Chapter 105: Holocene


    Although smaller in size, the Scandinavian Ice Sheet was simr to the Laurentide in character. At times, it covered most of Great Britain, where it incorporated several small British ice caps, and extended south across central Germany and Pnd and then northeast across the northern Russian in to the Arctic Ocean.


    To the east in northern Siberia and on the Arctic Shelf of Eurasia, a number of small ice caps and domes developed in hignd areas, and some of them may have coalesced to form ice sheets on the shallow shelf areas of the Arctic Ocean.


    ciers and small ice caps formed in the Alps and in the other high mountains of Europe and Asia. In the Southern Hemisphere, the Patagonia Ice Cap developed in the southern Andes, and ice caps andrger valley ciers formed in the central and northern Andes. ciers also developed in New Zend and on the higher mountains of Africa and Tasmania, including some located on the equator.


    The results of ciation varied greatly, depending on regional and local conditions. cial processes were concentrated near the base of the cier and in the marginal zone.


    Material eroded at the base was transported toward the margin, where it was deposited both at the cier bed and in the marginal area. These processes resulted in the stripping ofrge quantities of material from the central zones of the ice sheet and the deposition of this material in the marginal zone and beyond the ice sheet.


    The Laurentide and Scandinavian ice sheets scoured and eroded bedrock terrain in their central areas, leaving behind manykes and rtively thin cial drift.


    On the other hand, the Central Lond and the northern Great ins of the United States and the western ins of Canada, as well as northern Germany and Pnd, southern Sweden, and portions of eastern and northern Russia, contain rtively thick deposits of till and other cial sediment. Thendscape of such areas is t to gently rolling. Today, these areas are among the great agricultural regions of the world, which is inrge part attributable to ciation.


    The effects in mountainous terrain were even more dramatic. cial processes were concentrated in the upper regions where snow umted and in the valleys through which the ciers moved to lower elevations.


    These valley ciers carved towering peaks (such as the Matterhorn in the Alps),rge rock basins, and sweeping U-shaped valleys and left some of the most spectacr scenery on the Earth, with many high-levelkes and waterfalls.


    The lower portions of the valleysmonly contain ridges of cial drift. Ridges of this sort that form along valley slopes are calledteral moraines, while those that loop across a valley at the lower end of a cier are termed end moraines.


    The earliest observations and interpretations of more extensive Pleistocene ciation were made on such deposits andndforms in the Alps during the early part of the 19th century.


    The environment around the ice sheets was markedly different from that of today in these formerly ciated areas. Temperatures were much lower, and a zone of permafrost (perennially frozen ground) developed around the southern margin of the ice sheets in both North America and Eurasia.


    This zone was rtively narrow in central North America, on the order of 200 kilometres, but in Europe and Russia it extended many hundreds of kilometres south of the ice margin.


    Mean annual temperatures near the ice margin were about 6 C or colder and increased away from the ice margin to about 0 C near the southern extent of the permafrost. Compared with present-day conditions, the mean air temperature was on the order of 12  to 20 C colder near the ice margin.


    These conditions are indicated by ice-wedge casts andrge-scale patterned ground, which are relict forms of ice wedges and tundra polygons that form today only in areas with continuous permafrost.


    Frost activity through freezing and thawing was intensified, and in areas of more relief talus umtions andrge block fields formed along escarpments and valley sides.


    Mass-wasting processes also were intensified and much material was eroded from slopes in pericial areas. Deposits andndforms from such activity are known from the British Isles, northern Europe, and what was formerly the Soviet Union.


    Largekes, usually many times bigger than their modern counterparts, weremon during the Pleistocene. They fluctuated in level in response to the major climatic cycles or the opening and closing of outlets due to ciation and vertical movements ofnd areas. Somekes were closely tied to ciation.


    In North America a series ofrge procialkes formed around the margin of the Laurentide Ice Sheet during backwasting (recession) of the ice margin into Hudson Bay.


