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PHYSICAL
See also:GEOGRAPHY]
and deltas on the enclosing slopes, hundreds of feet above the See also:present See also:lake surfaces; the abandoned See also:shore lines, as studied by G
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K
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See also:
Lake Chelan, See also:long and narrow, deep set between spurless ridges with See also:hanging lateral valleys, and evidently of glacial origin, ornaments one of the eastern valleys
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The range is squarely transected by the See also:Columbia See also:river, which bears every See also:appearance of antecedent origin : the cascades in the river See also:gorge are caused by a sub-See also:recent landslide of great See also:size from the mountain walls
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See also:Klamath river, draining several lakes in the north-See also:west part of the Basin Range province and traversing the Cascade Range to the Pacific, is apparently also an antecedent river
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The Cascade Mountains present a marked example of the effect of See also:relief and aspect on rainfall; they rise across the path of the prevailing See also:westerly winds not far inland from a great ocean; hence they receive an abundant rainfall (80 in. or more, annually) on the west-See also: The highlands and uplands between the chief valleys are but moderately dissected; many small side streams still flow on the highland, and descend by steeply incised See also:gorges to the valleys of the larger See also:rivers . Some of the chief valleys are not cut in the floors of the old valleys of the former cycle, because the rivers were displaced from their former courses by623 lava flows, which now stand up as table mountains . Glacial erosion has been potent in excavating great cirques and small See also:rock-basins, especially among the higher southern surmounting summits, many of which have been thus somewhat reduced in, height while gaining an Alpine sharpness of See also:form; some of the short and steep canyons in the eastern slope have been converted into typical glacial troughs, and huge moraines have been laid on the desert See also:floor below them . Some of the western valleys have also in part of their length been converted into U-shaped troughs; the famous See also:Yosemite Vailey, eroded in massive granite, with side cliffs 1000 or 2000 ft. in height, and the smaller Hetch-Hetchy Valley not far away, are regarded by some observers as owing their See also:peculiar forms to glacial modifications of normal preglacial valleys . The western slope of the Sierra Nevada bears See also:fine forests similar to those of the Cascade Range and of the Coast Range, but of more open growth, and with the redwood exchanged for groves of " big trees " (See also:Sequoia gigantea) of which the tallest examples reach 325 ft . The higher summits in the south are above the See also:tree See also:line and expose great areas of See also:bare rock: See also:mountaineering is here a delightful summer recreation, with camps in the highland forests and ascents to the lofty peaks . See also:Gold occurs in See also:quartz See also:veins traversing various formations (some as young as See also:Jurassic), and also in gravels, which were for the most part deposited previous to the uplift of the Sierra " block." Some of the gravels then occurred as piedmont deposits along the western border of the old mountains; these gravels are now more or less dissected by new-cut valleys . Other auriferous gravels are buried under the upland lava flows, and are now reached by tunnels driven in beneath the rim of the table mountains . The reputed See also:discovery of traces of See also:early See also:man in the lava-covered gravels has not been authenticated . The northernmost part of the coast ranges, in Washington, is often given See also:independent See also:rank as the Olympic Range (Mt See also:Olympus, 8150 ft.) ; it is a picturesque mountain See also:group, bearing snowfields and glaciers, and suggestive of the See also:dome-like uplift of a previously worn-down mass; but it is now so maturely dissected as to make the suggested origin uncertain . Farther south, through Oregon and northern California, many members of the coast ranges resemble the Cascades and the Sierra in offering well-attested examples of the uplift of masses of disordered structure, that had been reduced to a tame surface by the erosion of an earlier cycle, and that are now again more or less dissected . Several of the ranges ascend abruptly from the See also:sea; their base is cut back in high cliffs ; the Sierra See also:Santa See also:Lucia, south of See also:San Francisco, is a range of this See also:kind; its seaward slope is almost uninhabitable . Elsewhere moderate re-entrants between the ranges have a continuous See also:beach, See also:concave seaward; such re-entrants afford imperfect See also:harbour-See also:age for vessels; See also:Monterey See also:Bay is