Andes
- Please note that the geology in this article reflects views from the first decade of the 20th Century. It is a good example of the descriptive geology of that time, but because it is pre-plate tectonics cannot be relied on when explaining the existence of these ranges and some of the structural commentary.
The
Andes is a vast
mountain system forming a continuous
chain of
highland along the western coast of
South America. It is roughly 7000 km (4400 miles) long, 160 km (100 miles) wide in some parts, and of an average height of about 4000 m (13,000 feet).
The connection of this system with that of the
Rocky Mountains, which has been pointed out by many writers, has received much support from the discovery of the extensive eruptions of granite during Tertiary times, extending from the southern extremity of
South America to
Alaska.
The
Andean range is composed of two great principal chains with a deep intermediate depression, in which, and at the sides of the great chains, arise other chains of minor importance, the chief of which is that called the
Cordillera de la Costa of
Chile. This starts from the southern extremity of the
continent and runs in a northerly direction, parallel with the coast, being broken up at its beginning into a number of islands and afterwards forming the western boundary of the great central
valley of Chile. To the north this coastal chain continues in small ridges or isolated hills along the
Pacific Ocean as far as
Colombia, always leaving the same valley more or less visible to the west of the western great chain.
Tierra del Fuego
Of the two principal chains the eastern is generally called Los Andes, and the western La Cordillera, in Colombia, Peru and Bolivia, where the eastern is likewise known as Cordillera Real de los Andes, while to the south of parallel 23°S in Chile and Argentina, the western is called Cordillera de los Andes. The eastern disappears in the centre of Argentina, and it is therefore only the Cordillera de los Andes that is prolonged as far as the south-eastern extremity of the continent.
The Cordillera de la Costa begins near \nCape Horn, which is composed principally of crystalline rocks, \nand its heights are inconsiderable when compared with \nthose of the true Cordillera of the Andes. The latter, \nas regards its main chain, is on the northern coast of the \nBeagle Channel in Tierra del Fuego, bounded on the north \nby the deep depression of Lake Fagnano and of Admiralty Sound. Staten Island appears to be the termination to the east. The Cordillera of the Andes in Tierra del Fuego is \nformed of crystalline schists, and culminates in the snow-capped \npeaks of Mount Darwin and Mount Sarmiento 2200 m(7200 feet), which \ncontains glaciers of greater extent than those of Mont Blanc. The extent of the glaciers is considerable in this region, which, geographically, is more complex than was formerly supposed. Although in the explored portion of the Fuegian \nchain the volcanoes which have been mentioned from time to \ntime have not been met with, there seem to have existed to the \nsouth, on the islands, many neo-volcanic rocks, some of which \nappear to be contemporaneous with the basaltic sheet that covers \na part of eastern Patagonia. The insular region between Mount \nSarmiento and the Cordillera de los Andes, properly so called, \ni.e. that which extends from Magellan Strait northwards, is \nnot fully explored, and all that is known of it is that it is \nprincipally composed of the same rocks as the Fuegian section, \nand that the greater part of its upper valleys is occupied \nby glaciers that reach down to the sea amid dense forest.
