4 Soils That Are Sediments
4.4 DEPOSITS FROM ICE
4.4.1 An Outrageous Proposal
The suggestion made in 1840 by a Swiss naturalist, Louis Agassiz, that large portions of the northern continents at one time were covered by
Case History
Creep was observed but was a futile warning for a huge landslide that occurred in 1964 in northern Italy. The slide completely filled the reservoir above Vaiont Dam and created such a monstrous splash that the wind broke windows over a mile (1.6 km) away. The wave overtopped the dam by about 100 m (300 ft) and washed down through the valley, taking the lives of over 2000 people. Creep was observed and monitored prior to the slide, and an attempt was made to drain the reservoir, but drainage did not keep up with the rate of soil creep so the lake level kept rising. As buoyancy reduced friction at the toe, creep turned into a landslide encompassing an area of about 1.6 2.4 km (1 1.5 miles). Some of the engineers at the dam were convicted of negligence because of failure to initiate a timely evacuation.
glaciers, may have made some people question his sobriety. Aggasiz’s spe-cialty at the time was the study of fossil fish. However, he was a competent and critical observer, and he saw similarities between deposits in North America and glacial deposits in the Alps. In particular he saw linear drag marks with a roughly north-south orientation scored into bedrock, which reminded him of home.
Boulders within glacial deposits also show scrape marks and often are flat-tened on one or more sides, Fig. 4.3. Agassiz became a professor at Harvard and revolutionized the teaching of natural sciences by emphasizing field study, an emphasis that also has a home in geotechnical engineering.
4.4.2 Extent
Prior to continental glaciation the Missouri River flowed north into what now is Hudson Bay, which occupies a basin that was pushed down below sea level by the weight of the glacial ice.
Approximately 30 percent of the continental land mass has been covered at one time or another by continental glaciers. Nearly 10 percent still is covered, including Greenland, Antarctica, and the northern islands of Canada. The top of the Greenland icecap is at an elevation of over 3000 m (10,000 ft) and the bottom is below present sea level, so the maximum thickness of continental glaciers would be measured in kilometers or miles. Further glacial melting therefore is a major concern because of the rise in sea level.
Figure 4.3 Boulders that have been faceted and striated are evidence for dragging by moving ice. The grinding action of continental glaciers was the ultimate source for most agricultural soils, whether deposited by ice or by wind or water.
Continental glaciers once covered broad northern areas of the continents.
In Europe, most of the British Isles, Scandinavia, and northern parts of Germany, Poland, and Russia were glaciated. Glaciation in North America pushed the Missouri and Ohio Rivers to their present locations and left broad areas of glacially derived sediments shown in Fig. 4.4.
Glaciation was a temporary inconvenience for mankind, but after the development of agriculture it has been a huge plus because it created a mantle of fertile soil.
Several soil deposits are associated with continental glaciation—soil deposited from the ice itself, soil deposited by water from melting ice, and soil picked up by winds crossing exposed river bars that deposited dust across broad areas of uplands.
4.4.3 Glacial Erosion
Alpine glaciers are confined along the edges by mountains, and scoop out characteristic U-shaped valleys. In contrast, continental glaciers are free to spread, stripping soil from bedrock, leveling hills, plucking out lake basins, and carrying the glacial sediment southward to be deposited wherever the ice runs out and melts. It is no coincidence that the Great Lakes of the north-central U.S.
are elongated in the directions of former glacial movement. Farther north, large seas such as Hudson Bay and the Baltic Sea owe their existence to depression of the Earth’s crust under the weight of glacial ice, and even now, thousands of years after the ice has melted, the crust still is rebounding upward.
Figure 4.4
Glaciated areas in the U.S. Vertical shading indicates younger
(Wisconsin age) glacial advances that are dominated by glacial features.
Cross-hatching shows earlier glacial deposits that are incised by streams and covered with wind-blown silt or loess.
4.4.4 Beaches and Strandlines
The extent of depression of the earth’s crust is indicated by the inclination of uplifted beaches that originally were level. Such beaches are called strandlines.