the most pronounced example of this kind . On still other parts of the coast a recent small elevatory movement has exposed part of the former sea bottom in a narrow coastal See also:plain, of which some typical harbourless examples are found in Oregon . Most of the recent movements appear to have been upward, for the coast presents few embayments such as would result from the depression and partial submergence of a dissected mountain range; but three important exceptions must be made to this See also:rule . In the north, the Strait of Juan de Fuca and the intricately branching waterways of See also:Puget See also:Sound between the Cascade and the Olympic ranges occupy trough-like depressions which were filled by extensive glaciers in Pleistocene times; and thus See also:mark the beginning of the great stretch of fiorded coast which extends northward to Alaska . The waterways here afford excellent harbours . The second important embayment is the See also:estuary of the Columbia river; but the occurrence of shoals at the mouth decreases the use that might otherwise be made of the river by ocean-going vessels . More important is San Francisco Bay, situated about midway on the Pacific coast of the United States, the result of a moderate depression whereby a trans-See also:verse valley, formerly followed by Sacramento river through the outermost of the Coast ranges, has been converted into a narrow strait—the " See also:Golden See also:Gate "—and a wider intermont See also:longitudinal valley has been flooded, forming the expansion of the inner bay . The Coast Range is heavily forested in the north, where rainfall is abundant in all seasons; but its lower ranges and valleys have a scanty tree growth in the south, where the rainfall is very tight: here grow redwoods (Sequoia sempervirens) and live oaks (Quercus agrifolia) . The chief metalliferous deposits of the range are of See also:mercury at New See also:Almaden, not far south of San Francisco . The open valleys between the spaced ranges offer many tempting sites for See also:settlement, but in the south See also:irrigation is needed for cultivation . The See also:belt of relative depression between the inner Pacific ranges and the Coast range is divided by the fine See also:volcano Mt Shasta (14,380 ft.) in northern California into unlike portions . To the north, the floor of the depression is for the most part above baselevel, and hence is dissected by open valleys, partly longitudinal, partly transverse, among hills of moderate relief . This district was originally for the most part forested, but is now coming to be cleared and farmed . South of Mt Shasta, the " Valley of California " is an admirable example of an aggraded intermont depression, about 400 M. long and from 30 to 70 M. wide . The floor of this depression being below baselevel, it has necessarily come to be the seat of the mountain See also:waste brought down by the many streams from the newly uplifted 62+ Sierra Nevada on the east and the coast ranges on the west; each stream forms an alluvial See also:fan of very See also:gentle slope; the fans all become laterally confluent, and incline very gently forward to meet in a nearly level axial belt, where the See also:trunk rivers—the Sacramento from the north and the San Joaquin from the south-east—wander in braided courses; their tendency to aggradation having been increased in the last half See also:century by the gravels from gold washing; their See also:waters entering San Francisco Bay . See also:Kings river, rising in the high southern Sieria near \'It Whitney, has built its fan rather actively, and obstructed the See also:discharge from the part of the valley next farther south, which has thus come to be overflowed by the shallow waters of Tulare Lake, of See also:flat, reedy, uncertain See also:borders . A little north of the centre of the valley rise the Marysville Buttes, the remains of a maturely dissected volcano (2128 ft.) . Elsewhere the floor of the valley is a featureless, treeless plain . (W . M . D.) 11.-See also:GEOLOGY All the great systems of rock formations are represented in the United States, though See also:close correlation with the systems of See also:Europe is not always possible . The general See also:geological See also:column for the See also:country is shown in the following table: Eras of Time . Periods of Time . See also:Groups of Systems . Systems of Rocks . (Present . Pleistocene . See also:Pliocene . See also:Miocene . Oligocene . See also:Eocene . Transition (Arapahoe and See also:Denver formations) . Upper Cretaceous . Widespread unconformity . Comanchean (Lower Cretaceous) . Jurassic . Triassic . i See also:Permian . See also:Coal See also:Measures, or Pennsylvanian . Widespread unconformity . Subcarboniferous, or Mississippian . Palaeozoic . . Devonian . See also:Silurian . Widespread unconformity . Ordovician . See also:Cambrian . Great unconformity . Keweenawan . Widespread unconformity . Upper Huronian . Widespread unconformity . See also:Middle Huronian . Il Widespread unconformity . Lower Huronian . Great unconformity . Great Granitoid See also:Series (intru- sive in the See also:main, Laurentian) . Archeozoic . . . Archean . Great Schist Series (See also:Mona, '` Kitchi, See also:Keewatin, Quinnissec; Lower Huronian of some L authors) . Archeozoic (Archean) Group.