Chile-Argentina, 52°-38°S
As Admiralty Sound and Lake Fagnano bound the Cordillera to \nthe north in Tierra del Fuego, so at the eastern side of the \nCordillera in the southernmost part of the continent there is \na longitudinal depression which separates the Andes from some \nindependent ridges pertaining to a secondary parallel broken \nchain called the pre-Cordillera. This depression is occupied \nin great part by a series of lakes, some of these filling \ntransversal breaches in the range, whilst others are remains \nof glacial reservoirs, bordered by morainic dams, extending \nas far as the eastern tableland and corresponding in these \ncases with transversal depressions which reach the Atlantic Ocean. Between the larger lakes, fed by the Andine glaciers \nof the eastern slope of the Southern Andes, are Lakes Maravilla, 260 km² (100 mile²) , and Sarmiento, 67 km² (26 mile²) , 51°S, which overflow into Last Hope Inlet; Argentino,1,476 km² (570 mile), 50°S; and Viedma, (450 mile²) 1,165 km², 49°30'S, which empty into the river Santa Cruz; the fjordian Lake San Martin/O'Higgins, 49°S, and Lakes Nansen, 47 km² (18 mile²) ; Azara,21 km² (8 mile² ); and Belgrano, 47 km² (18 mile²) (), which are dependents of \nLake San Martín/O'Higgins 984 km²(380 mile²)), and Lakes Pueyrredon/Cochrane 254 km² (98 mile²)) \nand Buenos Aires/General Carrera 1,813 km² (700 mile²)), which now overflow into the \nPacific, through one of the remarkable inlets that are found \nthroughout the Cordillera, the Calen Inlet, which is the \nlargest western fjord of Patagonia. To the north of Lake Buenos Aires there is Lake Elizalde, which, while situated \non the eastern slope, sends its waters to the Pacific Ocean, \nand Lakes Fontana 78 km² (30 mile²)) and La Plata 88 km² (34 mile²)), 45°S, which feed the river Senguerr, which flows to the Atlantic. Lake General Paz 171 km² (66 mile²)) on the eastern slope of the Andes, at 44°S, is the principal source of \nthe Palena River, which cuts all the Cordillera, while Lakes \nFutalaufquen 52 km² (20 mile²)) Menendez 73 km² (28 mile²)), Rivadavia 26 km²(10 mile²)), and other smaller lakes, also situated between 43°30', and 42°30'S on the eastern slope send their waters to the Pacific by the river Futaleufu which cuts \nthrough the Andes by a narrow gorge. The waters of Lake \nPuelo 47 km² (18 mile)) likewise flow into the same ocean through the river of that name, which also cuts the Cordillera, and of which the principal affluent (the Manso River) likewise drains the waters of a system of small lakes, the largest of which, Lake \nMascardi, measures 44 km² (17 mile²), which in comparatively recent times formed part of the basin of Lake Nahuel Huapi 536 km²(207 mile²)), 41°S. An extensive area of glacial deposits shows that a sheet of ice formerly covered the whole eastern slope to a great distance from the mountains. To the west another sheet reached at the same time the Pacific Ocean.
From the Strait of Magellan up to 52°S, the western \nslope of the Cordillera does not, properly speaking, exist. \nAbrupt walls overlook the Pacific, and great longitudinal and \ntransversal channels and fjords run right through the heart of the \nrange, cutting it generally in a direction more or less oblique \nto its axis, the result of movements of Earth's crust.
The mountains forming the Cordillera between Magellan Strait \nand 41°S are higher than those previously mentioned \nin Tierra del Fuego. Generally composed of granite, gneiss \nand Palaeozoic rocks, covered in many parts by rugged masses \nof volcanic origin, their general height is not less than 2000 m\n(6500 feet), while Mount Geikie is 2300 m (7500 feet) and Mount Stokes \n2200 m (7100 feet). To the north are Mounts Mayo 2300 m(7600 feet), Agassiz 3200 m (10,600 feet), and Fitzroy (also called Chalten),\nin 49°S (11,120 feet). The \nsection from 52°to 48°S is a continuous ice-capped \nmountain range, and some of the glaciers extend from the \neastern lakes to the western channels, where they reach the \nsea-level. The level of the lakes begins at40 m (130 feet) at Lake Maravilla and gradually ascends to nearly 200 m (700 feet) at Lake San Martin/O'Higgins. Passing the breach through which Lake San Martin empties itself into Calen Inlet, in 48°S, is found a wide \noblique opening in the range, through which flows the river Las \nHeras, fed by Lake Pueyrredon/Cochrane, which is only 125 m (410 feet) above the \nsea-level to the east of the Andes, while Lake Buenos Aires/General Carrera, \nimmediately to the north, is 220 m (710 feet). The Andes continue to be \nto the west an enormous rugged mass of ice and snow of an average \nheight of 2700 m (9000 feet), sending glaciers to all the eastern fjords.
