The Northern Andean EnvironmentAuthor(s): James J. ParsonsReviewed work(s):Source: Mountain Research and Development, Vol. 2, No. 3, State of Knowledge Report onAndean Ecosystems. Vol. 3: The Northern Andes: Environmental and Cultural Change (Aug.,1982), pp. 253-264Published by: International Mountain SocietyStable URL: http://www.jstor.org/stable/3673089 .Accessed: 23/11/2011 14:33

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Mountain Research and Development, Vol. 2, No. 3, 1982, pp. 253-262

THE NORTHERN ANDEAN ENVIRONMENT

JAMES J. PARSONS

Department of Geography University of California

Berkeley, CA 94720, U.S.A.

INTRODUCTION

The Andean Cordillera, one of the world's great moun- tain ranges, dominates the landscape and life of Ecuador, Colombia, and Venezuela (Frontispiece). These three coun- tries, with a comrbined population approaching 50 million, were briefly united politically as Gran Colombia in the years immediately following the nineteenth century wars of independence. They continue to retain a sense of common identity and interest that arises as much from the similarity of their physical environments as from their his- tory (Romero, 1965). Their traditions and their economies are mountain-based. Half the population, and consider- ably more than half in Colombia, still lives within the mountains, despite a continuing downslope migration towards the surrounding tropical lowlands. Continuing high rates of population growth and the attractions of urban life in such major cities as Quito, Bogota, Cali, Medellin, and Caracas underlie the persistence of their political and cultural dominance and of their dense rural settlement pat- terns.

This northernmost part of the Andes, extending like the neck and head of a giant sea-horse some 4,000 km from

about Cajamarca, Peru, to the Parla Peninsula in Vene- zuela, contrasts with the broader central part of the cordil- lera on account of higher rainfall and more abundant vege- tation cover (Gomez Molina and Little, 1981). Originally it was quite certainly forested, at least to the limits of the paramo some 3,200-3,500 m above sea level. In spite of extensive man-induced deforestation these are still moun- tains of greenness with much of the surface now in pas- ture grass or leguminous trees planted as shade for coffee.

Agriculture is practised without irrigation in most areas and artificial terracing is rare. Except in the wettest areas where an effective dry season is lacking, these highlands are closely settled by farmers of mestizo or Indian stock. Maize, manioc, and potatoes are the most common crops, along with introduced pasture grasses and, below about 2,000 m, coffee. Within these temperate highlands, modem urban centres, including the capital cities of Quito, Bogota, and Caracas, reflect their attractiveness as human habi- tats. These same highlands also were attractive to the numerous aboriginal populations that the Spaniards en- countered more than four centuries ago.

GEOMORPHOLOGY

The geology and history of the Northern Andes are undoubtedly distinct from that of the better studied central and southern part of the system and conclusions drawn from one sector are of limited validity for another (Irving, 1975). It is clear, however, that the entire cordillera has evolved through the subduction of the crumpled margin of the East Pacific (Nazca) plate and, to the north, the Caribbean plate, under the more rigid but lighter (sialic) South American plate, represented by the ancient crystalline Guiana and Brazilian shields (Ericksen, 1973). Sediments accumulated in the great geosyncline on the western margin of the continent have been compressed, deformed, and faulted by oblique subduction, underthrust- ing the continent at the rate of approximately 90 mm a

year, with associated thrust faulting and uplift. Geologists believe that these tectonic forces, similar to those operating elsewhere around the Pacific rim, were initiated during the Pliocene. That they continue today is evidenced by the

seismicity and the active vulcanism that characterize much of the cordillera.

The Andes are among the world's most youthful mountain ranges. Mountain building has been concen- trated along the plate margins with associated deep ocean trenches. It finds its driving forces in sea-floor spreading along the East Pacific rise (James, 1973). In the north, where the South American land mass abuts the smaller Caribbean plate, the tectonic relationships are but vaguely understood. Here the east-west trending Venezuelan mountains in some way seem to be associated with Central American and Antillean vulcanism and perhaps with that of the Galapagos Islands off Ecuador. The latter mark a submarine rift that separates the Nazca plate to the south from the smaller Cocos plate that extends northward along the Colombian and Central American coasts.

Parallel systems of ranges dominate the Andean struc- ture, converging at certain points in massive knots or nudos

254 / MOUNTAIN RESEARCH AND DEVELOPMENT

FIGURE 1. Major physical lineaments and interior plains of the Northern Andes.

(Figure 1). In Colombia, where it is most complex, the innermost ranges, facing the Amazon and Orinoco lowlands, are comprised predominantly of younger and less altered (Tertiary) sediments while the more western ranges tend to be of metamorphic rocks (gneisses, schists), intrusive granitic batholiths, or volcanics (one acid type of which has been named "andesite" for the mountains where it was first described). The existence of high-level erosion surfaces in the Eastern Cordillera of Colombia, often on soft sedimentary rocks, suggests that it experienced major vertical uplift in late Tertiary times and present altitudes reflect epeirogenic uplift after the principal Andean folding.