—The See also:oldest group of rocks, called the Archean, was formerly looked upon, at least in a tentative way, as the See also:original crust of the See also:earth or its downward See also:extension, much altered by the processes of See also:metamorphism . This view of its origin is now known not to be applicable to the Archean as a whole, since this See also:system contains some metamorphosed sedimentary rocks . In other words, if there was such a thing as an original crust, which may be looked upon as an open question, the Archean, as now defined, does not appear to represent it . The See also:meta-sedimentary rocks of the Archean include metamorphosed See also:limestone, and See also:schists which carry carbonaceous See also:matter in the form of See also:graphite . The See also:marble and graphite, as well as some other indirect evidence of See also:life less susceptible of brief statement, have been thought by many geologists sufficient to See also:warrant the inference that life existed before the close of the era when the Archean rocks were formed . Hence the era of their formation is called the Archeozoic era . Most of the Archean rocks fall into one or the other of two great series, a schistose series and a granitoid series, the latter being in large part intrusive in the former . The rocks of the granitoid series appear as great masses in the schist series, and in some places form great protruding bosses . They were formerly regarded as older than the schists and were designated on this See also:account " See also:primitive," " fundamental," &c . They have also been called Laurentian, a name which is still sometimes applied to them . Nearly all known sorts of schist are represented in the schistose part of the system . Most of them are the metamorphic products of[GEOLOGY igneous rocks, among which extrusive rocks, many of them pyroclastic, predominate . Metamorphosed sedimentary rocks are widely distributed in the schistose series, but they are distinctly subordinate to the meta-igneous rocks, and they are so highly metamorphic that stratigraphic methods are not usually applicable to them . In some areas, indeed, it is difficult to say whether the schists are meta-sedimentary or meta-igneous . The likeness of the Archean of one part of the country to that of another is one of its striking features . The Archean appears at the surface in many parts of the United States, and in still larger areas north of the See also:national boundary . It appears in the cores of some of the western mountains, in some of the deep canyons of the west, as in the See also:Grand See also:Canyon of the See also:Colorado in northern See also:Arizona, and over considerable areas in northern See also:Wisconsin and See also:Minnesota, in New See also:England and the piedmont See also:plateau east of the Appalachian Mountains, and in a few other situations . Wherever it comes to the surface it comes up from beneath younger rocks which are, as a rule, less metamorphic . By means of deep borings it is known at many points where it does not appear at the surface, and is believed to be universal beneath younger systems . Locally the Archean contains See also:iron ore, as in the See also:Vermilion district of northern Minnesota, and at some points in See also:Ontario . The ore is mostly in the form of See also:haematite . Proterozoic (Algonkian) Systems.—The Proterozoic group of rocks (called also Algonkian) includes all formations younger than the Archean and older than the Palaeozoic rocks . The See also:term Archean was formerly proposed to include these rocks, as well as those now called Archean, but the subdivision here recognized has come to be widely approved . The Proterozoic formations have a wide See also:distribution . They appear at the surface adjacent to most of the outcrops of the Archean, and in some other places . In many localities the two groups have not been separated . In some places this is because the regions where they occur have not been carefully studied since the subdivision into Archeozoic and Proterozoic was made, and in others because of the inherent difficulty of separation, as where the Proterozoic rocks are highly metamorphosed . On the whole, the Proterozoic rocks are predominantly sedimentary and subordinately igneous . Locally both the sedimentary and igneous parts of the group have been highly metamorphosed ; but as a rule the alteration of the sedimentary portions has not gone so far that stratigraphic methods are in-applicable to them, though in some places detailed study is necessary to make out their structure . The Proterozoic formations are unconformable on the Archean in most places where their relations are known . The unconformity between these groups is therefore widespread, probably more so than any later unconformity . Not only is it extensive in See also:area, but the stratigraphic break is