A blockfieldWhittow, John (1984). ''Dictionary of Physical Geography''. London: Penguin, 1984, pp. 66 and 190. . (also spelt block fieldLeser, Hartmut, ed. (2005). ''Wörterbuch Allgemeine Geographie'', 13th ed., dtv, Munich, pp. 107 and 221. . ), felsenmeer, boulder field or stone field is a surface covered by boulder- or block-sized angular rocks usually associated with

alpine Alpine may refer to any mountainous region. It may also refer to: Places Europe * Alps, a European mountain range ** Alpine states, which overlap with the European range Australia * Alpine, New South Wales, a Northern Village * Alpine National Pa ...

and

subpolar climate The subarctic climate (also called subpolar climate, or boreal climate) is a climate with long, cold (often very cold) winters, and short, warm to cool summers. It is found on large landmasses, often away from the moderating effects of an ocean, ge ...

s and periglaciation. Blockfields differ from screes and

talus slope Scree is a collection of broken rock fragments at the base of a cliff or other steep rocky mass that has accumulated through periodic rockfall. Landforms associated with these materials are often called talus deposits. Talus deposits typically h ...

in that blockfields do not apparently originate from mass wastings. They are believed to be formed by

frost weathering Frost weathering is a collective term for several mechanical weathering processes induced by stresses created by the freezing of water into ice. The term serves as an umbrella term for a variety of processes such as frost shattering, frost wedg ...

below the surface. An alternative theory that modern blockfields may have originated from chemical weathering that occurred in the

Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...

when the climate was relatively warmer. Following this thought the blockfields would then have been reworked by periglacial action. Most known blockfields are located in the northern hemisphere. Examples can be found in

Abisko National Park Abisko National Park ( sv, Abisko nationalpark) is a National Park in Sweden, established in 1909. Geography Abisko is situated in the Swedish province of Lapland near the Norwegian border (distance approx. by railway), and belongs to K ...

in Sweden, Snowdonia National Park in Wales, the Great End- Scafell Pike ridge in England, and Hickory Run Boulder Field and River of Rocks in the Appalachian Mountains of the United States. All examples except the first one are outside present day subpolar climate areas, and have thus traditionally been seen as relict

landform A landform is a natural or anthropogenic land feature on the solid surface of the Earth or other planetary body. Landforms together make up a given terrain, and their arrangement in the landscape is known as topography. Landforms include hills, ...

s from past times when these areas were under periglaciation. The term "felsenmeer" comes from the German meaning "sea of rock". In a felsenmeer or blockfield, freeze-thaw weathering has broken up the top layer of the rock, covering the underlying rock formation with jagged, angular boulders. Freeze-thaw or frost weathering occurs when water that is trapped along microcracks in rock expands and contracts due to fluctuations in temperature above and below the freezing point. Felsenmeers are formed ''in situ'', meaning that they are not transported during or after their creation.

Characteristics

Surface

Felsenmeers only form on slopes of 25° or lower. A steeper angle results in transport of the blocks due to gravity. This creates a talus slope, rather than a felsenmeer. Crude sorting with boulder

imbrication Imbrication is the arrangement of planar bodies such that they stack in a consistent fashion - rather like a toppled run of dominoes. *In roofing, imbrication is employed in the Imbrex and tegula system. *Imbrication (sedimentology) In sedime ...

can occasionally be seen on the surface of felsenmeers.

Profile

The depth of the boulder field depends on the slope angle, rock types, age, and erosional history. However, a reasonable average for felsenmeer depth is approximately 1m. Ballantyne (1998) defines three types of felsenmeer profiles: Type 1 consists of boulders overlying a matrix of fines at some depth below the surface. Type 2 consists of boulders supported by a cohesionless sandy matrix that continues from the surface down through the profile. Type 3 also consists of boulders supported by a matrix, but differs from Type 2 in that the matrix consists of silt and/or clay rather than sand.

Occurrence

Due to the slope requirements they are most commonly found on plateaus. Basalt and sedimentary rocks often produce larger, more numerous felsenmeers than other types of rock. Blockfields are most often found in high mountain periglacial regions near the Arctic Circle, especially in Iceland, the Canadian arctic and Norway and are still active in parts of Central Europe that were not covered by ice sheets. In the Southern Hemisphere inactive blockfields can be found in the Lesotho Highlands, and the Prince Edward Islands.

Age

Felsenmeers are, typically, relatively young geomorphological features. Most felsenmeers formed during or since the last ice age (approximately 20,000 years ago). Others may be pre-glacial features that survived glaciation beneath cold-based glaciers.Matthews, John A. (ed.), ''Encyclopedia of Environmental Change'', 3-volume set, Swansea: Sage, 2014. Their specific age can be determined using surface exposure dating, a technique that works best on materials which have been exposed to cosmic rays with little interference from trees or soils.

References

{{Authority control Rock formations Geomorphology Geotopes

Characteristics

Surface

Profile

Occurrence

Age

See also

References