.
First off is that, in New England, we have lots of stones to begin with. The advancing glaciers did a good job scraping off whatever loam might have been here and replacing it with a jumble of sand, gravel, and stones. If your pitchfork brings up a stone that’s in any way rounded or smoothed, chances are good that it was left there by the glaciers. If your fork hits something sharp and angular, you’re probably digging up a piece of the bedrock itself, and you can thank the glaciers for that, too, since they scraped off everything else.
Consider the lower Mississippi Valley for contrast. The glaciers never advanced that far south, so soils there have been accumulating without interruption for millions of years. Sometimes the soil can be hundreds of feet thick, with nary a rock all the way down.
The second reason we harvest so many stones in New England is because of winter’s frost. Without frost, all those stones would stay right where the glaciers left them, safely tucked in the ground. Instead, the freezing and thawing of winter works the stones loose and brings them to the surface, meaning that, no matter how well you cleaned the fields last spring, you’re likely to have a fresh crop waiting this year.
How the stones actually make their way to the surface is a matter of some debate. There’s the “they were pushed” school and the “they were pulled” school. The “pushers” believe that, because a stone conducts cold more efficiently than soil (it’s solid and denser, without any insulating air space), a stone causes the soil beneath itself to freeze faster than nearby soil at the same depth. This frozen soil expands as the water in it freezes, and the expansion acts like a jack, pushing the stone up from below through the as-yet-unfrozen surrounding earth.
The “pullers” concede that the freezing of water in soil is what causes a stone to move upward, but they believe it’s the soil freezing next to a stone, not under a stone, that causes the movement. As the soil freezes, it grabs the sides of the stone and pulls it upwards, yanking it out like a loose tooth. Unfrozen soil then oozes into the gap under the stone and prevents it from falling back into place come springtime.
Regardless of which theory is more accurate, the third reason we harvest lots of stones in New England is because we till the soil in the first place. Think about our forests for a moment, which nobody has tilled; how come, after ten thousand winters of freezing and thawing, our forest floors aren’t completely covered in loose stone?
The answer is that a few inches of organic matter – leaves, twigs, moss, roots, and everything else that ecologists call “duff” – provide pretty good insulation for the soil. Even the grass in your lawn insulates the ground somewhat, meaning that only where the soil is bare will the freeze/thaw action be powerful enough to lift stones to the surface. Home gardeners can help themselves, therefore, by spreading a thick mulch of leaves or straw in the fall to prevent the frost from penetrating too deeply into the ground.
But what if there is no natural duff to protect the soil? The question isn’t entirely hypothetical. Picture New Hampshire’s Presidential Range for a moment, and Mount Washington in particular. The soils are thin, there’s very little insulating, organic material above treeline, and the entire landscape is blanketed by loose rock and stone.
Incredibly enough, there are so many stones on Mount Washington that they take turns on the surface, rising up one year only to be covered by more stones rising up in later years – circulating sort of like a thick, bubbling stew. Each stone takes hundreds or possibly a thousand years to make a complete round trip from the soil to the surface and back into the soil again.
So as you lever yet another stone out of your garden this spring and catch yourself thinking, “this couldn’t get any worse,” remember the Alpine Garden on Mount Washington.