Geography ofUnited States Pacific Mountain System
For purposesdescription,physical geography ofUnited Statessplit into several major physiographic divisions, one beingPacific Mountain System. Please refer toGeography ofUnited States forother areas.
| Tablecontents |
|
|
The Pacific Ranges
Strong mountain ranges followtrend ofPacific coast, 150 or 200 miles inland. The Cascade Range enters from Canada, trending southward across the international boundary through WashingtonOregonlatitude 41°. The Sierra Nevada extends thence south-eastward through Californialatitude 35°. The lower coast ranges, nearerocean, continuelittle farther southward thanSierra Nevada, before giving waythat part ofBasin Range province which reachesPacificsouthernmost California.
Cascade Range
The Cascade Rangein essencematurely dissected highland, composedpartupwarped Colombian lavas,partolder rocks,crownedseveral dissected volcanoes,whichchief(beginning innorth) Mount Baker (10,827 ft), Mount Rainier (14,363 ft), Mount Adams (12,470 ft.)Mount Hood (11,225 ft). The first threein Washington andlastnorthern Oregon. These bear snowfieldsglaciers. The dissected highlands,ridgesvery irregular arrangement,sculptured everywhere infashion that strongly suggestsworknumerous local Pleistocene glaciers as an important supplementpreglacial erosion. Lake Chelan, longnarrow, deep set between spurless ridgeshanging lateral valleys,evidentlyglacial origin, ornaments one ofeastern valleys. The rangesquarely transected byColumbia River, which bears every appearanceantecedent origin. The cascades inriver gorgecaused bysub-recent landslidegreat size frommountain walls. The Klamath River, draining several lakes innorthwest part ofBasin Range provincetraversingCascade Range toPacific,apparently also an antecedent river.
The Cascade Mountains presentmarked example ofeffectrelief and aspect on rainfall. They rise acrosspath ofprevailing westerly winds not far inland fromgreat ocean. They receive an abundant rainfall (80 in. or more, annually) onWestward or windward slope,there theyheavily forested. The rainfalllight oneastward slope andpiedmont districtdry soforests thin out on that side ofrangetreeless lava plains follow immediately eastward.
Sierra Nevada
Generally speaking,Sierra Nevada isgreat mountain block, largely composedgranitedeformed metamorphosed rocks, reducedmoderate reliefan earlier (CretaceousTertiary) cycleerosion. Sub-recently, it has been elevated withslant towest,in this position sub-maturely dissected. The region was by no meanspeneplain before its slanting uplift. Its surface then was hillyinsouth mountainous. In its centralstill moreits northern part,was overspreadlavas which flowed westward alongbroad open valleys from many vents ineastern part. Nearnorthern end ofrange, eruptions have continued inpresent cycle, forming many conesyoung lava flows. The tilting ofmountain mass was presumably notsimple orsingle movement. It was probably slow, forPitt River (headwaters ofSacramento) traversesnorthern part ofrangeantecedent fashion. The tilting involvedsubdivision ofgreat block into smaller ones, innorthern half ofrange at least. Lake Tahoe (altitude 6225 ft) near the range crestexplained as occupyingdepression between two block fragments. Farther north similar depressions now appear as aggraded highland meadows. The tilting ofgreat block resultedpresentingstrong slope toeast andlong moderate slope towest. Toeast,facesdeserts ofBasin Range provincealsoin large measure responsible for their aridity. The altitudes alongupraised edge ofblock, or range crest,approximately 5000 ft innorth11,000 ft insouth. The mountains insouthern part ofblock, which had been reducedsubdued forms informer cycleerosion, were thus given a conspicuous height. There,formHigh Sierra andgreatly sharpened by revived erosion, both normalglacial. In this way Mount Whitney (14,502 ft.) camebehighest summit inUnited States (excluding Alaska). Innew altitude ofmountain mass, its steep eastern face has been deeply carvedshort canyons. Onwestern slope, an excellent beginningdissection has been made inerosionmany narrow valleys, whose greatest depth lies between their headwaters which still flow onhighland surface,their mouths atlow western base ofrange. The highlandsuplands betweenchief valleys are but moderately dissected. Many small side streams still flow onhighlanddescend by steeply incised gorges tovalleys oflarger rivers. Some ofchief valleysnot cut infloorsthe old valleys offormer cycle, becauserivers were displaced from their former courses by lava flows, which now stand up as table mountains. Glacial erosion has been potentexcavating great cirques and small rock-basins, especially amonghigher southern surmounting summits, manywhich have been thus somewhat reduced in, height while gaining an Alpine sharpnessform. Some ofshortsteep canyons ineastern slope have been converted into typical glacial troughs, and huge moraines have been laid ondesert floor below them. Somethe western valleys have alsoparttheir length been converted into U-shaped troughs. The famous Yosemite Valley, erodedmassive granite, with side cliffs 1000 or 2000 ft.height, andsmaller Hetch Hetchy Valley not far away,regarded by some observers as owing their peculiar formsglacial modificationsnormal preglacial valleys.