That the Northern Andes has been undergoing con- tinuing uplift is confirmed by fresh fault traces and by the high-level alluvial valley fill emplaced in many localities as aberrant hanging terraces high above present-day stream courses. For example, the deep alluvial and volcanic debris

that clogs the intermontane depression of Ecuador has been notably incised by headward eroding streams draining both towards the Pacific and towards the Amazon. They have left river terrace systems of considerable extent that com- prise some of the best agricultural land in the highlands. Some are lacustrine in origin, the result of drainage block- age by glaciers or by volcanic eruptions. The most extensive tracts of undissected valley fill are in the upper Cauca River (Valle del Cauca) and the high sabanas of Bogota in Colom- bia and the Lake Valencia basin in Venezuela.

THE ECUADORIAN ANDES From southern Ecuador to the Nevado de Ruiz

(5,400 m) in the Central Cordillera of Colombia the Andes are crowned by nearly sixty volcanoes or strato-volcanoes, many of them active, with strikingly symmetrical cones that reach into the zone of permanent snows. Cinder and lava ejecta from them mantle most of highland Ecuador and

J.J. PARSONS / 255

a significant part of the Colombian Andes, providing the fertile soils that support a dense farming population. In southernmost Ecuador and adjacent Peru gently sloping flows of pyroclastic materials cover a substantial area. Recent vulcanism has been lacking in Venezuela and in both the Eastern and Western Cordilleras of Colombia.

The Andes of Ecuador are the narrowest section of the entire cordillera. Their simplicity contrasts with the com- plexity of the system farther north. Two parallel chains, the Eastern and the Western Cordillera, topped with many volcanoes, most of which are snow-capped, enclose more than twenty intermontane basins (hoyas) that collectively comprise the Callejon Andina. The crests of the two ranges are about sixty kilometres apart and average 3,500 m ele- vation, except for the higher volcanic cones superimposed upon them. The basins, each occupied by its own regional urban centre (such as Loga, Cuenca, Riobamba, Ambato, Quito, Otavalo, and Ibarra), range in elevation from 2,000 m to 3,200 m and are separated from each other by cross ranges. These have developed at intersections where transverse faults or lineaments have cut across the fault- lined graben of the Callejon. The positioning of the larger volcanoes on alternate sides of the north-south depression favours the basin-like structure; the passes between them usually rise to about 3,500 m.

The culminating massif of Chimborazo (6,310 m), an inactive, glacier-adorned strato-volcano in the Western Cordillera near Riobamba, is the highest point in the Northern Andes. Once believed to be the highest moun- tain in the world, it early attracted the attention of visiting scientists. It inspired Alexander von Humboldt's classic study of the altitudinal belts of tropical vegetation which followed his unsuccessful attempt to reach the summit in 1802. Chimborazo is seen in best perspective from the Pacific lowlands. Set in a mountain chain that rarely falls below 3,200 m, it loses half its true height when seen from the populous inter-Andean depression.

Eight of Ecuador's volcanoes have been active in historic times (Hall, 1977; Smith. Inst., 1981). Three others are in a latent state, having erupted in the recent geologic past, and are susceptible to renewed activity. Cotopaxi (5,897 m), within sight of Quito, is one of the highest active volcanoes in the world. It was extremely active in the last half of the nineteenth century; its eruption in 1877 melted the perennial ice at its summit to produce mud flows that covered much of the inter-Andean valley below. Sangay (5,230 m) and Tungurahura (5,016 m) are other impres- sive members of this avenue of volcanoes. Two others, Reventador (3,485 m) and Sumaco (3,828 m) are basaltic cones that dominate a third, lower range to the east, which rises from a forested base at approximately 1,500 m. Reventador and Sangay have been almost continuously active in recent years. Although such eruptions occasionally threaten surrounding populations, the benefits of vulcanism clearly outweigh the problems and dangers it presents. They have provided not only the fertile and often nearly level soils of the highlands but also the cangahua (tuff) so important for the construction of houses and walls and the rocky materials employed in highway and airport construction.

in the highland basins of Ecuador is spectacularly exposed by barrancas that are being eroded headwardly into them. Among these, those formed by the Guayllabamba River and Chota River, which cut deeply into the intermontane fill north of Quito on their way to the Pacific lowlands, and form extensive salients of tierra caliente, are perhaps the most impressive.

THE COLOMBIAN ANDES A few kilometres north of the Ecuador-Colombian

border the parallel ranges and intermontane depression merge into the Nudo de Pasto (Volcan Galeras, 4,276 m), itself under attack by headward-eroding tributaries of the Patia River which flows westward to the Pacific near Tumaco. From Pasto the Colombian Andes fan out north- ward in three distinct prongs. Two of these eventually peter out on the Caribbean coast while the third curves eastward in a giant arc at latitude 7°30' North to form the Venezuelan Andes. A spur continues northward as the Sierra de Perija, forming the boundary between Colombia and Venezuela west of the Lake Maracaibo depression.

Two great river valleys, the Magdalena and the Cauca, separate the Colombian ranges and provide avenues of penetration from the Caribbean coastal lowlands into the heart of the Colombian Andes. Late Tertiary or Quater- nary volcanic activity blocking the middle course of the Cauca formed a great lake that for a time filled the western inter-Andean trough for about 200 km south of Cartago. The river eventually broke through this dam to leave the level floor of the Valle del Cauca (elevation about 980 m), today one of the most productive agricultural areas of Colombia.

