GLACIAL MELT-WATER EROSION
DEPOSITION AND LANDFORMS
James S. Aber
|Morphologic sequences||Glaciolacustrine sediments|
|Glaciomarine sediments||Stratified sediment landforms|
A morphologic sequence is a progression of melt-water deposits and landforms graded to a discrete ice-margin position. Many possible morphologic sequences may be developed--see Fig. 6-1. A given sequence exhibits systematic downstream changes in sediment texture, composition, thickness, and morphologic expression. As the ice margin periodically retreats, a series of morphologic sequences may build up in an overlapping arrangement--see Fig. 6-2.
|Poorly sorted, unstratified sediment from ice-contact position of morphologic sequence, Chenango valley, south-central New York. Note angular clasts and broken shale mass (right of trowel) within this sediment, which exhibits minimal influence of melt-water transportation. Photo date 6/76; © by J.S. Aber.|
|Stratified drift deposited by glacial melt-water stream, Chemung valley, south-central New York. Note variable texture and sorting of interbedded silt, sand, pebbly sand, and gravel. Cliff-swallow nests are located in silty sand beds. Scale pole marked in feet. Photo date 6/77; © by J.S. Aber.|
|Overview of gravel pit in outwash (sandur) plain, eastern Germany. The Pomeranian end moraine (ice margin) can be seen in the left background. Sediment of the sandur plain was deposited from braided river channels. Photo date 8/95; © by J.S. Aber.|
|Closeup view of stratified sand and gravel beneath surface of Pomeranian sandur plain. Note horizontal bedding and relatively good sorting of individual layers. Section is about 10 feet (3 m) high. Photo date 8/95; © by J.S. Aber.|
A large volume of glacial sediment accumulates in proglacial lakes of
different types. This sediment is derived from both surface melting of the
glacier and subglacial drainage, and non-glacial sediment may also enter
such lakes--see Fig. 6-3. Glacial lakes often have a milky color
because of the high concentration of
suspended silt, called rock flour. Sediment buildup normally takes
place late in a glacial cycle, as the glacier retreats from the lake basin.
Sediments laid down in contact with the ice may display little effect of
melt-water transportion and sorting. In contrast, sediments that
accumulate away from the glacier may be essentially lacustrine in
character. Sediment in the center of large glacial lakes is often laid
down in annual layers called varves.
|Peyto Lake, Canadian Rockies, Alberta. The milky, blue-green color of the lake results from a high content of suspended sediment (rock flour) washed in from nearby Bow Glacier. Such lakes have very high rates of sediment accumulation. Photo date 8/84; © by J.S. Aber.|
|Glaciolacustrine sediment exposed in bluffs along west side of Lake Michigan, Wisconsin. Note rhythmic layering; individual thick-thin couplets are varves. Each varve represents one year of sediment accumulation--thick layer during summer and thin layer during winter. Also note small pod of pebbles toward left side. Such coarse sediment may have been dropped by a melting iceberg or could have been deposited by a turbidity current. Photo date 7/89; © by J.S. Aber.|
Glaciolacustrine sediments accumulated in vast lake systems of central and eastern North America--see Fig. 6-4. The central region included glacial Lake Agassiz, one of the largest fresh-water lakes that every existed. Lake Agassiz covered during different phases most of Manitoba, and parts of Saskatchewan, Ontario, North and South Dakota, and Minnesota--see Fig. 6-5.
Overflow from Lake Agassiz and other large proglacial lakes cut spectacular spillway channels across the northern Great Plains. Similar spillways were formed in the Great Lakes region--see Fig. 6-6. Much of the modern Mississippi, Ohio and Missouri drainage systems, as well as the Hudson and St. Lawrence systems, were created by such melt-water floods during the last deglaciation--see Fig. 6-7. The existence of large proglacial lakes strongly influenced subglacial drainage and could have fed water into subglacial lakes. Subglacial lakes in the Hudson Bay region may have facilitated glacier surges--see Fig. 6-8.
|Glacial melt-water spillway channel, Dell Rapids, South Dakota. This deep channel was cut through Sioux Quartzite (pink rock). Erosion of such highly resistant and massive bedrock demonstrates the power of melt-water flooding. Photo date 7/96; © J.S. Aber.|
Glaciomarine sediments differ from glaciolacustrine deposits mainly in
their content of marine fossils--foraminifera and mollusks, etc.
The saline chemistry of seawater may also influence sedimentation by the
process of flocculation. Mass movements often occur on steep slopes
of deltas and along mountain shorelines and shelf edges. Finally wind, waves,
and tides are usually more important in marine environments than for lakes.
|Exposure within glaciomarine delta at Hegra, central Norway. Note the long, inclined foreset beds of sand and gravel that were deposited on the outer slope of the delta. Exposure is approximately 30 m high. Photo date 3/87; © by J.S. Aber.|
|Closeup view of foreset bedding within delta at Hegra. Note irregular pods or lenses of flow till interbedded with sand and gravel layers. The till was derived from a nearby glacier margin and moved down the delta slope as debris flows. Photo date 3/87; © by J.S. Aber.|
|Laminated silt and fine sand in a glaciomarine delta. Small folds could be the result of sediment compaction and slumping or a small ice advance. Pencil shows axis of one small folds. Herdla Island, western Norway. Photo date 5/87; © by J.S. Aber.|
In fjords and on the continental shelves a delicate balance exists between glacial deposition (grounded ice) and melt-water sedimentation beneath a floating glacier. Slight changes in sea level, ice thickness, or crustal depression/uplift may lead to repeated grounding and floating of a glacier. The sedimentary result may be a complicated interbedding of till and glaciomarine sediments, so-called till-tongue stratigraphy--see Fig. 6-9.
