Chapter 11—Glaciers and Glaciation
To aid in your understanding of glaciers, it is important to be familiar with terms that
describe glacial landforms.
Cirques—steep sided, bowl-shaped depressions where snow and ice accumulate in the
upper reaches of valleys
Tarns—small, ice-carved depressions in glacial valleys that fill with water when ice
melts
Horn—triangular peak formed when three or more cirques flank a mountain summit
Headwall—near vertical wall around a cirque; can be the upper slope of a horn; intense
mechanical weathering on headwalls result in very irregular topography (think “sawtooth
peaks”)
Arete—knife-like ridge separating adjacent cirques and U-shaped valleys
Cols—isolated low areas or passes along an arete
U-shaped valleys—glacial erosion carves large, U-shaped valleys, as opposed to
streams, which carve V-shaped valleys
Hanging valleys—formed at the intersection of tributary glaciers and parent glaciers
because parent glaciers erode deeper valleys than tributary glaciers; when the ice recedes,
there is usually a steep wall left at the intersection, and waterfalls result
Glacial till—unsorted and unstratified glacial debris deposited directly on the ground
after ice melts
Glacial outwash—glacial till transported by melt water for some distance and then
deposited in a sorted and stratified fashion
Ground moraine—unsorted, unstratified material laid down directly beneath melting ice
Terminal moraine—elongate mounds of glacial till that mark the farthest advance of the
ice (deposited at the end, or terminus, of the glacier as it begins to recede)
Recessional moraine—same as terminal moraine, but marks a place where the ice stood
stationary during an otherwise recessional phase
Lateral moraine—till deposited along the edges of glacier
Medial moraine—till deposited along the center of glacier
Outwash plain—in front of melting glacier, meltwater deposits glacial material here
Kettles/Kettle lakes—depressions in the outwash plain, lakes if they fill with water from
buried or trapped blocks of ice
Drumlins—if glaciers re-advance over previously deposited glacial till, the ice may
mound the till into these small hills, having an elliptical shape
Glacial erratics—large boulders left behind by retreating glaciers that appear ‘out of
place.’ Ex.—large glacial erratics in otherwise flat N. Missouri
Last episode of glaciation in N. America—began ~3 million years ago, eventually nearly
all of Canada and N. US (southern boundary of glaciation is ~ Missouri River). Ice
retreated and advanced many times; last extensive episode reached its peak ~18,000 years
ago.
Alpine Glaciation—shapes pre-existing mountain ranges
Continental Glaciation—more widespread, covers all topographic features. Rounds or
flattens hills.