Of the three Colombian ranges the non-volcanic Western Cordillera, which forms a barrier between the rain- drenched Pacific lowlands and the Cauca drainage, is the lowest and the least populated. Superhumid tropical air, lifted orographically, saturates its west-facing midslopes with up to 6,000 mm of rainfall annually. Between Cali and Buenaventura two passes of less than 1,600 m mark depressions in the range. Another to the north, the 2,560 m Boqueron de Toyo, is crossed by the road from Medellin to Turbo on the Gulf of Uraba. Elsewhere the crest is much higher, with maximum elevations approaching 4,000 m as far north as the little-known Paramillo in Antioquia, on the slopes of which was located the legendary Indian and colonial mining camp of Buritica. From here the cordillera fingers north into the three distinct serranias of Abibe, San Jeronimo, and Ayapel which drop gradually to the pied- mont plains of the Caribbean coast.

The Central Cordillera, narrower and loftier, is a continuation northward of the Ecuadorian volcanic range. Crystalline rocks are exposed at several places on its flanks, and are the bases of local gold and silver mineralization as at Popayan and Mariquita. Elsewhere the underlying basement is overlain by folded Tertiary sandstones and shales. From Pasto north to the Antioquia border near Sonson the older range has been capped by ash and lava derived from a line of some 20 Quaternary volcanoes (Ramirez, 1969). Several, like their counterparts in Ecua- dor, reach well into the area of permanent snow above 4,500-4,800 m. These include the Purace-Conucos The accumulated depth of ash and pyroclastic materials

256 / MOUNTAIN RESEARCH AND DEVELOPMENT

complex behind Popayan, the Nevado de Huila (5,750 m) southeast of Cali, and the Rufz-Tolima or Quindio complex between Manizales and Ibague. Seven of these have been active in historic times and four more are in the fumarolic stage. The fertile andesitic ash from their erup- tions has produced the often steep and unstable slopes that support most of Colombia's coffee fincas (family-operated coffee farms), including those of the highly productive Quindio district around the cities of Pereira and Armenia. The heavily travelled Quindio Pass (3,485 m) links Bogota and the Magdalena valley with the Valle del Cauca and the Pacific port of Buenaventura.

Beyond Sonson the volcanic Central Cordillera gives way to the deeply weathered quartz-diorites of the Mesozoic Antioquia batholith, a tableland covered by unproductive clay soils with an average elevation of about 2,500 m. It is divided into two parts by the deep transverse cleft of the antecedent Porce River (Medellin River) that occupies a U-shaped valley in which is situated the expanding metrop- olis of Medellin (1,500 m). This intrusive batholith, shot through with auriferous quartz veins, was the source of the gold-bearing gravels that gave rise to an active colonial mining economy both in the highlands and along the lower Cauca River and Nechi River (Zaragoza, Caceres). Where the Cauca turns east below Santa Fe de Antioquia it occu- pies a spectacular V-notched canyon that has been estimated to have an enormous potential for hydroelectric development.

The Eastern or Bogota Cordillera, separating the Magdalena valley from the eastern plains (Llanos) is blocky and massive, comprised chiefly of folded and faulted marine sediments that overlie older schists and gneisses. Narrow in the south, it broadens out in the high, largely unsettled massif of Sumapaz, with elevations up to 4,300 m, and is an important reserve water supply for the Bogota conurbation (Giihl, 1964). The sabana of Bogota (2,660 m), part of an extensive Pleistocene lake system that extends northward to Tunja, has been the traditional centre of Chibcha, Spanish, and Colombian culture and govern- ment. The gently rolling uplands that surround these high plains extend above the treeline into the paramos. Farther north, beyond the deep gorges cut by the Chichamocha River and its tributaries flowing towards the Magdalena, tower the culminating peaks of the Sierra Nevada de Cocuy (5,493 m).

From its higher eastern crest the cordillera drops abruptly to the Llanos, its flank cut by deep, short can- yons. The Bogota-Villavicencio highway provides a mag- nificent perspective on this east-facing scarp, which con- trasts with the more gradual descent westward in the direc- tion of the Magdalena valley.

Beyond the Nudo de Pamplona the Eastern Cordillera bifurcates into two much narrower ranges, the higher of which becomes the Venezuelan Andes. The Serrania de Perija or Motilones (3,750 m), the boundary range between Colombia and Venezuela, has extensive coal deposits now under development in its northern foothills. Towards the Caribbean it gradually descends to become the low hills that form the backbone of the Guajira Peninsula.

The isolated Sierra Nevada de Santa Marta, with its im-

rising abruptly from the Caribbean coast to snow-capped and glaciated peaks, is not strictly a part of the Andes. Some geologists, however, have interpreted it as an extension of the Central Cordillera, from which it is separated by the down-dropped Mesozoic graben of the Mompos Depression in the lower Magdalena valley.