Mount San Lorenzo, detached from the main chain in the \nPre-Cordillera, is 3600 m (11,800 feet) high. Mount San Valentin \n3900 m (12,700 feet) is the culminating point of the Andes in the \nregion extending from 49° to 46°S, a little north of \nwhich is the river Huemules which is followed by the breach \nof the river Aisen. These two rivers have emptied a large \nsystem of lakes, which in pre-Glacial times occupied the \neastern zone, thus forming a region suitable for colonization \nin the broad valleys and hollows, where the rivers, as in \nthe case with those in the north, cut through the Andes by \nnarrow gaps, forming cataracts and rapids between the snowy \npeaks. Volcanic action is still going on in these latitudes, \nas the glaciers are at times covered by ashes, but the \npredominant rocks to the east are the Tertiary granite, \nwhile to the west gneiss, older granite and Palaeozoic rocks \nprevail. The highest peaks, however, seem to be of volcanic \norigin. Farther north, up to 41°S, the water gaps are \nsituated at a lesser distance one from the other, owing mainly \nto more continuous erosion, this section of the continent \nbeing the region of the maximum rainfall on the western \ncoast to the south of the equator. Between the gaps of the \nriver Aisen and river Cisnes or Frias, which also pierces the \nchain, is found a huge mountain mass, in which is situated \nMount la Torre 2200 m (7150 feet). These form the continental \nwatershed, but in this region erosion is taking place so \nrapidly that the day is not far distant when Lakes La Plata and \nFontana, situated to the east at a height of 900 m (3000 feet) and now \ntributaries of the Atlantic, may become tributaries of the \nPacific. Already filtrations from the former go to feed \nwestern affluents through the granitic masses. To the north \nof Mount la Torre flows in the river Cisnes, 44°48'S, \nacross another water gap, continuing the range to the north \nwith high peaks, as Mount Alto Nevado 2240 m (7350 feet) and Mount Cacique 2100 m (7000 \nfeet). The glaciers reach almost the western channels, as is \nthe case at the river Quelal. The northern glaciers, descending \nnearly to sea-level, are situated at 43°40'S. To the north, a well-defined western longitudinal valley, at some recent time occupied by lakes and rivers, \ndivides the Cordillera into two chains, the eastern being \nthe main chain, to which belong Mounts Alto Nevado, Cacique, \nDentista, Maldonado, Serrano, each over 2100 m 7000 feet high; and \nTorrecillas 2250 m (7400 feet), Ventisquero23002300 m (7500 feet), and Tronador 3400 m\n(11,180 feet); while the western chain, broken into imposing \nblocks, contains several high volcanic peaks such as Mounts \nTanteles, Corcovado, Minchimahuida, Hornopiren and Yates. The \nrivers Palena, with its two branches, Pico and Carrenleufu, \nFutaleufu, Puelo and Manso cut the two chains, while the rivers \nRenihue, Bodadahue and Cochamo have their sources in the main \neastern ridge. Mention has been made of active volcanoes in \n51°, 49°, and 47°S, but these have not been properly \nlocated. The active volcanoes south of 41°, concerning \nwhich no doubt exists, are the Huequen, in 43°, and the \nCalbuco, both of which have been in eruption in modern times.
The surroundings of Mount Tronador, consisting of Tertiary \ngranite and basalt, form one of the most interesting regions \nin the Patagonian Andes for the mountaineers of the future. To \nthe east extends the large and picturesque lake of Nahuel Huapi, \nto the west is Lake Todos Los Santos 130 km² (50 mile²)), to which \nthe access is easy and of which the scenery is of surpassing \nbeauty. Between 41°and 38°S, among other smaller \nlakes, are Lakes Traful 117 km²(45 mile²)), Lacar 83 km²(32 mile)), \nwhich, properly belonging to the system of Atlantic lakes, \nempties itself by the only water gap that occurs in this zone \nof the Cordillera into the river Valdivia, a tributary of \nthe Pacific, Lake Lolog 40 km²(15 mile²)), Huechulafquen 117 km² (45 mile²)), and Lake Alumine 54 km² (21 mile²)). The volcanoes of Lanin 3700 m\n(12,140 feet), Quetrupillan 2800 m (9180 feet), Villarrica 3200 m(10,400 feet), \nLlaima and Tolhuaca are all more or less active; the first \nis in the main chain, while the others are on the western \nslope. The scenery in the neighbourhood is magnificent, the \nsnowy cones rising from amidst woods of araucaria, and being \nsurrounded by blue lakes. While the scenery of the western \nslope of the Andes is exceedingly grand, with its deep fjords, \nglaciers and woods, yet the severity of its climate detracts \nconsiderably from its charm. The climate of the eastern \nslope, however, is milder, the landscapes are magnificent, with \nwooded valleys and beautiful lakes. The valleys are already \npartly settled by colonists. Between 52°and 40°S \nerosion has carried the watershed of the continent from the \nsummit of the Cordillera to the eastern plains of Patagonia.