    Thekes were confined in part by the ice margin and in part by highernd to the south, east, and west. One of thergest was Lake Agassiz, which covered sizable areas of Manitoba, Ontario, and Saskatchewan and extended into North Dakota and Minnesota. The Great Lakes also formed as a result of ciation as lobes of ice moved down preexisting londs and scoured out the weak rocks in the basins.


    Otherkes formed in the Chamin and Hudson valleys in eastern North America during deciation. Simr cialkes developed around the Scandinavian Ice Sheet and in other ciated regions.


    Of equal interest was the development ofrgekes in areas that today have arid to semiarid climatic regimes and generallyckkes or have modernkes that are much reduced in size and are saline in character.


    Suchkes are referred to as pluvialkes, and the climate under which they existed is termed a pluvial climate. Most of thesekes existed in closed basins thatcked outlets, and thus their levels were rted to rtive amounts of precipitation and evaporation. A record of fluctuatingke levels is provided by ancient shorelines and beach deposits that are present along the slopes of the enclosing mountains as well as by the sediment and soil record preserved in the subsurface deposits of theke basins.


    The history ofke fluctuations varies somewhat locally within a region but may be much different from one region of the world to another, depending on the local and regional climate.


    In the Great Basin of Utah, Nevada, California, and Oregon and in other areas of the western and southwestern United States and Mexico, about 100 basins containedkes during the Pleistocene. Thergest of these was Lake Bonneville, the predecessor of the modern Great Salt Lake in Utah. At its highest stage Lake Bonneville covered an area of about 52,000 square kilometres, and its maximum depth was approximately 370 metres.


    These conditions existed about 15,000 years ago during the interval of thest major Pleistocene ciation. Lake Bonneville shrank rapidly in size and, by 12,000 years ago, had permanently shrunk to a point where it had be smaller than the Great Salt Lake.


    A long record of fluctuatingke levels is evident from a 930-metre core taken in the Searles Lake basin in California. Parts of the sediment record from the core sample indicate a deepke withcustrine silts and ys and freshwater fossils. Other parts contain unusual evaporite minerals which indicate that theke was shallow and highly saline or even evidence of sediment exposure indicative of theplete desation of theke. The inferred climatic record from the core is simr to the marine oxygen isotope record but differs in that it shows more variation in the amplitude of the climatic cycles.


    Pluvialkes in these areas were most extensive during times of widespread ciation in the Northern Hemisphere and were low or dry during times of reduced cial cover. Paleoclimatic modeling suggests that the Laurentide Ice Sheet forced the pr jet stream south of its present-day position during ciation. This brought more moisture from the Pacific into the desert areas of the southwestern United States, causing greater precipitation as well as producing more cloud cover, which, together with lower temperatures, resulted in less evaporation.


    Pluvialkes also weremon in other dry regions of the world, particrly in the subtropical zones, including eastern and northern Africa and portions of Australia, Asia, and the Middle East. Examples of these pluvial bodies are the Dead Sea in Jordan and Israel and Lake Chad in the southern Sahara.


    Thetter, now a shallow salineke, covered some 300,000 square kilometres and was about six times the size of Lake Bonneville.


    A number ofkes in the rift valleys of East Africa wererger and deeper than they are today. Among the better-known and better-understood are Lakes Rudolf, Victoria, Nakuru, Naivasha, Magadi, and Rukwa. Most of thesekes in the tropical and subtropical regions were not in phase with those in the Great Basin of North America.


    They were rtively high for some 20,000 or more years immediately before thest ciation and again just after thest ciation in the early Holocene.


    A long climatic record inferred from sediments in Lake George in southeastern Australia has characteristics simr to those of the marine oxygen isotope record.


    Alternating humid and arid climatic cycles were more rhythmic and of greater magnitude in the middle andte Pleistocene than earlier, and a major change in basin hydrology urred approximately 2.5 million years ago.