very great, as shown by (I) the excess of metamorphism of the lower group as compared with the upper, and (2) the amount of erosion suffered by the older group before the deposition of the younger . The first of these See also:differences between the two systems is significant of the dynamic changes suffered by the Archean before the beginning of that part of the Proterozoic era represented by known formations . The extent of the unconformity is usually significant of the geographic changes of the See also:interval unrecorded by known Proterozoic rocks . The Proterozoic formations have been studied in detail in few great areas . One of these is about Lake See also:Superior, where the formations have attracted See also:attention on account of the abundant iron ore which they contain . Four See also:major subdivisions or systems of the group have been recognized in this region, as shown in the preceding table . These systems are separated one from another by unconformitics in most places, and the lower systems, as a rule, have suffered a greater degree of metamorphism than the upper ones, though this is not to be looked upon as a hard and fast rule . The commoner sorts of rock in the several Huronian systems are See also:quartzite and See also:slate (ranging from shale to schist) ; but limestone is not wanting, and igneous rocks, both intrusive and extrusive, some metamorphic and some not, abound . Iron ore occurs in the sedimentary part of the Huronian, especially in Minnesota, See also:Michigan, Wisconsin and parts of Canada . The ore is chiefly haematite, and has been See also:developed from antecedent ferruginous sedimentary deposits, through concentration and See also:purification by ground See also:water . The lower part of the Keweenawan system consists of a great See also:succession of lava flows, of prodigious thickness . This portion of the system is overlain by thick beds of sedimentary rock, mostly See also:conglomerate and See also:sandstone, derived from the igneous rocks beneath . A few geologists regard the sedimentary rocks here classed as Keweenawan as Palaeozoic; but they have yielded no fossils, and are unconformable beneath the Upper Cambrian, which is the oldest sedimentary formation of the region which bears fossils . The aggregate thickness of the Proterozoic systems in the Lake Superior region is several See also:miles, as usually computed, but there are obvious difficulties in determining the thickness of such great systems, especially when they are much metamorphosed . The See also:copper of the Lake Superior region is in the Keweenawan system, chiefly in its sedimentary and amygdaloidal parts . The Proterozoic formations in other parts of the See also:continent cannot be correlated in detail with those of the Lake Superior region . The number of systems is not everywhere the same, nor are they every-where alike, and their definite correlation with one another is not See also:Cainozoic . Mesozoic . Proterozoic possible now, and may never be . The Proterozoic formations have yielded a few fossils in several places, especially See also:Montana and northern Arizona; but they are so imperfect, their See also:numbers, whether of individuals or of See also:species, are so small, and the localities where they occur so few, that they are of little service in correlation throughout the United States . The See also:carbon-bearing shales, slates and schists, and the limestone, are indications that life was relatively abundant, even though but few fossils are preserved . Among the known fossils are vermes, See also:crustacea and probably brachiopods and pteropods The See also:character of the sediments of the Proterozoic is such as to show that mature weathering affected the older rocks before their material was worked over into the Proterozoic formations . This mature weathering, resulting in the relatively See also:complete separation of the quartz from the See also:kaolin, and both from the See also:calcium carbonate and other basic materials, implies conditions of rock decay comparable to those of the present time . In all but a few places where their relations are known, the Proterozoic rocks are unconformable beneath the Palaeozoic Where conformity exists the separation is made on the basis of fossils, it having been agreed that the oldest rocks carrying the Olenellus See also:fauna are to be regarded as the base of the Cambrian system . The Palaeozoic and later formations are usually less altered,12,000 ft. in eastern New See also:York, and almost as much in the southern Appalachian Mountains (See also:Georgia and See also:Alabama) ; but its See also:average thickness is much less . In Wisconsin, where the Upper Cambrian only is present, the thickness is about See also:i000 ft . The greater thickness in the east appears to be due in part to the fact that an extensive area of See also:land, Appalachia. See also:lay east of the site of the Appalachian Mountains throughout the Palaeozoic era, and quantities of sediment from it were accumulated where these mountains were to arise later . The greatness of the thickness, as it has been measured, is also due in part to the oblique position in which the beds of sediment were originally deposited . The Cambrian formations have not been notably metamorphosed, except in a few regions where dynamic metamorphism has been effective . The system is without any notable amount of igneous rock . As in other parts of the See also:world, the system here contains abundant fossils, among which See also:trilobites, brachiopods and See also:worms are the most abundant . The range of forms, however, is great . Ordovician System.