THE VENEZUELAN ANDES The Cordillera of Merida is set off from the broader

Colombian ranges and the Paramo de Tama on the Venezuela-Colombia boundary by the Tachira Depression (800 m), the lowest pass through the Andes north of the Chilean lake district. This low hill country, in which are situated the cities of San Cristobal (Venezuela) and Cucuta (Colombia), greatly facilitates communications between the rapidly developing Lake Maracaibo lowlands and the western Llanos, especially with the completion of the highway along the margin of the Llanos which links the border areas with Barinas, Guanare, Caracas, and easternmost Venezuela.

The steep and rugged Merida Cordillera has a charac- teristic Andean structure and grandeur. It extends to the paramos and permanent snow belts at several places, most conspicuously in the massive Sierra Nevada de Merida (Pico de Bolivar 5,007 m) immediately above the city of that name to which it is linked by an aerial cable. Some 450 km long and 80 km wide, the cordillera is comprised chiefly of ancient schists and gneisses with granitic intru- sions exposed at the higher points. On its southeast margin it drops precipitously from the summits directly to the Llanos, its flanks deeply incised by short streams. On the northwest it is in part bordered by the lower Tertiary hills and dissected plateaus of the Segovia Highlands that extend through the state of Lara to the arid Caribbean littoral (Jahn, 1921).

These tortuously rugged mountain lands were the early heartland of what was to become Venezuela. Colonial cities such as Merida, Valera, and Trujillo, nestled in long, linear, fault-controlled valleys, were linked by a major mule trail to both Bogota and Caracas. Almost the only flat lands are the river terraces and mesas along the incised canyons of rivers such as the Mototan and the Chama. Like the major fault systems these run parallel to the predominant northeast-southwest orientation of the ranges until cutting through deep transverse canyons to the lowlands around Lake Maracaibo. Isolation from markets, increasing frag- mentation of land holdings, and severe soil erosion have led to the area's continuing economic decline. The Venezuelan Andes today are the most backward and impoverished part of the country, the source of much of the emigration to the oil fields and the burgeoning metro- politan areas of Caracas and Maracaibo (Watters, 1967).

Behind Barquisimeto and the headwaters of the northeasterly flowing Yaracuy River, the Merida Cordil- lera merges into the low hills of the Lara Depression. Here the Andes proper, in Venezuelan usage, are said to termi- nate. Their topographic trend lines, however, continue eastward as the Central Highlands and the Northeastern Highlands, El Sistema Orograifica Central and El Sistema Montafioso Oriental of Pablo Vila (Vila, 1960).

posing granite massif (5,775 m, the highest in Colombia) The first of these, in reality two distinct ranges, is an

J.J. PARSONS / 257

east-west oriented block about 400 km in length that abuts the Caribbean shore. The higher and unbroken Serranfa de la Costa reaches above 2,600 m at the Silla de Caracas and the Pico Nigueta immediately north of the capital city. Both it and the Serrania del Interior, composed predomi- nantly of crystalline and metamorphic rocks, terminate on the coast where they turn south to form the western side of the Gulf of Barcelona. The straight northern margin of the Serrania de la Costa, mostly cliffed, is controlled by faulting. So are the intermontane valleys that lie between the two major ranges, including the 90 km long Lake Valencia Depression (elevation 450 m) and, to the east, the smaller valley of the Tuy River which drains eastward to the Caribbean. It is a tributary of the latter, the Guaire River, with headwaters in the coastal range only 10 km from the Caribbean, that drains the elongated valley of Caracas (elevation 900 m), now almost completely urbanized. The anticlinal ridges of the Serrania del Interior to the south and west, overlooking the Llanos, are distin-

guished locally by a number of steep, sharp limestone peaks known as morros.

Eastward, beyond the Gulf of Barcelona, the Serrania de la Costa re-merges to form the two peninsulas of Araya and Parfa, which rise above the straight, cliffed coast to elevations of about 1,000 m. They, too, are predominantly of metamorphic rocks. Beyond the Parfa Peninsula and the Bocas del Drago channel the same structure re-emerges as the northern range of the island of Trinidad. South of the structural depression in which Cumana and the gulf of that name are located, between Barcelona and the Gulf of Parfa, lie the higher, sparsely settled Cretaceous highlands that are the eastward extension of the Serrania del Interior (Pico de Turimiquire, 2,596 m). Between these Northeastern Highlands and the Central Highlands only a low range of hills, drained by the northward flowing Unare River, separate the interior Llanos from the Caribbean Sea (Jahn, 1921; Vila, 1960).

CLIMATE AND VEGETATION

In equatorial latitudes where the Northern Andes lie the seasons are delimited by rainfall. There is little temperature difference between the warmest and coldest month. Such markedly isothermal conditions, together with the temperate climate of the mid-slopes and the brilliant trans- lucence of the Andean air, qualifies these mountains as "a land of eternal spring."

The decrease in temperature with increasing altitude approximates 0.6° C for each 100 m so that the topographic map serves roughly as a temperature map, with contour lines equating with isotherms. But different flanks of the same mountain may vary markedly in precipitation, cloudiness, and radiation. The rapid succession of life forms has deeply impressed observers travelling through these mountains since the days of Humboldt.

