Geography Reference
In-Depth Information
occur in large numbers across an area, such as the 10,000 or so that
occur in west central New York State. There is considerable debate
as to the mechanisms that cause drumlin formation, including that
they were formed during really large flood events.
The outwash plain downstream of a glacier is often rich in
gravels and boulders, and the meltwater system also contains a lot
of rock flour and has a milky colour. Braided river systems are
common in these environments. When large blocks of ice get
swept down the river system and are deposited and buried they can
later form
kettle holes
. The slowly melting ice leaves a depression
in the surface where elsewhere in the surrounding landscape sedi-
ment had been deposited by the river system. On the other hand,
braided rivers may deposit sediments that are several hundred
metres thick in some locations, masking underlying features.
Permafrost
Permafrost refers to soil or bedrock that is frozen for longer than
two years. It covers around 25 per cent of the land surface,
although this is mainly in the northern hemisphere where there are
significant land masses at high latitudes. More than half of the land
area of both Russia and Canada, the world's two largest countries,
is covered with permafrost. Permafrost usually consists of soil
which contains ice within its pore spaces. Most of the Earth's per-
mafrost is very susceptible to climate change because it exists at
temperatures just a few degrees below 0ºC.
At high latitudes, but outside of ice sheets and glaciers, perma-
frost tends to be continuous and often extends several hundred
metres in depth. At slightly lower latitudes the permafrost may not
be continuous and is also thinner, perhaps just being a few metres
in depth. Continuous permafrost moves to discontinuous perma-
frost at a mean annual air temperature of around -6° to -8°C while
discontinuous permafrost extends to a mean annual temperature of
around -1°C within continental interiors. Close to the surface
there is an annual melt and freeze cycle in both continuous and
discontinuous permafrost. Temperatures measured in permafrost
are typically lowest close to the ground surface, excluding the
active layer
, and increase with depth, reaching the melting point
at the base of the permafrost.