—The succeeding Ordovician (Lower Silurian) system of rocks is closely connected with the Cambrian, geographically, stratigraphically and faunally . Its distribution is much the same as that of the Upper Cambrian, with which it is conform-able in many places . The Ordovician system contains much more more accessible, and better known than the Proterozoic and Archeozoic, and will be taken up by systems . Cambrian System.—The lower part of the Cambrian system, characterized by the Olenellus fauna, is restricted to the borders of the continent, where it rests on the older rocks unconformably in most places . The middle part of the system, characterized by the Paradoxides fauna, is somewhat more widespread, resting on the lower part conformably, but overlapping it, especially in the south and west . The upper part of the system, carrying the Dicellocephalus fauna, is very much more extensive; it is indeed one of the most widespread series of rocks on the continent . The lower, middle and upper parts of the system all contain marine fossils . This being the See also:case, the distribution of the several divisions indicates that progressive submergence of the United States was in progress during the See also:period, and that most of the country was covered by the sea before its close . The system is composed chiefly of clastic rocks, and their See also:composition and structure show that the water in which they were deposited was shallow . In the interior, the upper part of the system, the See also:Potsdam sandstone, is generally arenaceous . It is well exposed in New York, Wisconsin, See also:Missouri and elsewhere, about the out-crops of older rocks . The system is also exposed in many of the western mountains or about their borders, especially about those the cores of which are of Archean or Proterozoic rock . The thickness of the system has been estimated at io,009 toJurassic&Triassic 1ilermwn.Pennsylvanin pp~~~~& Mississippian Y11/~Devonian&Silurian Ordovician & mbrian „Metamorphic rocks "Age undetermined vv°v v Proterozoic& V V "Archaeozoic 25 7 limestone, and therefore much less clastic rock, than the Cambrian, pointing to clearer seas in which life abounded . The succession of beds in New York has become a sort of See also:standard with which the system in other parts of the United States has been compared . The succession of formations in that See also:state is as follows See also:Richmond beds (in See also:Ohio Upper Ordovician (or and See also:Indiana) . Cincinnatian) See also:Lorraine beds . See also:Utica shales . Middle Ordovician (or ( Trenton limestone . Mohawkian) jl See also:Black River limestone . Lowville limestone . Lower Ordovician (or ( Chary limestone . See also:Canadian) jl Beekmantown limestone . (=Calciferous) . The See also:classification in the right-See also:hand column of this table is not applicable in detail to regions remote from New York . There is in some places an unconformity between the Richmond beds (or their See also:equivalent) and underlying formations, and this unconformity, together with certain palaeontological considerations, has raised the question whether the uppermost part of the system, as outlined above, should not be classed as Silurian (Upper Silurian) . Over the interior the strata are nearly See also:horizontal, but in the mountain regions of the east and west, as well as in the mountains of See also:Arkansas Ordovician and See also:Oklahoma, they are tilted and folded, and locally much metamorphosed . The outcrops of the system appear for the most part in close association with the outcrops of the Cambrian system, but the system appears in a few places where the Cambrian does not, as in southern Ohio and central See also:Tennessee . The thickness of the system varies from point to point, being greatest in the Appalachian Mountains, and much less in the interior . The oil and See also:gas of Ohio and eastern Indiana come from the middle portion of the Ordovician system . So also do the See also:lead and See also:zinc of south-western Wisconsin and the adjacent parts of See also:Iowa and See also:Illinois . The lead of south-eastern Missouri comes from about the same See also:horizon . The fossils of the Ordovician system show that life made great progress during the period, in numbers both of individuals and of species . The life, like that of the later Cambrian, was singularly See also:cosmopolitan, being in contrast with the provincial character of the life of the earlier Cambrian and of the early (Upper) Silurian which followed . Beside the expansion of types which abounded in the Cambrian, vertebrate remains (fishes) are found in the Ordovician . So, also, are the first See also:relics of See also:insects . The departure of the Ordovician life from that of the Cambrian was perhaps most pronounced in the great development of the molluscs and crinoids (including cystoids), but See also:corals were also abundant for the first time, and See also:graptolites came into prominence . Silurian System.