Rocks

As many gardeners know, it is in late autumn that granite breeds. Its gestation period generally lasts through the winter, with the resulting offspring appearing by planting time. At leaf-drop we will notice again just how many rocks there are. Bradford, New Hampshire, where I live, has, by one reckoning, at least 250 miles of stone walls.

Stones, however, were not always in such obvious abundance.

What is now central New Hampshire was once the northwestern coast of Africa, while most of Vermont is part of the original North American continent. This accounts for the differences in geology in general and the rocks in particular between the two states, but perhaps not for the differences in politics.

The stones in New Hampshire formed at the core of the Appalachian Mountain range created when Africa and North America collided 680 million years ago. A similar collision is taking place today, as India is slowly being pushed into the soft underbelly of Asia. The Himalayas are currently rising at a rate of about ½ inch per year.

New England didn’t become rocky until the appearance of the Laurentide Ice Sheet. The ancient soils were scoured down to bedrock. Stone slabs were heaved, broken, and scattered about. The lush temperate forests eventually began to return 10 thousand years ago, continuing the slow buildup of organic soils begun by the enterprising lichens and which would again bury a great many of the smaller stones.

The pervasive mythology of this region relates that heroic pioneers cleared a rocky wasteland to create farms. Though not without extensive exposed outcroppings, early hill farms were not heart-breakingly stony. According to historians, geologists, and historical records of the time) upland farms were largely fertile in the early 1700s, and remained so if treated with care.

In a crude oversimplification of complicated and profound events, one could say that the social and political changes in colonial agriculture from communal food production to self-sufficiency, the exploding population of Europeans and removal of native peoples, the deforestation of New England, and the culmination of the “Little Ice Age” by the early 1900s all contributed to the emergence of slumbering hordes of stonewall-sized rocks.

The first European coastal settlements were built on soils first cleared for cultivation by the Native Americans. These lands were sandy and largely devoid of rocks. Most stonewalls were built further inland in the first half of the 1800s, well after the American Revolution.

Although many walls were built to delineate ownership, most walls needed the addition of wood (and later, wire) to function as animal barriers. Disrespectful though it may sound, many stone walls were in fact only linear waste heaps for the surfacing surplus. Rocks were moved to the sides of pasture, hayfield, and cropland according to the preponderance of stone, availability of labor, and the turning radius of the given farm equipment. The average stone-walled field is between two and five acres for these reasons.

The heaving of rocks is a complicated and much-studied process. In heavily forested areas, normal winter snow, a heavy layer of leaves, and a modicum of topsoil prevents the ground from freezing more than a couple of inches deep. By contrast, a pasture or thinly wooded area may freeze to the depth of several feet.

Frost heaving begins when water in the surface soil freezes. Water expands as it turns to ice, thus soils expand during freezing. Water in liquid or vapor form is attracted toward soil that is already frozen and each speck of ice between grains of soil consolidates the particles into a single rigid mass. As the frost line deepens it reaches the tops of rocks.

When the “head” of the rock is frozen into the descending frost layer a small void is created beneath the base of the rock. Because spring thawing happens from below as well as from above (deep subsoil remains at about 55 degrees F.), soil adjacent to this void falls in while the stone itself is still frozen in place from above. The stone, once the thawing ground above releases it, is unable to return to its original niche and thus over time may eventually poke through the thawing and descending upper soil.

A distinct process called “frost push” can happen at shallower depths. Cold is conducted at a faster rate through stone than through the surrounding soil. This causes the bases of these rocks to freeze; the cradling soil then freezes and expands, pushing rocks up.

These forces of frost heave and frost push after the large-scale felling of the original forests were not the only processes luring stones up into the sunlight. Loss of topsoil and compaction caused by overgrazing, especially on slopes, were other significant factors.

What farmers, scientists, and even amateur naturalists such as Thoreau documented in the 1800s in North America (and much earlier in Northern Europe) was that the supply of stones, not too troublesome at first, appeared within a few decades after the trees came down.

The clearing of stones at the first half of the 19th century was an annual chore for several generations of farmers, though a picnic compared to the job of supplying 20 or more cords of wood to heat the average home. After that time the rate of rock “production” slowed.

Though not yet in my garden.

By J. Ann Eldridge, UNH Cooperative Extension Wildlife Coverts Cooperator

2/23/06