Chapter 106: Lithospheric te movement


    Rivers and the valleys that they upy were affected strongly by the changing climates of the Pleistocene. River channels and their sediment record are controlled inrge part by the amount and type of load that is supplied by their drainage basins and the discharge or quantity of water avable for flow.


    Both are closely rted to climate, which not only includes precipitation, evaporation, and seasonality but also controls the extent of the vegetative cover of thend and the type and intensity of weathering processes.


    In addition, because of sea-level changes rted to ciation, the base level of rivers in coastal regions also fluctuated by significant amounts. As a result, river environments were dynamic and variable.


    This was true for most rivers, but particrly so for those rivers that drainedrge quantities of meltwater and sediment from the cier margins. During ciation, rivers of thetter kind developed braided-channel patterns in response to the input ofrge quantities of sediment derived from the melting ciers and subcial waters and to therge fluctuations in the quantity of water flowing at any one time, which varied because of seasonal and diurnal controls on the generation of meltwater. During times of ciation many of these rivers deposited thick sequences of sand and gravel in their valleys; examples include those of the Hudson, Mississippi, and Ohio rivers in the United States and of the Thames, Elbe, Rhine, and Seine rivers in Europe. Simr valleys have been buried by younger cial deposits and are no longer evident at the surface. They exist today as bedrock valleys with thick fills of fluvial sand and gravel orcustrine silt in localities wherekes existed in the valleys as a result of cial damming. The sand and gravel fill in the surface valleys provide aggregate material for construction, and much groundwater is derived from the fills of both surface and buried valleys.


    Some cial valleys, as well asrge und areas, were sites of major catastrophic floods that resulted from the sudden drainage of procial and subcialkes. Such floods are known as jkulups, an dic term for subcialke outbursts. Thergest and best-known floods of this type urred in the Channeled Scand of the Columbia teau region in eastern Washington state. Ice tongues flowing south from the Cordilleran Ice Sheet periodically dammed the rk Fork River, forming cial Lake Misso. At times, Lake Misso stretched more than 200 kilometres upvalley and was about 600 metres deep near the ice dam. Sudden failure of the ice dam released over 2,000 cubic kilometres of water, which flooded westward and southward across the Columbia teau and down the Columbia River valley. The floods cut through a loess cover into basalt and left a system ofrge dry channels with waterfalls, potholes, and longitudinal grooves in the basalt. Associated with the dry channels are huge, coarse gravel bars and giant current ripples. Otherrge catastrophic floods resulted from the sudden drainage of cial Lake Agassiz and from the ancestral Great Lakes, as well as from some noncialkes such as Lake Bonneville in the Great Basin (see above).


    During the AnglianElsterian ciation in Europe arge ice-dammedke formed in the North Sea, andrge overflows from it initiated cutting of the Dover Straits.


    During the transition from cial to intercial conditions, river channel patterns evolved from braided to meandering as a result of decreased load and possibly discharge. Near ciated areas, rivers eroded into cial outwash and left a system of stream terraces along the sides of most valleys.


    These modern intercial rivers are much smaller than their cial counterparts and are underfit (i.e., appear too small) with respect to therge valleys in which they flow. In contrast, near coastal areas rivers actively built up their channels during the transition to intercial conditions in response to rising sea level.


    Eolian deposits are important in the Pleistocene record and indicate widespread wind action at certain times and in certain areas of the world. Mention has already been made of the importance of loesspaleosol records in working out regional chronologies and paleoclimatic history. Loess nketsrge portions of the central and northwestern United States, ska, the east European in of Russia, and southern Europe, where it is closely rted to episodes of ciation or to the cold pericial climate beyond the ice sheet margins or to both. The loess was derived primarily from the broad floodins of the braided rivers draining meltwater and sediment away from the ciers as well as from newly exposed cial drift. Locally, sand dunes and sheets of sand ur near the valley sources and in some cases coverrge und areas, as in central and northern Europe. The loess in China, on the other hand, is considered to have been deted mostly from such desert areas as the Gobi.