From 40°S. southward the Chile-Argentine Boundary Commission \nunder Sir T. H. Holdich carried out important investigations in \n1902; and between 38°and 33°S the Andes were somewhat \nextensively explored about the close of the 19th century by \nArgentine and Chilean Commissions. The highest peaks in the \nlatter section are volcanic and their eruptions have sensibly \nmodified the character of the primitive ridges. Outflows \nof lava and tufa cover the mountain sides and fill up the \nvalleys. The Jurassic and Cretaceous formations, which in \nthe Southern Cordillera are situated outside of the range \nto the east, form to a considerable extent the mass of the \ngreat range, together with quartz porphyry, the Tertiary, \ngranite and other eruptive rocks, which have been observed \nalong all the chain in South America up to Alaska in the \nnorth. Gneiss is seldom met with, but there are crystalline \nrocks, belonging chiefly to the pre-Cordillera of the eastern \nand to the Cordillera de la Costa on the western side.
Chile-Argentina from 38°S northward
About 38°S the Andes take a great transversal extension; \nthere are no wide intermediate valleys between the different \nridges but the main ridge is perfectly defined. Volcanic \ncones continue to predominate, the old crystalline rocks \nalmost disappear, while the Mesozoic rocks are most \ncommon. The higher peaks are in the main chain, while \nthe Domuyo 4700 m (15,317 feet) belongs to a lateral eastern \nridge. The principal peaks between this and Mount Tupungato \nat 33°S are: Mount Cochico 2100 m (8255 feet), Campanario, \n4000 m (13,140 feet), Peteroa 4050 m (13,297 feet), Tinguiririca, Castillo 5000 m \n(16,535 feet), Volcano Maipu 5350 m (17,576 feet), Alvarado 4450 m (14,600 feet), Amarillo 4670 m (15,321 feet), Volcano San Jose 6050 m(19,849 feet), \nPiuquenes 5430 m (17,815 feet), and Volcano Bravard 5980 m (19,619 feet).
North of Maipu volcano, ascended by R. P. Gussfeldt in \n1883, the Cordillera is composed of two huge principal ridges \nwhich unite and terminate in the neighbourhood of Mount \nTupungato. The valley between them is 2700 m (9000 feet) high; and in \nthat part of the Cordillera are situated the highest passes \nsouth of 33°S, one of which, the Piuquenes Pass, \nreaches 4060 m (13,333 feet), whilst the easiest of transit and almost \nthe lowest is that of Pichachen 1980 m (6505 feet), which is the most \nfrequented during winter. Mount Tupungato reaches 6800 m (22,329 \nfeet), according to Argentine measurement. To the north of this \nmountain, situated at the watershed of the Andes, extends \na lofty region comprising peaks such as Chimbote 5680 m (18,645 \nfeet) and Mount Polleras 6180 m (20,266 feet). The Pircas Pass is \nsituated at a height of 5170 m (16,962 feet). The gaps of Bermejo and \nIglesia, in the Uspallata road, the best known of all the \npasses between Argentina and Chile, are at 3970 m (13,025 feet) and \n4100 m (13,412 feet) altitude respectively, while the nearest peaks, \nthose of Juncal and Tolorsa, are 5900 m and 6100 m(19,358 and 20,140 feet) high.