Strandlines bordering Hudson Bay, the Great Lakes, and the Baltic Sea now are tilted in directions consistent with a hypothesis of crustal rebound. The directions of bedrock striations indicate that the Hudson Bay area was at the center of the Laurentide ice sheet, and measurements and dating of strandlines show that the Gulf of Bothnia in the northern part of the Baltic Sea is rising at a rate of about 1 cm (0.5 in.) per year.
Tilted strandlines also indicate rebound around Great Salt Lake as the lake level lowered as a result of desiccation. The ancestral lake, known as Lake Bonneville, was about 330 m (1000 ft) higher during the Pleistocene, and the land has risen about 70 m (230 ft).
4.4.5 Mechanics of Glacial Sliding
Rock debris concentrated in basal ice is a kind of ‘‘glacial sandpaper.’’ The drag marks, deep longitudinal gouges left in bedrock, are called striations.
Nevertheless the overall frictional resistance to sliding had to be low to carry the ice for such long distances with a low slope angle, and must have been aided by pressure-melting of ice at the bottom of the glacier. This would transfer the weight to liquid water trapped between the ice and the soil, thereby creating a positive pore-water pressure sufficient to support the ice. A similar decrease in friction from positive pore-water pressures also plays an important role in landslides.
4.4.6 Life in the Pleistocene
The time of glaciation is referred to as the Pleistocene epoch of the Cenozoic (recent life) era, and extended from about 2.5 million to 10,000 years ago.
Subarctic cold and a generous food supply led to many experimental models of cats, cave bears, sloths, mammoths, and mastodons. A few mammoths survived in miniature on islands off the northern coast of Siberia almost into historical times, 5000 years ago. Mammoths found in permanently frozen ground, or permafrost, yield frozen tissues that are being studied for their DNA.
The early Pleistocene saw the emergence of man, who now probably would be referred to as intellectually challenged. The stocky and large-brained Neanderthal man appeared about 100,000 years ago, and the taller and equally large-brained Cro-Magnon man first appeared about 35,000 ybp (years before present) and threw his weight around. Modern man is Cro-Magnon with shoes and a haircut.
4.4.7 River Valleys in the Pleistocene
An indirect consequence of continental glaciation was a lowering of sea level, as much water was locked up in cold storage in the glaciers. The most telling evidence that this occurred is thick deposits of sediments in modern river valleys, particularly close to their outlets into the sea. These sediments only could have been deposited if sea level were lower because of a basic requirement for water to run downhill.
This influence of continental glaciation was world-wide because as sea level lowered, rivers were free to cut downward and create deep, entrenched river valleys; then as the glaciers melted and sea level rose, river valleys close to the sea became drowned as estuaries or, if scoured out by glacial ice, fiords.
4.4.8 Sedimentation of River Valleys
Rivers draining glaciated areas carried large amounts of sediment, a process that still can be observed in Alaska. During Pleistocene periods of glacial retreat the availability of this glacio-fluvial sediment and the simultaneous rise in sea level caused valleys to be filled with sediment, creating the base for broad floodplains that now extend hundreds of miles upstream from the sea. The broad valley of the Lower Mississippi River south of Cairo, Illinois, is an example. The thick sedi-ment fill in river valleys directly influences foundation designs for bridges, etc., because of the large depth to bedrock.
Question: What volume of glacial ice would be required to lower sea level 100 m?
The total surface area of the Earth is approximately 509,600,000 km2, of which oceans cover about 71 percent.
Answer: 36,200,000 km3¼8,700,000 cubic miles. This does not include displace-ment as the weight of the ice pushed down the Earth’s crust.
4.4.9 Glacial Drift, the Deposit
Deposits from glaciers are collectively called glacial drift. The rock and mineral composition of drift reflects the source area from which it came, which in the case of continental glaciation may be only a few hundred kilometers to the north.
Where glacial erosion attacked mainly granite, the resulting glacial drift will contain large quantities of sand, gravel, and boulders. Where the glacial ice gouged into shale, the resulting drift has a high percentage of clay. Incorpora-tion of ground-up limestone by glaciers causes much glacial drift to be calcareous, meaning that it contains calcium carbonate. Copper and traces of silver and gold
have been found in glacial deposits, and back-tracking has led to the discovery of valuable diamond-bearing kimberlite rocks in the Northwest Territories.