In generalized terms convention distinguishes the tierra caliente (below 1,000 m), the tierra templada (1,000-2,200 m) and the tierra fria (2,200-3,200 m), above which is the treeless paramo, habitat of the distinctive giant Composi- tae, Espeletia spp. (frailejdn). Vertical contrasts in tem- perature also play a critical role, along with annual pre- cipitation and potential evapotranspiration, in defining the well-known biological life zones mapped by the Hold- ridge system: Ecuador (MAG, 1978); Colombia (IGAC, 1962); and Venezuela (Ewel, Madriz and Tosi, 1976). Although widely adopted, the system does not take into account the length or severity of the dry season or the extent to which the original vegetation cover may have been modified by man. Based upon the life zone maps for the three countries, a generalized map of the Northern Andes is presented in Figure 2.

The position of the Intertropical Convergence Zone (ITCZ) contributes directly to the perennial humidity of southern Colombia and northern Ecuador, with most of the area having the double rainfall maximum and mini- mum characteristic of the inner tropics. Its seasonal shift northward in June-July and southward in Decem- ber-January controls the seasonality of precipitation

elsewhere. In areas close to the Caribbean the single dry season (verano) centres on the first months of the year while in most of Ecuador this occurs in the middle months. Precipitation at Quito, where agriculture in some years may suffer from drought, has been shown to be related with statistical significance to the January intensity of the Icelandic low pressure cell and this with January temper- atures in Greenland and Norway, the intensity of the trade winds, and the position of the equatorial zone of rains

(Knapp, 1980). Thus, short-term forecasts of some reliability, and even the reconstruction of the climatic history of this part of the Andes, may be within reach. Weather conditions would be influenced also by the be- haviour of the cold Peru (Humboldt) Current along the Peruvian coast and the "El Nifio" phenomena which occa- sionally impinges on the weather conditions of southern Ecuador (Blandin Landivar, 1976).

These Andean mountains and valleys, although generally drier than the surrounding lowlands to the east and west, are much better watered than the cordillera to the south, in Peru. Irrigation is needed only in restricted areas where there are severe rain-shadow conditions. Be- cause of the abruptness and complexity of the topography there are numerous such dry areas and important ecological changes may occur over very short distances. The deepest valleys, as those of the Chama River (Venezuela), the Chichimocha and Patia rivers (Colombia), and the Chota River (Ecuador) have true desert conditions (the BS and B W of the K6ppen system) with cactus and other xerophytes. Desert conditions also exist in a narrow zone along the coastal mountain fringes of Venezuela where pre- vailing winds parallel to the shoreline lead to atmospheric divergence and subsidence. The upper Cauca and Mag- dalena valleys and the intermontane basins of Ecuador are also significantly less rainy than the surrounding mountains.

Human habitation of the Andes for thousands of years has led to major alterations in the vegetation cover. Forests

258 / MOUNTAIN RESEARCH AND DEVELOPMENT

FIGURE 2. Life zones of the Northern Andes generalized from ecological maps following the system of world vegetation formations of L.R. Holdridge (1979).

originally covered all but the highest and driest areas and localities edaphically unsuited to support them. Today they are restricted to the steepest, most inaccessible slopes and areas of especially high rainfall. Elsewhere pasture, crops, or degraded scrub and grass have replaced the original cover of broadleaf evergreen trees. The first chroniclers often described the inner Andes as being but sparsely wooded, "rough, with naked trees and few hills." They frequently interpreted these conditions as the product of Indian agriculture and burning. In more recent times, with the increasing numbers of European cattle, the area of grassland has been vastly extended. Introduced species of African origin are particularly conspicuous and some, like kikuyu grass, are spreading aggressively.

Even in the most lush forest tracts that remain, as in the superhumid environments of the outer cordilleran flanks in the eastern and western ranges of Ecuador and Colombia, there is much evidence of earlier human occu- pancy. These wet montane forests are characterized by

mosses, lianas, bromeliads, and such economically valued members as cinchona, the latex-bearing Sapotaceae (balata, chicle), ivory nut or tagua (Phytelephus), and the giant American bamboo (Guadua). Lumbering has had a very minor role here due to the singular difficulty of access as well as the absence of massed trees of any one commercial species.

The paramo is a distinctive biome of the superhumid equatorial high mountains, reaching from the upper limits of the cloud forests (about 3,200 m) to the belt of perma- nent snows (4,700 m). It is unique to the Ecuador and Colombian Andes and their lesser extension into Venezuela. Paramo-like environments also extend a short distance into northern Peru and into the Cordillera de Tala- manca of southeastern Costa Rica. Its closest counterpart elsewhere is found in the high mountains of East Africa and New Guinea (Troll, 1968; Salgado-Labouriau, 1979; Lauer, 1981). This alpine-type vegetation association is characterized by tussock grasses (pajonales), cushion plants,

J.J. PARSONS / 259

and the tree-like Espeletia, a curious looking, hairy-leafed genus represented by 50 different species (Cuatrecasas, 1968; Lauer, 1979). It is this last, fire-resistant and adapted to low temperature and high humidity, that gives special character to the paramo landscape, especially in Colombia and Venezuela.

