By Sally O’Byrne, Teacher-Naturalist
Last Wednesday, I attended the weekly bird survey at the Delaware Nature Society’s Flint Woods Preserve near Centreville. Mike Weaver, a Land Manager who works there, joined me on the walk. We were impressed with the amount of water that had seeped into the ground with this year’s excessive precipitation.
Another unusual feature was how last night’s ice formed in areas with saturated soil that didn’t have standing water. Along a path that was damp, the leaves were pushed up by the ice. Looking closely, we saw that the ice was in the form of miniature columns.
When we plucked an ice column from the mud, it was obvious that the ice had formed from the ground up and had “leached” mud from the dirt into the columns by some sort of capillary action. Some of the columns were 6 to 8 inches high. This was a new find for both Mike and I.
Epilogue (By Joe Sebastiani): What Sally and Mike found was a form of Frost Heave. This usually happens in moist, fine grained soil, like silt, and it is not just simply due to freezing and the associated volume expansion of water turning to ice. Vertical displacement of soil from Frost Heave is far greater than the volume expansion associated with water freezing, which is about 9%. Why does this Frost Heave occur then? Simply put, the column of ice is drawing in liquid water trapped in surrounding small pores of soil. This happens as the ice purges itself of tiny foreign particles, raising the ice upwards while it sucks in surrounding water from the soil. This liquid water is added to the column as more ice. So, the ice moves upwards and grows at the same time. (This is according to Wikipedia, so you can look it up yourself if you really want the dirt, uh, ice on this phenomenon).
Frost Heave occurs when you have the following conditions…freezing temperatures, a supply of water, and soil that has the ability to conduct water, a high affinity for it, and is saturated. Clay soils are not very water conductive and sandy soils have a low affinity for water. Silty soils are the middle ground (no pun intended), and tend to be where Frost Heave occurs.
Now, go out there and see if you have some Frost Heave where you live. Watch out! Frost Heave can damage plant roots, cause erosion on steep slopes, and even crack concrete as it pushes up the soil!