The western slope ofSierra Nevada bears fine forests similarthose ofCascade RangeofCoast Range, butmore open growth,withredwood exchangedgrovesbig trees (Sequoia gigantea)whichtallest examples reach 325 ft. The higher summits insouth are abovetree lineexpose great areasbare rock. Mountaineering is heredelightful summer recreation,camps inhighland forests and ascents tolofty peaks. Gold occursquartz veins traversing various formations (some as young as Jurassic),alsogravels, which were formost part deposited previous touplift ofSierra block. Some ofgravels then occurred as piedmont deposits alongwestern border ofold mountains. These gravelsnow more or less dissected by new-cut valleys. Other auriferous gravels are buried underupland lava flows, andnow reached by tunnels drivenbeneathrim oftable mountains.
Other
The northernmost part ofcoast ranges,Washington,often given independent rank asOlympic Mountains (Mount Olympus, 8150 ft.). It ispicturesque mountain group, bearing snowfieldsglaciers,suggestive ofdome-like uplift ofpreviously worn-down mass. Farther south, through Oregonnorthern California, many members ofcoast ranges resembleCascades andSierraoffering well-attested examples ofupliftmassesdisordered structure, that had been reduced totame surface byerosionan earlier cycle,thatnow again more or less dissected.
Several ofranges ascend abruptly fromsea. Their basecut back in high cliffs. The Santa Cruz Mountains, southSan Francisco, isrange of this kind. Theremoderate re-entrants betweenranges that havecontinuous concave seaward beaches such as Monterey Bay. On still other parts ofcoastrecent small elevatory movement has exposed part offormer sea bottom innarrow coastal plain,which some typical harborless examplesfoundOregon. Most ofrecent movements appearhave been upward, forcoast presents few embayments such as would result fromdepressionpartial submergence ofdissected mountain range. Therethree important exceptions must be madethis rule. Innorth,StraitJuan de Fuca andintricately branching waterwaysPuget Sound betweenCascade andOlympic ranges occupy trough-like depressions which were filled by extensive glaciersPleistocene timesmarkbeginning ofgreat stretchforded
coast which extends northwardAlaska. The second important embayment isestuary ofColumbia river. Finally,more important oneSan Francisco Bay, situated about midway onPacific coast ofUnited States,result ofmoderate depression wherebytransverse valley, formerly followed by Sacramento river throughoutermostthe Coast ranges, has been converted intonarrow straitGolden Gate andwider intermontane longitudinal valley has been flooded, formingexpansion ofinner bay.
The Coast Rangeheavily forested innorth, where rainfallabundantall seasons. However, its lower rangesvalleys havescanty tree growth insouth, whererainfallvery light. Here, coast redwoodslive oaks (Quercus agrifolia) grow. The chief metalliferous deposits ofrangeof Mercury at New Almaden, not far southSan Francisco. The open valleys betweenspaced ranges offer many tempting sitessettlement, but insouth irrigationneededcultivation.
The beltrelative depression betweeninner Pacific ranges andCoast rangedivided byfine volcano Mount Shasta (14,380 ft) in northern California into unlike portions. Tonorth,floorthe depressionformost part above baselevel,hencedissected by open valleys, partly longitudinal, partly transverse, among hillsmoderate relief. This district was originallythe most part forested, butnow comingbe clearedfarmed.
SouthMt Shasta,California Central Valleyan admirable examplean aggraded intermontane depression, about 400 miles longfrom 3070 miles wide. The floorthis depression being below baselevel, it has necessarily comebeseat ofmountain waste brought down bymany streams fromnewly uplifted Sierra Nevada oneast andcoast ranges onwest. Each stream forms an alluvial fanvery gentle slope. The fans all become laterally confluent and incline very gently forwardmeet innearly level axial belt. Theretrunk riversSacramento fromnorth andSan Joaquin fromsoutheast wanderbraided coursestheir waters entering San Francisco Bay. Kings River, rising inhigh southern Sieria near Mt Whitney, has built its fan rather actively A little north ofcenter ofvalley riseMarysville Buttes,remains ofmaturely dissected volcano (2128 ft). Elsewherefloor ofvalley isfeatureless, treeless plain. (W. M. D.)
