Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 244

FIGURE 5.5 Freshly upheaved stones on a gentle slope at 3,400 m near Deluge Lake in the Gore
Range near Vail, Colorado. (Photo by J. R. Janke.)
Frost heave and thrust are often evidenced by the upheaval and ejection of rocks
from depth. Primary frost heave occurs at the frost line and is the result of ice expan-
sion. Secondary frost heave occurs as ice continues to expand over time because of the
attraction of water and concentration of ice in soil layers. Rocks that are freshly up-
heaved have newly exposed unweathered surfaces that are lichen-free. Heave is made
evident by disrupted vegetation and soil surfaces. In some cases, rocks may be heaved
1-2 m above the surface and stand like lonely tombstones amid the tundra (Price 1970)
(Fig. 5.5).
The principal processes involved in the movement of stones through frost heave and
thrust fall into two groups: frost pull and frost push. Frost pull operates when the soil
freezes and expands upward, pulling rocks with it. Upon thawing, the rocks do not re-
turn to their original positions, as soil fills the voids beneath the rock. Thus, the stones
migrate toward the surface by increments with each freeze and thaw cycle. Frost push
results from ice lenses growing underneath stones and pushing rocks upward. Because
rocks have greater conductivity than soil, heat (or cold) can pass through them more
quickly than through the soil. Therefore, ice lenses can accumulate at their bases and
cause differential heaving. As with the frost pull mechanism, finer material then seeps
into the cavity underneath and prevents the full return of the rock to its initial position
(Washburn 1980). Ample water and a stable freezing front are critical for the develop-
ment of ice lenses that produce heave (Smith and Patterson 1989). Otherwise, pore ice
will form if the freezing front moves rapidly or water supply is cut off. Segregated ice