4.4.10 Glacial Outwash
Glacially derived water deposits that are carried beyond the limits of glacial advances are called outwash. These are mainly sand and gravel and are important sources of aggregate for use on roads and in concrete.
4.4.11 Glacial Till
The most abundant glacial deposit is glacial till, which is deposited by slow melting of ice such that there is little sorting by running water. Till deposited by modern glaciers is a mud that readily flows under its own weight, and gradually settles into a solid mass as the the soil loses water and consolidates.
Subglacial till has been been run over and compressed into a hard mass by the weight of the glacier. In engineering terms such a soil is said to be overconsolidated, meaning that it has been consolidated under a pressure that is in excess of that which exists today. The pressure involved in overconsolidation is called the overconsolidation pressure, also called the preconsolidation pressure.
This is an important measure in foundation engineering because it represents a pressure that can be replaced without causing appreciable settlement. It is surcharge imposed by the weight of a glacier.
4.4.12 Overconsolidation Pressure and Ice Thickness
The overconsolidation pressure determined from laboratory tests may not represent the maximum pressure imposed by the weight of glacial ice because of restricted drainage as water is trapped between soil on the bottom and ice on the top. This is another evidence for the existence of positive pore-water pressure that aided glacial movement. The overconsolidation pressure determined from laboratory testing may better reflect the maximum ice thickness where the glacier has overridden a porous rock such as limestone.
4.4.13 Retreat of an Ice Front
Retreat of a glacier does not mean that the glacier backed up, but signifies retreat of a glacial margin when the rate of melting exceeds the rate of ice advance.
During the transition from advance to retreat, the rate of advance temporarily equals the rate of retreat, so the ice front is stationary even though the ice still is moving. This causes a large pile-up of sediment called a terminal moraine. Periodic surges during the final retreat result in hills called recessional moraines that have a similar origin.
During the final retreat large blocks of ice may stagnate and be incorporated into the moraines. Later when the blocks of ice melt they leave steep-sided depressions called ‘‘kettle lakes.’’ Kettle lakes are common in the northern U.S. and in the British Isles.
4.4.14 Two Kinds of Till
Terminal and recessional moraines are not overridden by a glacier so they are not overconsolidated. Glacial till that is not overconsolidated sometimes is called superglacial till, not because it is super but because it was deposited by melting from the top. A close association with water draining from the melting ice causes superglacial till to be more sandy than subglacial till, and to contain irregular pockets and layers of sand.
Superglacial till, being less dense than subglacial till, more readily weathers, so a distinction also may be made on the basis of soil color, brown on top of gray.
4.4.15 Ground Moraine
The uniform layer of glacial sediment left during a steady retreat of an ice front is called a ground moraine. Ground moraines have a gently to moderately rolling topography with internal drainage, meaning that streams collect into pools and have no exit. The resulting wet conditions in the swale areas give a ground moraine a mottled appearance on aerial photographs. Ground moraines also can show fingerprint-like patterns caused by seasonal retreats of the ice front.
Erosion by surface runoff water gradually removes soil from the shallow hills of a ground moraine and deposits it in adjacent swales, where the soil tends to be wet, clayey, and highly compressible. Swale soils also can contain expansive clay minerals, so drainage may dry them out and make them vulnerable to later rewetting.
4.4.16 Peat in the Swales
Peat is vegetation that has grown and died in bogs, and is protected from decay by being under water. Peat is common in ground moraine areas. As peat is mostly water, it is a very difficult soil for the engineer. Piles or piers are used to support structures such as bridges, and road embankments for roads may be built high enough that they can sink and displace the peat until it reaches solid soil at the bottom. The process is speeded up by drilling through the embankment and placing dynamite charges in the peat layer.
Another approach is to float an embankment on foamed plastic. The least expensive alternative may be to simply go around the bog. Most important is to recognize areas of peat in time to influence location, design, and construction.