    The deserts of the subtropical regions also experienced eolian activity during the Pleistocene. In Australia, the time of peak aridity and maximum dune activity (about 20,000 to 12,000 years ago) corrtes with the time of peak ciation in the Northern Hemisphere. This also was the case in the Sahara and other deserts in Africa, India, and the Middle East.


    One estimate is that the tropical arid zones were five timesrger during times of peak ciation. Sea level was lower at these times, the water was colder, and tropical cyclones were less extensive, resulting in decreased rainfall.


    These episodes of intensified eolian activity are recorded in other Pleistocene records. Ocean cores taken downwind of these regions contain windblown sediment in the portions of the core that umted during times of maximum eolian activity.


    In addition, microparticles ur in ice cores taken from the Greend and Antarctic ice sheets and are concentrated at times of maximum ciation and aridity in the subtropical deserts.


    At other times, the climate was less arid and the desert areas contracted, and vegetation developed to stabilize the dunes under more humid (pluvial) conditions.


    The lithospheric tes continued to shift during the Pleistocene, but the continents essentially were in their modern position at the start of the epoch. Of more importance to subsequent Quaternary events were thete Tertiary tectonic movements that affected the evolution of climate toward that of the Quaternary. Among these were the formation of the Isthmus of Panama, which affected oceanic cirction, and the uplift of the Tibetan teau and broad regional areas of the western United States, which affected atmospheric cirction, particrly the position and configuration of the pr jet stream.


    Vertical movements of the Earth''s crust also were caused by the formation and melting ofrge ice sheets. The area beneath an ice sheet subsides during ciation because the crust is not able to sustain the weight of the cier.


    These isostatic movements take ce through the flow of material in the Earth''s mantle, and the amount of subsidence amounts to about one-third the thickness of the ice sheetfor example, about one kilometre in the central area of the Laurentide Ice Sheet in Canada.


    Melting of the ice sheet removes the load and causes the ground to rise, or rebound. Such uplift is rapid at first but decreases with time. More than 300 metres of uplift has urred in the eastern Hudson Bay area since that area was deciated.


    Substantial uplifting also took ce prior to theplete melting of the ice sheets, and upward crustal movement continues today at a maximum rate of about 1.3 centimetres per year.


    A simr record of cio-isostatic adjustments is encountered in Fennoscandia, where the greatest depression and subsequent uplift rted to the Scandinavian Ice Sheet is located in the Gulf of Bothnia.


    The nts and animals of the Pleistocene are, in many respects, simr to those living today, but important differences exist. Moreover, the spatial distribution of various Pleistocene fauna and flora types differed markedly from what it is at present. Changes in climate and environment causedrge-scale migrations of both nts and animals, evolutionary adaptations, and in some cases extinction.


    Study of the biota provides not only data on the past paleoenvironments but also insights into the response of nts and animals to well-documented environmental change.


    Of particr importance is the evolution of the genus Homo during the Pleistocene and the extinction ofrge mammals at the end of the epoch.


    ording to Xin Evolutionary changes during the Pleistocene generally were minor because of the short interval of time involved. They were greatest among the mammals. In fact, the epoch has been subdivided into mammalian ages on the basis of the appearance of certain immigrant or endemic forms.


    Mammalian evolution included the development ofrge forms, many of which became adapted to Arctic conditions. Among these were the woolly mammoth, woolly rhinoceros, musk ox, moose, reindeer, and others that inhabited the cold pericial areas.


    Large mammals that inhabited the more temperate zones included the elephant, mastodon, bison, hippopotamus, wild hog, deer, giant beaver, horse, and ground sloth.


    The evolution of these as well as of much smaller forms was affected in part by three factors: (1) a generally cooler, more arid climate subject to periodic fluctuations, (2) new migration routes resultingrgely from the emergence of intercontinental connections during times of lower sea level, and (3) a changing geography due to the uplift of teaus and mountain building.