James S. Aber
|Glacial sediment||Glacial till|
Glacial sediment can be divided in two general categories: till and stratified sediment. Till is defined as "... a sediment that has been transported and is subsequently deposited by or from glacier ice, with little or no sorting by water" (Dreimanis and Lundqvist 1984, p. 9). It is material that was released from glacier ice usually by melting and was deposited without significant transportation or sorting due to water or gravity movements.
|First Cromer till at West Runton, United Kingdom. Strongly banded (sheared) till with pebbles of white chalk and black flint. Small secondary faults are present. Red pocket knife for scale. Photo date 9/88; © by J.S. Aber.|
|Stratified sand and gravel deposited by glacial melt water in south-central New York. Note variable bedding, sorting, and grain size of sediment. Scale pole marked in feet. Photo date 6/77; © by J.S. Aber.|
Where glacial sediment is transported and deposited by mass movement or from water, stratification and sorting are usually evident. Such sediment should not be called till, even though it may superficially resemble till. Likewise pre-existing bedrock and/or sediment that was deformed by ice movement should not be called till unless it was penetratively sheared and mixed together with some new glacial sediment--glacial mélange.
|Chalk-banded glacial mélange on the island of Møn, southern
Denmark. Chalk, till, and sand are sheared and folded around the granite
cobble. Ice movement from right to left; scale pole marked in 20-cm intervals.
Photo date 5/79; © J.S. Aber.|
The word "till" has many different meaning in English. In this context, it refers to
sediment deposited directly from glacier ice. Several general observations may be given about the
occurrence of till (adapted from Goldthwait 1971):
Lower gray till exposed at Cedar Bluffs, eastern Nebraska. Note
fragment of spruce wood preserved in till. The ice advance that deposited
the till apparently overran the remains of a boreal spruce forest.
Photo date 7/85; © by J.S. Aber.
|Oriented boulder embedded in lower gray till at Atchison, Kansas. Striations on the boulder top trend N60E, parallel to the boulder's long axis. Such features indicate the direction of ice movement (NE to SW) at the time of till deposition. Needle is 14 inches (35 cm) long. Photo date 10/87; © by J.S. Aber.|
The genesis of till deposits can be illustrated graphically by the till prism--see Fig. 4-2. Primary tills are created directly from glacier ice by a combination of lodgement, meltout, and/or deformation of pre-existing sediment. Secondary tills are reworked by mass movements (flow). The best field evidence for ancient glaciation consists of tillite containing erratics and striated stones resting on a glacially abraded or deformed substratum. Such tillite/pavement associations, if widespread, are unequivocal proof of former glaciation. Direct glacial deposits are frequently associated with melt-water sediments laid down at the ice margin.
Ground moraine is deposited as a nearly flat plain beneath an ice
sheet; whereas various types of moraine ridges or hills build up in
ice-margin positions. The latter category includes: end, lateral,
interlobate, ribbed, and hummocky moraine--see
Fig. 4-3. These moraines and other glacial
landforms are nicely illustrated on the Glacial Map of Canada
(Prest et al. 1967).
Landforms of glacial deposition are many and varied, including several
kinds of moraines. This term is a French word that originally
referred to rubbly piles of debris in front of alpine glaciers. It is now
used in many different contexts referring to constructional landforms
(American) or deposits (European). Moraines may be composed of diamicton
and/or stratified sediment.
Ground moraine is deposited as a nearly flat plain beneath an ice sheet; whereas various types of moraine ridges or hills build up in ice-margin positions. The latter category includes: end, lateral, interlobate, ribbed, and hummocky moraine--see Fig. 4-3. These moraines and other glacial landforms are nicely illustrated on the Glacial Map of Canada (Prest et al. 1967).
|Massive end moraine of Late Mérida (Wisconsin) age, located behind the Los Frailes hotel in the Venezuelan Andes Mountains. This moraine was deposited by a large valley glacier that advanced from the left. Photo date 6/96; © by J.S. Aber.|
|Hummocky moraine landscape in southwestern Saskatchewan, Canada. The hummocky topography resulted when ice stagnated in the "Gap" between the western and central blocks of the Cypress Hills (high plateau on horizon to right). Irregular deposits and hollows formed as the ice wasted away. Photo date 5/93; © by J.S. Aber.|
|Hummocky moraine with many small lake basins at Kurtkowiec, Tatra Mountains, southern Poland. Small recessional and medial moraines form dams for lake basins of irregular size and shape. View in the downglacier direction. Photo date 8/93; © by J.S. Aber.|
Drumlins are among the most distinctive glacial
landforms. Drumlins are streamlined hills ideally having the shape of a
teardrop or inverted spoon--see Fig. 4-4. They occur in fields
containing dozens or hundreds to thousands of individual drumlins. They
are arranged en echelon in broad belts or arcs behind conspicuous
ice-margin positions, and the pattern of drumlins is thought to indicate
ice flow directions.
|Aerial view of classic drumlins in east-central Wisconsin. Elongated, streamlined hills with blunt upice ends. Ice movement from left to right. © JLM Visuals (134/07).|
Drumlins appear to be streamlined waves molded into the landscape in a most remarkable way by overriding ice. The origin of drumlins is controversial, and various mechanisms have been proposed over the years to explain their genesis (Shaw and Sharpe 1987):
|Cross section through core of large drumlin at Galway, western Ireland. Drumlin consists of crudely layered till with pockets of stratified sediments. Ice movement was from left to right. Section is about 10 m high. Photo date 5/91; © by J.S. Aber.|
|Closeup view of "boulder-clay" till within drumlin at Galway. Smooth dark stones are Paleozoic limestone. Red pocket knife for scale. Photo date 5/91; © by J.S. Aber.|
|Closeup view of stratified layer within drumlin at Galway. The sand and gravel were deposited by subglacial meltwater and then slightly deformed--note small faults in lower part of view. Photo date 5/91; © by J.S. Aber.|
|Typical drumlin in east-central Wisconsin. This overview shows asymmetrical longitudinal profile of drumlin form. Ice movement is left to right. Photo date 7/89; © by J.S. Aber.|
|Internal structure of drumlin in east-central Wisconsin (see above). Note the large overturned fold with red-brown till (right) wrapped around core of deformed sand. Ice movement is left to right. Photo date 7/89; © by J.S. Aber.|
Drumlins are examples of streamlined glacial landforms. Streamlined forms are developed at scales ranging in length from 10s of m to 100s of km--see Fig. 4-5. These streamlined forms are created by a combination of glacial erosion, deposition, and squeezing of sediment beneath the ice, and streamlining represents ice movement patterns. Streamlined landscapes may cover vast regions, such as large portions of the Canadian Shield and northern Great Plains.
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ES 331/767 © J.S. Aber (2008).
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