Mounts Tupungato, Aconcagua 7130 m (23,393 feet) and Mercedario 6700 m (21,982 feet) are the highest peaks of the central Argentine-Chilean \nAndes. These three peaks are formed of eruptive rocks, \nsurrounded by Jurassic beds which have undergone a thorough \nmetamorphosis. While in the west of the Andes, from the \nlatitude of Aconcagua, the central valley of Chile runs \nwithout any notable interruption to the south end of the \ncontinent, a valley which almost disappears to the north, \nleaving only some rare inflexions which are considered by \nChilean geographers and geologists to be a continuation of \nthe same valley; to the east in Argentina a longitudinal \nvalley, perfectly characterized, runs along the eastern \nfoot of the Cordillera, separating this from the pre- \nCordillera, which is parallel to the Cordillera de la Costa of \nChile. Between Aconcagua and Mercedario are the passes of \nEspinacito 4500 m(14,803 feet) and Los Patos or Valle Hermoso 3580 m (11,736 \nfeet), chosen by the Argentine General San Martin, when he \nmade his memorable passage across the chain during the War of \nIndependence. North of Valle Hermoso the Andean ridges, \nwhile very high, are not abrupt, and the passes are more \nnumerous than in the south; some of them descending 3000 m (10,000 \nfeet), but most of them between 4000 m and 4300 m ( 13,000 and 14,000 feet). The pass of Quebrada Grande is 3800 m (12,468 feet) in altitude; Cencerro, 3950 m (12,944 \nfeet); Mercedario, 4030 m (13,206 feet); Ojota, 4360 m (14,304 feet); Pachon, 4400 m (14,485 \nfeet); while Gordito is 3150 m (10,318 feet). Farther north the passes are \nhigher. Barahona Pass is 4600 m (15,092 feet); Ternera,4850 m ( 15,912 feet); \nSan Lorenzo, 5000 m (16,420 feet), while the peak of the volcano \nreaches 5530 m (18,143 feet); Mount Olivares, 6240 m (20,472 feet); Porongos, 5940 m (19,488 feet; Tortolas, 6130 m (20,121 feet); and Potro, 5900 m (19,357 feet).
Bolivia
As far as 28°S the Cordillera de los Andes has been \nprincipally formed by two well-defined ridges, but to the \nnorth, recent volcanic action has greatly modified its \norography. Only a single line of passes characterizes the \nmain ridge, and amongst them are the passes of Ollita 4580 m(15,026 \nfeet), Penas Negras 4400 m (14,435 feet), Pircas Negras 4150 m (13,615 feet), \nLa Gallina 4950 m (16,240 feet), Tres Quebradas 4735 m (15,535 feet), and \nAguita 4720 m(15,485 feet). To the north of Mount Potro the peaks \nin the Cordillera are not very prominent as far as the great \nmass of Tres Quebradas, but here are to be met with some \nthat may he considered as amongst the highest of the whole \nrange. Mount Aguita is 6000 m (20,600 feet), and the culminating \npeak of those of Tres Cruces reaches 6900 m (22,658 feet). To the \neast of the eastern longitudinal valley, at 27°S, \nbegins a high volcanic plateau between the Cordillera and \nthe southern prolongation of the Bolivian Cordillera Real, \nwhich contains lofty summits, such as Mount Veladero 6400 m (20,998 \nfeet), Mount Bonete 6700 m (21,980), Mount Reclus 6300 m (20,670), Mount \nPissis 6750 m(22,146), Mount Ojo del Salado 6600 m (21,653), and Incahuasi 6620 m\n(21,719). To the north of Tres Cruces is a transversal \ndepression in the Cordillera, which is considered to be \nthe southern termination of the high plateau of the Puna de \nAtacama. The Cordillera of the Andes borders the Puna to the \nwest, while the Bolivian Cordillera Real bounds it to the \neast. In that region the Cordillera of the Andes is of \ncomparatively recent origin, being principally constituted \nby a line of high volcanoes, the chief summits being those of \nJuncal, Panteon de Aliste, Azufre or Listarria 5680 m(18,636 feet), \nLlullaillaco 6620 m (21,720), Miniques 5950 m (19,357), Socompa 6080 m (19,948), \nLicancaur 6000 m (19,685), Viscachuelas 6280 m (20,605), Tapaquilcha 5950 m(19,520), \nOyahua 5860 m (19,242), Ancaquilcha 6200 m (20,275), Olca 5840 m(19,150), Mino 6130 m\n(20,112), Sillilica 6430 m (21,100), Perinacota 6380 m (20,918), Sagama 6800 m \n(22,339), Tacona 6020 