Clouds or fog enshroud the paramo for much of the year and evapotranspiration is low. The boggy, peat-like soils are black and high in organic matter. The vertical range of the paramo formation narrows on the more humid eastern exposure of the cordillera where the treeline is higher and the snowline lower (Troll, 1968; Salgado- Labouriau, 1979; Lauer, 1981). In Ecuador it may drop to 2,800 m on the drier west-facing slopes of the Eastern Cordillera facing the Callejon Andina.

Climatological data are limited but average annual precipitation may range from 750 to more than 2,000 mm. The Venezuelan paramos are drier than those of Colombia and Ecuador, and from late November to March they frequently extend into the clear air above the sea of clouds banked against the uppermost forest (ceja de la montana). The same may be true of the Sierra Nevada de Santa Marta. The paramo above Merida averages 180 days of precipitation a year while the Pairamo de Montserrat above Bogota and the high shoulders of Volcan Cotopaxi in

Ecuador both report some 250 rainy days (Lauer, 1979). In this super-saturated environment, with high daytime radiation, the average annual temperature ranges from 2 to 10°C. There is a high frequency of night-time frost and the diurnal freeze-and-thaw cycle stresses both plant life and the soil (Schnetter et al., 1976).

The lower paramo (sub-paramo) below 3,500 m is a transitional belt in which scattered clumps of trees are not uncommon, especially the cold-resistant Polylepis. It favours shallow, talus slopes where the soil tends to be warmer than elsewhere. These trees may occasionally reach as high as 4,000 m.

Despite its bleak and forbidding climate, the paramo areas have been significantly altered by human activity, especially wood cutting and burning to promote grazing. Agriculture has also impinged on its lower reaches. Potatoes are the paramo crop par excellence, being grown as high as 4,000 m in Colombia on the eastern slope of the Sierra Nevada de Cocuy. In Ecuador potato produc- tion is now being transformed by modern mechanized agriculture which is said to be obtaining good yields in spite of the low temperatures (Sampedro, 1975-1976). But extensive tracts of paramo remain relatively untouched by humans. Their principal future value may be as water catchment areas for the cities and farmlands below.

GLACIATION AND CLIMATIC CHANGE

Tropical mountains provide a unique opportunity for the study of climatic change through the advance and re- treat of glacial ice and snow. The South American Andes are one of only three places on earth where glaciation occurs in the vicinity of the equator. In each there are clear indica- tions of a recent sharp retreat of the lower limit of glacial ice. Today only small remnant glaciers remain on the

higher Andean peaks (Hastenrath, 1979). From available historical accounts there seems to have been no significant warming or cooling trend during the colonial period. The first indication of ice recession comes in the early years of the nineteenth century. A later ("neo-glacial") advance about 80 years ago has been followed by an accelerated retreat continuing to the present.

The present position of the permanent snowline varies with exposure. It lies at about 4,600-4,700 m in the Colombian Andes, only slightly higher in Ecuador (Wilhelmy, 1957). Glacial features such as moraines are found throughout the paramo and as low as about 3,000 m elevation. There is evidence that isolated valley glaciers in the past descended to 2,200 m on the massive Sierra Nevada de Santa Marta and to nearly 2,600 m on the Venezuelan Andes (Royo y Gomez, 1959). Cirque lakes,

a classic glacial feature of high mountains, are concentrated in a belt between 3,300 and 3,600 m in all three countries. The present climate appears to be warmer and drier than experienced in the recent past.

Relative chronology can be established for Andean glaciations on geomorphic evidence but it is difficult to extrapolate to the tropical zone from better known mid- latitude conditions. Two, three, or four glacial advances are generally recognized in the Northern Andes. Although usually interpreted as subdivisions of the late Wisconsin (Wiirm) glaciation of North America and Europe, evidence for pre-Wisconsin glaciation has been reported in Colombia where a lateral moraine on the Nevado de Ruiz lies under volcanic ash radiometrically dated at 100,000 BP (Herd and Naeser, 1974). The lack of evidences of earlier glacia- tion often has been attributed to the late uplift of the Andean range although on palynological evidence it has been argued that the Eastern Cordillera of Colombia, at least, has been at approximately its present elevation since the end of the Pliocene (Van der Hammen, 1979). More evidence, including confirmed dating by other techniques, is needed to verify this interpretation.

NATURAL HAZARDS

The combination of the geological background, the broken topography with over-steepened slopes, and the high incidence locally of torrential tropical rainfall makes the densely settled Northern Andes highly susceptibe to devastating ecological disruptions. Their position along

plate boundaries where continuing crustal deformation is occurring contributes especially to the high concentration of geologic hazards (Caviedes, 1981). The instability of the volcanic ash slopes and the glacio-fluvial deposits from the glaciated higher mountains contributes to this propensity

260 / MOUNTAIN RESEARCH AND DEVELOPMENT

to catastrophe, which is further aggravated by the high densities of population. The consequences of such events for the adjacent lowlands may be substantial, especially in the form of widespread flooding and accelerated sedimentation aggravated by man-induced hydrologic alterations (Brunnschweiler, n.d.).