Lønne (1995) elaborated three models for sedimentary facies and depositional architecture of glacial melt-water fan/delta accumulations in deep fjord or continental shelf environments--see Fig. 6-10. These models include: subglacial till, topset stream deposits, foreset delta beds, suspension fallout from melt-water plume, ice-rafted debris, and glaciotectonic deformation. The models differ according to position of the ice margin in relation to sea level and delta/fan surface.
Landforms of stratified sediments
Stratified sediment takes many interesting and distinctive landforms, such
as eskers and kames. Kames are hills or terraces that vary from
small conical peaks to broad, plateaus that are composed mainly of sand and gravel.
Kames are deposited in contact with stagnant ice from streams or as deltas and fans
in ice-marginal lakes or seas. When the stagnant ice later melts away, part
of the kame may collapse. A deep hole, called a kettle hole, may be
left where a buried mass of dead ice melts away. Kames and kettle holes
may be associated in a landform type called kame-and-kettle moraine.
|McMullen Hill, east-central Wisconsin. This conical hill is a classic moulin kame composed of stratified sand and gravel. It was presumably deposited at the bottom of a moulin (vertical melt-water shaft) within stagnant ice. When the ice later melted away, a symmetrical hill was left. Photo date 7/89; © by J.S. Aber.|
|Kame terrace, Eidslandet, western Norway. The terrace surface (farmsted) is underlain by stratified sand and gravel of a glaciomarine delta. The terrace scarp (foreground) represents the ice margin position and the terrace surface marks sea level in the fjord at the time the delta was deposited. Photo date 5/87; © by J.S. Aber.|
Kames may accumulate in lateral, end, or interlobate positions. Where many kames coalesce into a massive ridge, the landform is called a kame moraine. The Herdla Moraine of western Norway is a particularly good example of this kind of moraine, deposited largely in a marine environment--see Fig. 6-11. The moraine was formed during the Younger Dryas phase of late Weichselian glaciation around 10,000 radiocarbon years ago (Mangerud 2000). The moraine consists of glaciomarine deltas that built up along a grounded ice margin in shallow entrances to fjords. Minor readvances induced small glaciotectonic structures and caused overcompaction and consolidation of the stratified sediments--see Fig. 6-12. An analogous kame moraine is developing today in front of Malaspina Glacier, Alaska--see Fig. 6-13.
|Overview of Herdla Island, western Norway. This island is the type locality of the Herdla Moraine. The island represents a glaciomarine delta built on shallow bedrock knobs at the entrance to fjords of the Bergen vicinity. The church is located on a terrace surface that marks sea level (+32 m) at the time of delta deposition. Photo date 5/87; © by J.S. Aber.|
|Exposure of glaciomarine sediment on side of Herdla Island. Sediment is mainly silt and silty sand with small gravel bodies. The sediment was strongly compacted and locally deformed by overriding ice. Photo date 5/87; © by J.S. Aber.|
|Malaspina Glacier, Alaska. NASA space-shuttle photograph, STS066-117-014, 11/14/94, 70 mm format. Low-oblique view toward south (top). Malaspina is a classic large piedmont glacier that descends to tidewater from several mountain sources. This glacier has a history of surging--rapid advances separated by periods of stagnation. The glacier is mostly snow covered in this November view. NASA Johnson Space Center, Imagery Services.|
Eskers are long, fairly narrow ridges of sand and gravel. They may be straight or sinuous, continuous or beaded, single or multiple, sharp- or flat-crested. They vary from a few m to 10s of m high, and may be <1 km to 100s of km in length. Eskers are deposited from melt-water streams in several different situations: in open crevasses--see Fig. 6-14, within subglacial tunnels, and from tunnel outlets at retreating ice margins--see Fig. 6-15.
Eskers are generally found behind and leading toward major end moraines, deltas or outwash fans. Eskers may also be associated with valleys carved by subglacial meltwater erosion; such channels are called tunnel valleys--see Fig. 6-16. Extensive esker and tunnel-valley systems reflect the subglacial melt-water drainage network developed during late stages of glaciation and ice retreat.
|Lövviksnäset esker, a classic esker ridge composed of stratified sand and gravel. The esker is partly submerged in a lake, near Hoting, central Sweden. Photo date 6/87; © by J.S. Aber.|
|Near-modern esker ridge in front of Sléttjökull, northern margin of Mýrdalsjökull ice cap, southern Iceland. This cobbly esker was deposited from a melt-water stream in a subglacial tunnel. The esker has since been uncovered as the ice margin retreated during this century. Photo date 8/94; © by J.S. Aber.|
|Panoramic view of the Tatra Mountains in Slovakia. Expansive surface in foreground is composed of glaciofluvial fans washed out of the mountains during Pleistocene uplift and glaciations. Kite aerial photograph, July 2007 © J.S. Aber. More kite aerial photos of the Tatra Mountains. |
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ES 331/767 © J.S. Aber (2011).