—The Silurian system is much less widely distributed than the Ordovician . This and other corroborative facts imply a widespread emergence of land at the close of the Ordovician period . As a result of this emergence the stratigraphic break between the Ordovician and the Silurian is one of the greatest in the whole Palaeozoic group . The classification of the system in New York is as follows: See also:Manlius limestone . Cayugan (Neo- or Rondout waterlime . Upper Silurian) Cobleskill limestone . See also:Salina beds . `See also:Guelph See also:dolomite . Silurian . Niagaran (Meso- or See also:Lockport limestone . Middle Silurian) See also:Rochester shale . See also:Clinton beds . See also:Medina sandstone . See also:Oneida conglomerate . Shawangunk grit . The lower part of this system is chiefly clastic, and is known only in the eastern part of the continent . The middle portion contains much limestone, generally known as the See also:Niagara limestone, and is much more widespread than the lower, being found very generally over the eastern interior, as far west as the See also:Mississippi and in places somewhat beyond . The Niagara limestone contains the oldest known See also:coral reefs of the continent . They occur in eastern Wisconsin and at other points farther east and south . It is over this limestone that the Niagara falls in the world-famous See also:cataract . One member of the middle See also:division of the system (Clinton beds) contains much iron ore, especially in the Appalachian Mountain region . The ore is extensively worked at some points, as at See also:Birmingham, Alabama . The upper part of the system is more restricted than the middle, and includes the See also:salt-bearing series of New York, Ohio and See also:Pennsylvania, with its peculiar fauna . It is difficult to see how salt could have originated in this region except under conditions very different climatically from those of the present time . In the interior the thickness of the system is less than woo ft. in many places, but in and near the Appalachian Mountains its thickness is much greater—more than five times as great if the maximum thicknesses of all formations be made the basis of calculation . In the Great Plains and farther west the Silurian has little known See also:representation . Either this part of the continent was largely land at this time, or the Silurian formations here have been worn away or remain undifferentiated . Rocks of Silurian age, however, are known at some points in Arizona, Nevada and southern California . Corals, echinoderms, brachiopods and all groups of molluscs abounded . Graptolites had declined notably as compared with the Ordovician, and the trilobites passed their See also:climax before the end of the period . Certain other remarkable crustacea, however, had made their appearance, especially in connexion with the Salina series of the east . There are numerous outliers of the Silurian north of the United States, even up to the See also:Arctic regions .
These outliers have a See also:common fauna, which is closely related to that of the interior of the United States
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They give some See also:clue to the amount of erosion which the system has suffered, and also afford a clue to the route by which the animals whose fossils are found in the United States entered this country
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Thus, the Niagara fauna of the interior of the United States has striking resemblances to the See also:mid-Silurian faunas of See also:Sweden and Great See also:Britain
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It seems probable, therefore, that marine animals found migratory conditions between these regions, probably by way of northern islands
.
The fauna of the Appalachian region is far less like that of Europe, and indicates but slight connexion with the fauna of the interior
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Both the earlier and the later parts of the Silurian period seem to have been times when physical conditions were such as to favour the development of provincial faunas,, Upper { See also:Portage beds
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Devonian Senecan Genesee shale
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Tully limestone
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Erian
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{See also: Middle See also:Onondaga (Corniferous Devonian Ulsterian. limestone) 1 Schoharie grit . Esopus grit . Oriskanian See also:Oriskany beds . See also:Kingston beds . Lower Helderbergian Becraft limestone . Devonian I 1 New See also: |