This large rock finally rose to a level where the point of a garden plow at Two Toad Farm found it. Photo: Two Toad Farm
By Kevin McKeon, Maine Master Naturalist
Beginning about 2.6 million years ago, several glacial episodes scoured Maine’s bedrock surface with ice sheets two miles high, grinding against rock like a huge sandpaper machine. Stones of all sizes were created by these glacial advance/retreat cycles: Huge boulders were plucked from mountainsides and dug out from bedrock; some became smaller rocks by grinding against each other and against the bedrock. The weight of these massive ice sheets dragging along the bedrock with imbedded rocks created pebbles, sand, and silt. Later on, melting glacial waters deposited these imbedded rocks as the sheets retreated. Some dropped from the ice, remaining as large boulders called glacial erratics. The smaller stones and particles were carried along with glacial meltwater torrents, deposited where the water slowed a bit and could no longer carry them. Sand, silt, and clay particles were layered along these meltwater flows the same way, covering the earlier deposited, larger stones.
Slowly, vegetation migrated into the scoured, stony landscapes. Liverworts, hornworts, mosses, and lichens formed mycorrhizal relationships with the existing fungi by attaching to each other, anchored to rocks with roots that excreted acids capable of dissolving minerals, which the plants ate. Photosynthesis provided the sugars, and together these vegetative/mycorrhizal coverings went through multitudes of growth, death, and decomposition cycles, slowly building organic-rich soil within the landscape. Eventually, forests migrated and colonized, adding to the soil-building cycles.
Also migrating into the landscape were the animals able to sustain themselves by eating vegetation, and the animals that ate those animals. Among these creatures were the soil dwellers, like grubs, ants, and worms — tunnel-makers that move and mix soil particles. Other things that made tunnels were the plants’ roots, leaving voids after death and decay. Heavier rocks above these tunnels tended to crush them, causing an ever so slight sinking of the rocks. Over the millennia, rocks sank and became concentrated at depths more or less determined by the tunnels. All this while the forests and soils grew.
As forests evolved and thickened, they offered the soil a bit of protection from freezing, so the soils froze less deeply. As the organic soil thickened, the submerged rocks slept. Then came the settlers and pioneers who cleared the forests for their crops and pastures. The resultant forest openings created a path for cold to move deeply into these now-exposed soils. Freeze/thaw cycles caused expansion/contraction cycles — frost heaving. It’s these cycles that tend to pull and push rocks toward the surface, and agricultural practices of the time accelerated these movements.
The mechanical, chemical, and hydrological changes that accompanied agriculture combined to awaken our sleeping, soil-bound rocks. Forest-covered soils rarely freeze more than a few inches deep, and water remains mostly unfrozen. Deforestation for agriculture removed the soil-freezing buffer, allowing soil to freeze three feet deep or more, causing heaving actions among the long dormant rocks. Tilling increased the depth subject to these cycles; it also sped up soil thaw in the spring, allowing for earlier planting. But tilling also resulted in a persistent layer of ice below the tilled soil, preventing the early spring meltwaters from percolating through it, causing erosive surface soil runoff. With the loss of a bit of topsoil, our rocks became a bit closer to the surface, and more exposed to the effects of freeze/thaw soil heaving. So, what is heaving’s relationship to rocks?
When water freezes, it expands by about 9% and encapsulates soil particles. This ice and soil sheet also attracts and captures liquid and gaseous soil water from below, so the ice sheet grows from below. As this sheet — or frost zone — deepens and reaches the top part of a rock, it also gets captured, pulling — heaving — it up ever so slightly, leaving a bit of a void under the rock: This lifting effect happens near the maximum frost depth level.
Closer to the surface, within the frost area, the stone experiences an added pushing effect. As frost works its way downward, cold is conducted through the stone, causing the stone’s lower surface to freeze. Liquid and gaseous soil water then freezes to the bottom of the stone and expands, thus pushing the stone up.
Now comes the spring thaw, and the melting of soil ice. But soil thaw also happens from the bottom of the ice/soil sheet from Earth’s ground warmth. So, we have our lifted rock still captured by the ice/soil sheet, while ice begins to melt in the lower part of the sheet. As it does so, particles of soil released from the sheet travel with the melting ice into the void created by the lifted rock — and the voids left by the soil critters and rotted roots — filling in those spaces, leaving no place for the rock to settle, effectively “lifting” the rock toward the soil surface. And our other “pushed” rock is effectively supported by the soil under it.
So, both rocks — one within the frost zone and one a bit deeper — are now closer to the surface. Both these pull and push forces result in maybe a quarter-inch upward movement per freeze/thaw cycle, several inches per decade, making farmland quite rocky for a few generations as rocks breach the surface. Eventually, under constant tillage, most of the rocks within the frost zone are raised and cleared from the fields, making centuries-old farmland almost rock-free. But for most of us backyard gardeners, the spring rains wash off those emerged rocks, giving us the effect of our gardens growing rocks. Just don’t try to peel ‘em with a potato peeler!
Water freezing: https://www.youtube.com/watch?v=T4GCShGvw-M
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