m (19,740), Misti 5800 m (19,029); to the east \ncloses in the intermediary high plateau which begins at 28°S in Argentina. The principal peaks of the Bolivian \nAndes and its prolongation from south to north, are Famatina, \nin the centre of Argentina, 6200 m (20,340 feet), Languna Blanca 5600 m\n(18,307), Diamante 5500 m (18,045), (Cachi 6000 m(20,000), Granadas, Lipez 6000 m \n(19,680), Guadalupe 5760 m(18,910), Chorolque 5630 m (18,480), Cuzco 5465 m \n(17,930), Enriaca 5700 m (18,716), Junari 4940 m(16,200), Michiga 5300 m (17,410), \nQuimza-Cruz 5570 m (18,280), Illimani 6460 m (21,190) and Sorata 6550 m(21,490).
While the western range of the Cordillera is principally \nformed by volcanic rocks, the eastern (to the east of the \nrange is Cerro Potosi, (4700 m (15,400 feet) Andes of Bolivia are chiefly \ncomposed of old crystalline rocks. Between the ranges in the \nhigh plateau north to 27° are numerous isolated volcanoes \nwhich have been in activity in recent times, such as Peinado \n5760 m (18,898 feet), San Pedro 5700 m (18,701), Antuco 5800 m(19,029), Antofalla 6100 m\n(20,014), Rincon 5450 m (17,881), Pastos Grandes 5350 m(17,553), Zapalegui 5350 m \n(17,553), Suniguira 5870 m(19,258), Tahue 5320 m(17,458); volcanoes which \nhave been elevated from a lncustrine basin, which very recently \noccupied the whole extension, and the remains of which are, \nin the south, the Laguna Verde, at 28°, and in the north Lake \nTiticaca. The discovery of great Pampean mammals in the \nPleistocene beds of that region shows that this upheaval of the \nlatter is very recent, for in the heart of the Cordillera, as \nwell as on the west coast of Bolivia and Peru, there have been \ndiscovered, in very recent deposits, the remains of some mammals \nwhich cannot have crossed the high range as it now exists.
Peru-Ecuador
The two Cordilleras that formed the Andes to the north of 28°S are continued in Peru. The western, which \nreaches an altitude of about 3000 m (10,000 feet), then ceases to exist as a continuous chain, there remaining only a short, \nhigh ridge, called by Edward Whymper the "Pacific range of the equator," and between this ridge and the crystalline Andean axis, the "avenue of volcanoes," to use his words, arises amidst majestic scenery.
Mount Chimborazo, which is not in the main chain, reaches 6310 m (20,703 ft); since Earth bulges at the equator, its summit is further from Earth's centre than that of Mount Everest.
Cotopaxi 5897 m (19,348), Antisana 5704 m (18,715), Cayambe 5790 m (18,997) are in the eastern range, with many other peaks over 5000 m (16,000 feet) which still contain glaciers. Sangay 5230 m (17,160 feet), under the equator, according to Wolf, appears to be the most active volcano in the world. Pichincha 4800 m (15,804 feet) and Cotocachi 4970 m (16,297 feet) are the loftiest volcanoes of the western range. In Colombia the three principal chains are continuations of those under the equator, and show very slight traces of volcanic action,
Colombia
In the western chain, which is remarkable for its regularity, \nthe highest peak is 11,150 feet, and the lowest pass 2050 m (6725 \nfeet). The central chain, separated from the western chain by \nthe valley of the Cauca and from the eastern by the valley \nof the Magdalena, is unbroken; it is the more important \nowing to its greater altitudes and is of volcanic character. \nTo the south, near the equator, are Mounts Arapul 4070 m (13,360 \nfeet) and Chumbul 4790 m (15,720 feet). The volcanoes Campainero \n3800 m (12,470 feet) and Pasto 4270 m (14,000 feet) are also in that zone. \nFarther north is the volcano Purace, which presents a height \nof 5000 m (16,000 feet); then come Huila 5500 m (18,000), Santa Catalina 4930 m\n(16,170), and Tolima 5600 m (18,400), Santa Isabel 5100 m (16,760), Ruiz 5300 m \n(17,390) and Hervas 5590 m (18,340). The eastern chain begins north \nof the equator at 1800 m (6000 feet), gradually rises to the height \nof Nevado 4300 m(14,146 feet), Pan de Azucar 3700 m (12,140 feet), and in \nthe Sierra Nevada de Cochi attains to peaks of 5100 m 16,700 feet.