Earthquakes are a continuing and ever-present hazard to human life in all the Andean countries. Comparatively few such seismic events, however, are known to have been associated with surface fault movements. Nearly all that have their epicentres on land are deep, focused within the Benioff zone below the continental plate. Shallow quakes tend to occur offshore in association with ocean trenches. In these, surface ruptures seem likely to occur, but they cannot be observed. Tsunami waves, such as the one that damaged Tumaco, Colombia, in 1979 may be evidence of this.

Historical reconstructions suggest that "ruinous and disastrous" quakes have hit the Venezuelan Andes approxi- mately once every 30 years since 1530 (Centefio-Graii, 1940). Major quakes have three times destroyed Caracas (1641, 1766, and 1812) with the most calamitous being the last. A 6.5 Richter-scale temblor on 29 July 1967 did major structural damage to modern high-rise buildings in the capital and in La Guaira. Cumana, on the eastern coast, has been levelled four times (1530, 1706, 1853, and 1929) with seismic sea-wave damage contributing significantly to the toll on at least two occasions. Caruipano suffered similarly in 1874. Merida and the surrounding area in the Andes have been frequently shaken, most destructively in 1786.

A similar Colombian study (Ramfrez, 1969) lists 597 quakes in the period 1566-1963, of which some 70 were of VIII or greater intensity on the Rossi-Forel scale. Those of 1595 and 1845 accompanied the eruption of Ruiz with destructive mud flows (aluviones) onto the Tolima plains in the Magdalena valley taking many lives on the latter occasion. Quakes of 1566, 1595, 1644, 1735, 1785, and 1805 stand out among those in the colonial period and the last destroyed the Magdalena river port of Honda. Bogota has suffered damage on numerous occasions, but the most serious quake in Colombian history was probably that of 1875 that made rubble of the city of Cucuta on the Vene- zuelan border. The 1906 Tumaco quake was also extra- ordinarily severe. This and the more recent Tumaco quake were shallow rooted, with offshore epicentres (Herd et al., 1981).

Ecuador, perhaps even more prone to damaging seismic events, lists 315 "principal quakes" between 1534 and 1958, of which 99 are estimated to have been of 6.5 or greater magnitude on the Richter scale (OAE, 1959). The destruction of Ambato in 1698 and of Riobamba in 1797 (which killed between 5,000 and 6,000 people) stand out as probably the greatest tragedies in the country's seismic history. That there will be other and equally damaging quakes in the future is one of the facts of life that must con- tinually be taken into account by officials and planners as well as the common man.

Earthquakes have often triggered destructive landslides

or mud flows in addition to the more common structural damage to buildings that they inflict. Especially after heavy rains the steep slopes saturated with water are susceptible to massive slipping and sliding which is not always related to seismic activity. In Colombia entire hillsides and even towns have been wiped off the map. Precariously sited cities like Manizales and Bucaramanga have lost entire blocks of buildings in such events with major loss of life. Rio- bamba, an important colonial city in Ecuador at the present location of Cajabamba, was destroyed by such a landslide in 1797.

Road cuts or other construction activity may facilitate such mass movements, sometimes with cataclysmic results. The great Quiebra Blanca slide on the Bogota-Villavicen- cio road in June 1977 took the lives of some 200 people waiting in buses and cars blocked by smaller slides. It left that strategic highway link between the Colombian capital and the Llanos closed for four months, effectively isolating the department of Meta from the rest of the country. Such disruptions of communication are routine in Colombia, and the cultivation of excessively steep and unstable slopes often aggravates the problem. Many a Colombian campesino has "fallen" to his death while cultivating his maize field or manioc patch. But even well-forested Andean slopes are scarred with slips that may be unrelated to human activi- ties. These are caused by overloading with water during extreme rainfall events even in the absence of seismic move- ments. Such slide scars, reaching down to the regolith, may take many years to be healed over by the surrounding forest.

Garner (1959) counted more than 200 major slides and countless smaller ones in a single air reconnaissance over the water-saturated slope of the Serrania de la Costa near Caracas in February 1951. The larger ones were in excess of 100 m wide and up to 400 m long. They were inclined at 30° or more, with their bases in steep valleys. He esti- mated, based on the average dimension of the slides, that a minimum of 20 million cubic metres of material had been removed in a few weeks. Such phenomena apparently have played an important role in slope steepening in the Northern Andes.

Forest clearing, much intensified in recent years, has led to accelerated slope erosion, with resultant intensification of floods, especially in the extensive drainage basin of the Magdalena valley (Vidart, 1976). Such floods in the lower Magdalena system, as well as in the Valle del Cauca, have become annual occurrences, with damage reaching into many millions of dollars. The Magdalena has become vir- tually unnavigable during much of the year as a result of the changed hydrologic regime, and extensive shoaling during the dry season prevents the use of even shallow- draft vessels. Reservoirs, such as that on the Anchicaya River, near Cali, have silted up at alarming rates and in some cases have become almost useless for water storage. Lake Valencia in Venezuela is today but a fraction of its original size as a result of increased take-off of water for irrigation and reduced moisture retention in the soils within its watershed.

