are separated by bare ground. It has been conjectured that periglacial stripes on Salisbury Plain in England, that happened by chance to align with the solar sunrise at mid summer and sun set at mid winter , gave rise to awe and veneration by prehistoric people that eventually culminated in the building of the Stonehenge. Formation In periglacial areas and areas affected by seasonal frost, repeated freezing and thawing of groundwater forces larger stones toward the surface, as smaller stones flow and settle underneath larger stones. At the surface, areas that are rich in larger stones contain much less water than highly porous areas of finer grained sediments. These water-saturated areas of finer sediments have a much greater ability to expand and contract as freezing and thawing occur, leading to lateral forces which ultimately pile larger stones into clusters and stripes. Through time, repeated freeze-thaw cycles smooth out irregularities and odd-shaped piles to form the common polygons, circles, and stripes of patterned ground. Patterned ground occurs in alpine areas with freeze thaw cycles. For example, on Mount Kenya seasonal frost layer is a few centimetres (inches) below the surface in places. Patterned ground is present at to the west of Mugi Hill. These mounds grow because of the repeated freezing and thawing of the ground drawing in more water. There are blockfields present around where the ground has cracked to form hexagons. Solifluction occurs when the night temperatures freeze the soil before it thaws again in the morning. This daily expansion and contraction of the soil prevents the establishment of vegetation. Frost also sorts the sediments in the ground. Once the mantle has been weathered, finer particles tend to migrate away from the freezing front, and larger particles migrate through the action of gravity. Patterned ground forms mostly within the active layer of permafrost. See also Emergence Glacier Ice field Ice sheet Mima mounds References External links Periglacial landforms Soil landforms