The snow-line of the Andes is highest in parts of Peru \nwhere it lies at about 5000 m (16,500 feet). Its general range \nfrom the extreme north to Patagonia is 4300 m to 4700 m (14,000 to 15,500 \nfeet), but along the Patagonian frontier it sinks rapidly, \nuntil in Tierra del Fuego it lies at about 1500 m (4900 feet).
Structure
The structure of the Andes is least complex in the southern portion of the range. Between 33° and 36°S the chain consists broadly of a series of simple folds of \nJurassic and Cretaceous beds. It is probably separated on \nthe east from the recent deposits of the pampas by a great \nfault, which, however, is always concealed by an enormous \nmass of scree material. The Cretaceous beds lie in a broad \nsynclinal upon the eastern flank, but the greater part of \nthe chain is formed of Jurassic beds, through which, on the \nwestern margin, rise the numerous andesitic volcanic centres. \nThere is no continuous band of ancient gneiss, nor indeed \nof any beds older than the Jurassic. There is very little \nover-folding or faulting, and the structure is that of the \nJura mountains rather than of the Alps. The inner or eastern \nridge farther north of Argentina consists of crystalline rocks \nwith infolded Ordovician and Cambrian beds, often overlaid \nunconformably by a sandstone with plant-remains (chiefly \nRhaetic). In Bolivia this eastern ridge, separated from the \nwestern Cordillera by the longitudinal valley in which Lake \nTiticaca lies, is formed chiefly of Archaean and Palaeozoic \nrocks. All the geological systems, from the Cambrian to the \nCarboniferous, are represented and they are all strongly \nfolded, the folds leaning over towards the west. West of the \ngreat valley the range is composed of Mesozoic beds, together \nwith Tertiary volcanic rocks. (The Cordillera of Argentina \nand Chile is clearly the continuation of the western chain \nalone.) In Ecuador there is still an inner chain of ancient \ngneisses and schists and an outer chain composed of Mesozoic \nbeds. The longitudinal valley which separates them is \noccupied mainly by volcanic deposits. North of Ecuador the \nstructure becomes more complex. Of the three main chains \ninto which the mountains are now divided, the western \nbranch is formed mostly of Cretaceous beds; but the inner \nchains no longer consist exclusively of the older rocks, and \nCretaceous beds take a considerable share in their formation.
The great volcanoes, active and extinct, are not confined \nto any one zone. Sometimes they rise from the Mesozoic \nzone of the western Cordillera, sometimes from the ancient \nrocks of the eastern zone. But they all lie within the \nrange itself and do not, as in the Carpathians and the \nApennines, form a fringe upon the inner border of the chain.
The curvature of the range around the Brazilian massif, \nand the position of the zone of older rocks upon the eastern \nflank, led Suess to the conclusion that the Andes owe their \norigin to an overthrust from east to west, and that the \nVorland lies beneath the Pacific. In the south Wehrli \nand Burckhardt maintain that the thrust came from the \nwest, and they look upon the ancient rocks of Argentina \nas the Vorland. In this part of the chain, however, \nthere is but little evidence of overthrusting of any kind.
\nANDES is an acronym for an architecture with non-sequential dynamic execution scheduling.
Category:Mountain ranges
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