Question

This wall, located in New England, is built of diverse stones and boulders cleared from nearby fields. In 1914 , Robert Frost wrote a nowfamous poem titled "Mending Wall" about a feature like this one. It begins with these lines: Something there is that doesn't love a wall, That sends the frozen-ground- swell under it, And spills the upper boulders in the sun; And makes gaps even two can pass abreast. a. What is the likely weathering process causing the wall to swell and "spill" its boulders? (Think back to Chapter 6 .) b. Is it likely that the source of all the rocks in the wall is bedrock in the immediate vicinity? Explain. c. What term applies to the rocks composing the wall?

Short answer

a. Freeze-thaw weathering. b. Yes, likely from nearby bedrock due to glaciation. c. Fieldstone.

Step-by-step solution

Identify the Weathering Process

The lines from Robert Frost's poem describe a wall experiencing a swelling and the spilling of boulders due to the frozen-ground swell. This indicates freeze-thaw cycles, a physical weathering process, where water enters cracks, freezes, expands, and thus exerts pressure on the rocks, causing them to break apart and move.

Assess Local Rock Sources

Considering that the wall is built from stones and boulders cleared from nearby fields, it is likely that the source of these rocks is indeed the bedrock in the immediate vicinity. New England’s landscape was significantly shaped by glaciation, which left deposits of rocks and boulders scattered across the region.

Define the Type of Rocks in the Wall

The rocks composing the wall, which consist of different types and sizes collected from fields, are typically referred to as "fieldstone." Fieldstone is a term used for naturally occurring rock found on or near the surface in fields, often used for building structures like walls.

Key concepts

Freeze-thaw cycles

Freeze-thaw cycles are a natural process that plays a significant role in weathering, which is the breakdown of rocks, soils, and minerals. This phenomenon occurs in climates where temperatures fluctuate above and below the freezing point.
When water finds its way into cracks in rocks, it can freeze as temperatures drop. As water turns to ice, it expands by about 9%. This expansion exerts tremendous pressure on the rock, causing cracks to widen. This process repeats over many cycles, eventually leading to the rocks breaking apart.
In areas like New England, freeze-thaw cycles are particularly common due to the cold winter months. The expansion of water when it freezes can "spill" boulders and stones, as described in Robert Frost's poem, leading to gaps in structures like walls.

Fieldstone

Fieldstone refers to rocks that are found naturally on or near the ground's surface in fields. These rocks are typically irregular in shape and come in a variety of sizes. Fieldstone is often collected and used in construction, especially for building structures like walls and fireplaces.
These stones are usually gathered from the local environment, where they have been left by natural processes like glaciation (discussed further below). Because fieldstones are found naturally, they often blend well with the surrounding landscape, giving structures a rustic and natural appearance.
The use of fieldstone in New England walls is a great example of this, as the stones are often picked from nearby fields where they were deposited from the glaciers that once covered the region.

Glaciation

Glaciation is a powerful geological process that shapes landscapes by moving and depositing rock materials over vast areas. During a glaciation period, massive glaciers form and move across continents. The glaciers scrape and erode the surface of the earth, transporting a variety of rocks and sediments.
As glaciers advance and retreat, they leave behind these materials, creating deposits known as moraines. New England's landscape, for example, was greatly affected by the glaciations of the last Ice Age, leaving behind the characteristic pieces of fieldstone.
These glacial deposits are the source of many fieldstones used in the region. The distributed rocks from glaciation are a testament to this slow yet vastly transformative process.

Physical weathering

Physical weathering, also known as mechanical weathering, involves the breakdown of rocks without any change in their chemical composition. This type of weathering is typically caused by physical forces such as temperature changes, water, ice, and wind.
Freeze-thaw cycles are a prime example of physical weathering, as they cause rocks to fracture and disintegrate without altering their internal minerals. Other methods include root expansion, where plant roots grow into and push apart rocks, and abrasion from particles carried by wind or water.
Understanding physical weathering is crucial when considering landscapes affected by a high degree of environmental stressors, like those in New England. By recognizing these processes, we gain insights into how such natural forces sculpt the earth's surface.