GLOSSARY - Physical Geography - page 154

Holocene The last 10,000 years to the present day, an interglacial
(warm period) of the Quaternary.
Humus Soil organic matter which is very resistant to decomposition.
Hydrograph A graph showing the change in water discharge over time
for a water body.
Ice shelves A thick, floating mat of ice formed where ice flowing from
the land reaches the ocean. The floating ice remains connected to the
ice on land.
Ice wedges A V-shaped wedge of ice that forms in areas with no insu-
lating snow cover. When the ground becomes very cold it cracks to
create frost crack polygons. The cracks become filled with water,
which then freeze creating a wedge of ice which can grow through
time, expanding the crack.
Igneous rock Rock that has formed from melted material.
Iniltration capacity The upper rate at which water can flow into the
soil from the surface. This rate can change through time depending on
how wet the soil is and the surface conditions.
Iniltration-excess overland flow Where the rate of rainfall or irriga-
tion water supply to the soil surface exceeds the infiltration capacity,
leading excess water to flow across the surface. This is also known as
Hortonian overland flow.
Infiltration rate The time it takes for a unit of water added to a surface
to enter a unit of soil.
Infrared radiation The energy that is released by all solids, liquids and

gases as heat.
Interglacials Long, warm phases of the Quaternary (last 2.4 million
years), in which glaciers and ice sheets receded and became limited to a
few locations.
Intertropical convergence zone The region where the trade winds
from the northern and southern hemispheres converge. Conditions are
favourable for warm, moist, rising air, resulting in cloudiness and heavy
rainfall.
Jet streams Narrow, high speed winds caused by sharp temperature
gradients, located within Rossby waves in the upper atmosphere. They
can be thousands of kilometres long and hundreds of kilometres wide.
Karst A landscape shaped by the weathering of limestone rock, charac-
terised by underground drainage tunnels and surface depressions.
Kettle holes Depressions in the surface of a glacial sediment deposit
caused by blocks of ice which become surrounded by sediment and,
after melting, leave a hole in the sediment.
Keystone species A species which is connected to most levels of the