J.J. PARSONS / 261

CONCLUSIONS

That part of the Andes situated between Cajamarca in northern Peru and the coast of Venezuela is a dense mosaic of ecosystems that is distinguished from the broad central section of the cordillera in Peru and Bolivia by its higher humidity and the climatic symmetry between the east and west flanks of the range. Weathering in the Northern Andes is a predominantly chemical process that generates a fine detritus-clay, silt, and sand-that is naturally protected by vegetation from fluvial or aeolian erosion. When the vegetation is disturbed, change is triggered. The removal of the original plant cover in this "high energy" environ- ment of sharp relief where all processes are intensified has led to serious erosion. Rapidly accelerating human pres- sures are exponentially augmenting the downslope transfer of materials. In many cases people are beginning to follow.

Fortunately the recuperative powers of vegetation cover in these humid environments is considerable. However, the abundant rainfall and its higher intensities in the north tend to neutralize this apparent advantage. It is the soils of the drier, rain-shadowed pockets in Ecuador, Colombia, and Venezuela that show some of the most severe exam- ples of soil erosion, gullying, and land abandonment (Jungerius, 1975). The introduction of European livestock has had an especially severe impact on such areas, destroy- ing the ground cover and encouraging the selective spread of thorny, poisonous, less palatable plants. Animal tramp- ling on slopes became more damaging because the sheep, goats, cattle, donkeys, and horses had sharper hooves and were more numerous than the llamas and alpacas that they replaced (Ellenberg, 1979).

Colombia has taken the lead in studying the region's erosion problems. A general erosion map of the country has been prepared (scale of 1:1,000,000) to serve as a base- line document for planning purposes (Lecarpentier et al., 1977). In March 1981, Unesco and the Colombian Society of Geology co-sponsored a meeting of experts in Bogota' focused on the subject of "Erosion Processes of the Northern Andes," but with participants and papers from all Andean countries. The meeting emphasized the need for systematic

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dor. Quito: XI Asamblea General y Reuniones Panamericanas de Consulta Conexas.

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Caviedes, C., 1981: Natural hazards in Latin America: a survey and discussion. In Martinson, T.L. and Elbow, G.S. (eds.) Geographic Research on Latin America: Benchmark 1980. Proceed- ings of the Conference of Latin Americanist Geographers, Vol. 8, pp. 280-294.

Centefio-Graii, M., 1940: Estudios Seismol6gicos. Caracas: n.p. Cuatrecasas, J., 1968: Paramo vegetation and its life forms. In

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Eckholm, E., 1975: The deterioration of mountain environments. Science, 189: 764-770.

mountain hazards mapping for future land-use planning. New forms of environmental management offer hope for

the future. The environmental diversity of mountains has resulted in a wide range of income-producing and subsistence strategies available to mountain people (Ives, 1979). Experience with afforestation, even in drier areas of the Andes, shows a surprising potential for tree growth and incidentally provides evidence that most of the cordillera must once have been naturally forested. Another major potential which has hardly begun to be developed is hydroelectric power, something that will be increasingly welcome in an energy hungry world.

However, the green Andes of the north remain a fragile environment, highly sensitive to ecological disruption. And with some of the highest rates of natural population increase in the world, there have developed pressures on the land that are unlikely to lessen, even with the twin safety valves of outmigration to the lowlands and to the cities (Eckholm, 1975).

New methods of land use and management need to be directed towards long-term sustained yields. The agricul- tural and social systems that have developed on these steep slopes have not always been well adapted to hillside farm- ing. Rural development programmes have seldom given the hillside farmers much attention, although many of the basic tools are already available. Colombia, Ecuador, and Venezuela each have active mapping programmes and up- to-date national atlases (IGAC, 1977; IGM, 1978; MARNR, 1979). An international symposium held in December 1980 in Costa Rica on "Agricultural, Livestock, and Forestry Production in the Hill Lands of Tropical America" emphasized the need to study the land as ecological and watershed systems with the well-being of the people and their access to productive land a major consideration (Novoa B. and Posner, 1981). Future suc- cesses hinge on recognition of the need for multidisciplinary and integrative approaches in understanding the intricacies of the constraints facing the people who work on the Andean slopes (Glaser and Celecia, 1981).

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G6mez Molina, E. and Little, A.V., 1981: Geoecology of the Andes: the natural science basis for research planning. Mount. Res. Dev., 1(2):115-144.

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Giuhl, E., 1964: Aspectos geograficos y humanos de la region del Sumapaz en la Cordillera Oriental de Colombia. Rev. Acad. Col. Cien. Exac. Fis. Nat. (Bogota), 12:153-162.

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Herd, D.G., Youd, T.L., Meyer, H., Arango, J.L., Person, W.J. and Mendoza, C., 1981: The great Tumaco, Colombia earthquake of 12 December 1979. Science, 211:441-446.

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263

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Farming landscape south of Quito, Ecuador, altitude about 3,000 m. Photo by Gisbert Glaser.

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Landscape south of Otavalo, Ecuador, at about 3,400 m. This view shows the deep fluvial dissection that has occurred in the ash and pyroclastic fill of the intermontane basins (Callejon Andina). Photo by Gisbert Glaser.

264 / MOUNTAIN RESEARCH AND DEVELOPMENT

Ecuador's western range of the Andes. This view shows the characteristic dry watersheds of the Cordillera Oxidentale on the road between Latacunga and Quevedo. Photo by Gisbert Glaser.