Blockfield
Encyclopedia
A blockfield or felsenmeer is a surface covered by block-sized angular rocks usually associated alpine
and subpolar climates and periglaciation. Blockfields differ from scree
s and talus slope in that blockfields does not apparently originate from mass movement
s.
Most known blockfields are located in the northern hemisphere, examples can be found can be found in Abisko National Park
in Sweden, Snowdonia National Park in Wales, River of Rocks
, Hickory Run Boulder Field
in the Appalachian
s of United States
. All examples except the first one are outside present day subpolar climate areas, and have thus traditionally been seen as relict landform
s from past times when these areas were under periglaciation.
The term "felsenmeer" comes from the German meaning 'sea of rock'. In a felsenmeer (also known as a block field), freeze-thaw weathering has broken up the top layer of the rock, covering the underlying rock formation with jagged, angular boulders. Freeze-thaw 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.
slope, rather than a felsenmeer. Crude sorting with boulder imbrication
can occasionally be seen on the surface of felsenmeers.
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.
regions near the Arctic Circle, especially in Iceland
, the Canadian
arctic and Norway
. Due to the slope requirements they are most commonly found on plateaus. Basalt
and sedimentary rock
s often produce larger, more numerous felsenmeers than other types of rock.
(approximately 20,000 years ago). 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.
Alpine climate
Alpine climate is the average weather for a region above the tree line. This climate is also referred to as mountain climate or highland climate....
and subpolar climates and periglaciation. Blockfields differ from scree
Scree
Scree, also called talus, is a term given to an accumulation of broken rock fragments at the base of crags, mountain cliffs, or valley shoulders. Landforms associated with these materials are sometimes called scree slopes or talus piles...
s and talus slope in that blockfields does not apparently originate from mass movement
Mass movement
Mass movement refers to the political concept of a political party or movement which is supported by large segments of a population. Political movements that typically advocate the creation of a mass movement include the ideologies of communism and fascism...
s.
Most known blockfields are located in the northern hemisphere, examples can be found can be found in Abisko National Park
Abisko National Park
Abisko National Park is a National Park in Sweden.Geographically, Abisko is situated in the Swedish province of Lapland near the Norwegian border , and belongs to Kiruna Municipality, Sweden's northernmost and largest municipality...
in Sweden, Snowdonia National Park in Wales, River of Rocks
Hawk Mountain
Hawk Mountain is a mountain ridge, part of the Blue Mountain Ridge in the Appalachian Mountain chain, located in central-eastern Pennsylvania near Reading and Allentown. The area includes 13,000 acres of protected private and public land, including the 2,600 acre Hawk Mountain Sanctuary.The River...
, Hickory Run Boulder Field
Hickory Run State Park
Hickory Run State Park is a Pennsylvania state park in Kidder and Penn Forest Townships in Carbon County, Pennsylvania in the United States. The park is spread across the Pocono Mountains...
in the Appalachian
Appalachian Mountains
The Appalachian Mountains #Whether the stressed vowel is or ,#Whether the "ch" is pronounced as a fricative or an affricate , and#Whether the final vowel is the monophthong or the diphthong .), often called the Appalachians, are a system of mountains in eastern North America. The Appalachians...
s of United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. All examples except the first one are outside present day subpolar climate areas, and have thus traditionally been seen as relict landform
Landform
A landform or physical feature in the earth sciences and geology sub-fields, comprises a geomorphological unit, and is largely defined by its surface form and location in the landscape, as part of the terrain, and as such, is typically an element of topography...
s from past times when these areas were under periglaciation.
The term "felsenmeer" comes from the German meaning 'sea of rock'. In a felsenmeer (also known as a block field), freeze-thaw weathering has broken up the top layer of the rock, covering the underlying rock formation with jagged, angular boulders. Freeze-thaw 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.
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 talusScree
Scree, also called talus, is a term given to an accumulation of broken rock fragments at the base of crags, mountain cliffs, or valley shoulders. Landforms associated with these materials are sometimes called scree slopes or talus piles...
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 ....
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
Felsenmeers are most often found in high mountain periglacialPeriglacial
Periglacial is an adjective originally referring to places in the edges of glacial areas, but it has later been widely used in geomorphology to describe any place where geomorphic processes related to freezing of water occur...
regions near the Arctic Circle, especially in Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
, the Canadian
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
arctic and Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
. Due to the slope requirements they are most commonly found on plateaus. Basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
and sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
s often produce larger, more numerous felsenmeers than other types of rock.
Age
Felsenmeers are, typically, relatively young geomorphological features. Most felsenmeers formed during or since last ice ageIce age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
(approximately 20,000 years ago). Their specific age can be determined using surface exposure dating
Surface exposure dating
Surface exposure dating is a collection of geochronological techniques for estimating the length of time that a rock has been exposed at or near Earth's surface. Surface exposure dating is used to date glacial advances and retreats, erosion history, lava flows, meteorite impacts, rock slides, fault...
, a technique that works best on materials which have been exposed to cosmic rays with little interference from trees or soils.