An 1872 Department of Agriculture report estimated over 240,000 miles of stone walls in New England, long enough to stretch from here to the moon. Have you ever wondered about how these walls got here?
Natural History
New England’s bedrock consists of a chaotic mix of ancient sea bed, volcanic activity and prehistoric mountains thrust upwards during collisions between continental plates. For 50,000 years this area was buried under a sheet of ice 1 mile deep. Year after year, as the snows piled up, compressing the bottom layers of ice until they started to flow under the incredible weight; crushing and pulverizing the rocks below as the glaciers flowed south. During the ice age, glaciers scoured the bedrock, plucking and carrying massive amounts of rock and stone as they advanced, only to leave them behind as vast glacial deposits (called till) when the glaciers eventually receded. Over the 12,000 years since the glaciers retreat, weathering rocks, penetrating roots, and decaying organic matter eventually buried these rocks deep beneath a rich surface soil.
Robert Frost’s “something” is the freeze-thaw cycle of our changing seasons. As we all know, water expands when frozen, increasing its volume by 9%. When ice is formed underground, it can generate extreme force on its surroundings and cause frost heaving. Moisture laden soil freezes in the winter creating a sub-surface layer of ice called a lens. Capillary action brings more water percolating up from the unfrozen soil below to feed the expanding ice lens, which in turn pushes overlying rocks upwards as it expands. The "heave" refers to upward movement of the ground and the force of a growing ice lens is sufficient to lift a layer of soil with over 30,000 psi of pressure. They don’t call it a ‘heave” for nothing! In the spring thaw, as the ice melts from below, the gap underneath the rocks is partially filled in and the overlying rocks settle, but not to their original position. Hence, over many cycles, stones that had once been deeply buried were gradually raised to the surface, where they were encountered by farmers cultivating the land.
Curiously, the first Colonial settlers did not have to contend with rocky soil. After clearing forests to make fields for farming or pasture, they found a rich layer of soil with few rocks. Stone walls weren’t commonly constructed until the latter half of the 18th century when widespread deforestation of the New England landscape occurred. Ironically, it was the act of clearing woodlots and more intensive cultivation that caused these crops of stone (sometimes called “New England Potatoes”) to appear each spring. Rocks embedded in woodland soil are held in place by roots of trees and shrubs; expanding and contracting as a unit. After the fields were cleared, regular plowing prevented roots and trees from taking hold, thereby permitting the conditions where rocks were gradually lifted towards the surface. Frost heaving was also accelerated by a global climate phenomenon known as the Little Ice Age, which saw significantly colder temperatures than today causing the ground to freeze deeply. The peak intensity of the Little Ice Age coincided with the peak period of deforestation (heating a typical New England farmhouse required burning up to 35 cords of cut wood a year), which in turn coincided with the most intense period of stonewall building.
Modern day experiments have shown that there is a sequence of stone sizes and shapes that will appear in a pasture or field because the shape and size of the rock influences how fast they are heaved up. Slabs appear first as their shape allows them to be heaved more efficiently, followed by large, rough, angular granite which are raised faster than smaller smoother stones.
Human History
Farmers built stone walls as they cleared the land for pastures, hayfields and crop fields, and the walls served as fences to keep livestock in or out of those fields. As we tromp around the countryside a century and a half later, using billion dollar satellites to find geocaches hidden in these walls, pause to take a moment and contemplate the stories that are also hidden within the stones:
Crude walls of large rocks which lack small stones indicate an area that was not regularly plowed and likely used as livestock pasture.
Wide walls with large rocks framing the edges and small rocks filling the insides indicate a wall surrounding a crop field. The presence of numerous fist sized stones, and/or piles of small stones indicate where the farmers tossed their rocks as they plowed. A higher soil level on one side of the wall compared to the other also indicates frequent plowing.
Stone walls build around hayfields are generally not very wide and contain only a small amount of fist-size stones. Hayfields only required plowing a few times during its use, to smooth the terrain for the scythe; as opposed to crop fields which were plowed every spring before planting. Hence, there were fewer small rocks to discard.
Big, old trees found next to stone walls can also give us clues. Trees that grow in a forest quickly reach for the canopy as they compete for sunlight, whereas trees that grow in full sunlight can lazily extend outward. Old, wide-spreading trees once grew in the open and are sometimes called pasture trees as they were left to provide shade for the livestock. Border trees, in which one side has extended limbs and the other has rounded knobs from limbs that have died and healed, indicate that one side of the wall was was abandoned before the other.
Remnants of wire in and around the stone wall can tell us even more about the area. Wire indicates the area was abandoned after 1874 when wire was first mass produced. If the wire is barbed, the field likely held cows or horses, whereas sheep were penned in with smooth wire because their heavy fleece could get tangled in barbs and injure the animal. Walls that have no wire were likely abandoned between 1840 and 1865 during the migration westward to Ohio and beyond.
Logging Requirements:
Please send me an email with your answers to the following questions, and please do not post any spoiler photos.
The coordinates bring you to a T intersection of two stone walls. To make the necessary observations, you will need to explore the walls for about 50 ft in each in each direction from the intersection (north, south and east). It will be difficult to see the clues in deep snow. Then, using your powers of observation and what you have learned from this cache page, please tell the geological story of the landscape:
1. In your own words, describe how these rocks got here. Are they local, or were they carried here.
2. By what forces did they come to the surface, and can you observe any evidence of continued action by these forces after the fields were abandoned? Do you think those forces are still occurring today?
3. Knowing about the sequence of stone sizes and shapes that appear, which wall was built earlier, the East/West or North/South wall? How can you tell?
4. The T intersection of the N/S and E/W walls divides the area into three sections, North, South and West. Understanding the influence of cultivation on frost heaving, can you determine the activities occurred in each of these sections (crops, hay or pasture)? How can you tell?
Optional (Note: While this site presents a fascinating intersection between natural history and human history Earthcache guidelines do not allow me to require you to answer non-geological questions.) Using your powers of observation and what you have learned from this cache page, please tell the human story of the landscape
5. Which animal(s) were pastured here? How can you tell?
6. When were these fields abandoned? How can you tell?
7. There is a large tree near the posted coordinates. Does it give you a clue as to whether the wall was constructed in a forested or cultivated landscape? Was one field abandoned before the other? Again, what evidence do you see to support your conclusions?
Sources:
Wessels,T. Forest Forensics. The Countryman Press 2010
Wessels, T. Reading the Forested Landscape. The Countryman Press 1997
Thorson, R. Stone by Stone: The Magnificent History in New England's Stone Walls Walker & Company 2004
https://en.wikipedia.org/wiki/Frost_heaving
http://studentaccess.emporia.edu/~acourtne/FrozenGround.html