Felsenmeer

A thin accumulation of usually angular blocks, with no fine sizes in the upper part, over solid or weathered bedrock, colluvium, or alluvium, without a cliff or ledge above as an apparent source. ( Bates and Jackson, p. 76 )

Block fields are common in high mountain terrain or in the sub-polar regions of the globe. They are closely related to the cold climate phenomena, block flows, block streams and stone roses. Block flows are associated with solifluction, tend to form on slopes and migrate down slope over their underlying soils. Block streams fill the bottom of stream valleys and are emplaced mostly through flow in water or debris flows. Felsenmeer is formed primarily in situ in flat or low slope terrain and is primarily product of frost wedging. It is this in situ, immobile source of the blocks that distinguishes felsenmeer from these related forms. (Washburn, p 192)

Before I go on, a brief overview of the process of frost wedging should be discussed. Frost wedging is a mechanical weathering process where liquid water in the cracks of rocks, freezes with a drop in temperature and shatters the rock through its change in volume with phase. This phase change between water in ice results in a 10% volumetric increase, an increase which can ultimately exert up to 2000 lbs/in2 of pressure on the surrounding rock. (Twidale, p. 182) This pressure is rarely reached however as most rocks will fail and rupture at stresses far below this maximum. (Summerfield p 146) Frost wedging is most effective in regions where the temperature fluctuates back and forth across the freezing point regularly. It is not as effective in polar regions as the water is in permanent solid form and cant thaw to begin the wedging process. (Twidale, p. 182) Therefore frost wedging is most effective in the sub polar areas or on the high elevations of mountain chains. It is here where we return to our look at Felsenmeer.

The process of felsenmeer formation begins with the infiltration of water into cracks in the bedrock of a cold climate environment. Frost wedging occurs, shattering blocks from the parent material and often transporting the blocks upward as the water continues to expand. The size of these blocks is determined not by the intensity of the freezing and wedging, but by the depth of the infiltration of the ice into the cracks and fissures. (Washburn, p. 192) Consequently the initial size of the blocks is determined by the structure and lithology of the parent material from which the block field forms.

One of the key components of felsenmeer that keeps it from becoming one of its similar land forms is the amount of underlying soil. Many of the features that occur in the climatic zones that felsenmeer inhabits, are formed mainly due to the influence of frost needling and ice lifting in their soils. These processes work best in silty or sandy soils where the action of the ice produces stone circles, patterned ground and mud polygons (Tricart, p. 87-112) One of the key components of the formation of felsenmeer appears to be the removal of small grained constituents from the surface of the formation. This is accomplished by the washing away of the fines in the spring thaws, an event which is encouraged by the large quantity of void spaces in the block fragments. (Tricart, p. 100) With the removal of the fines, and the clearing of the voids so clay and silt accumulation does not occur, and the block field dies not freeze solid which would lead the way for further frost wedging and fragmentation of the cobble and boulders of the field.

As a result of this low silt requirement, Felsenmeer has been found to form in specific lithologies. Felsenmeer forms in coarse grained rocks, with few fines. Some fines can be present as they will be removed by the action of water, but excessive small grained erosion can cause the buildup of the conditions that allow the frost actions described above to occur, shattering the blocks, as well as the freezing of the block fields base which would cause a slick surface for migration to occur. (Block flows) In order to preclude these problems of solifluction and frost shattering, Felsenmeer tends to form in schists, granites, basalts and massive limestones. (Tricart, p. 109) If these conditions are met, then the formation and maintenance of a block field is possible.

So in summary, Felsenmeer is a cold climate process that is a result of the frost wedging of bedrock. It forms in crystaline rocks, with few fines, and is characterized by large angular blocks with absent soil in its upper layers. It is formed in sub-polar areas and in the high elevation reaches of mountains. It is closely related to Block flows and block streams but is distinguished mainly by its immobility. It is a rather fascinating process and is the main method of mechanical erosion for the regions in which it is found.

Bibliography:

Summerfield, Michael, A., Global Geomorphology, John Wiley and Sons, New York, 1991.

Tricart, Jean, Geomorphology of Cold Environments, Macmillan St. Martins Press, New York, 1970.

Twidale, C.R., Analysis of Landforms, John Wiley and Sons Australasia Pty Ltd, Nwe York, 1976.

Washburn, A.L., Periglacial processes and environments. St Martins Press New York, 1973.