QUATERNARY IN VENEZUELA

Little attention has been paid so far to the Quaternary in Venezuela in spite of its scientific and economic interest. The limits between Pliocene, Pleistocene and Holocene have not yet been clearly drawn. Considerable confusion exists on this point; various formations originally assigned to the Quaternary have had to be transferred to the Pliocene, and there are still some supposed Quaternary formations, such as the Mesa and El Milagro, whose Quaternary age (at least as a whole) is doubtful.

As examples of the confusion, we have the beds at Cabo Blanco, Maiquetia, D. F., which have attracted the attention of geologists since the time of Humboldt. Some have classified them as Quaternary, others as Miocene or Pliocene, whereas in reality there are two Tertiary formations separated by an unconformity, and three Quaternary raised beaches. To realize the general confusion of ideas, it is enough to compare the two editions of Liddle's Geology of Venezuela (1928, 1946), especially the chapters on the Pleistocene and Recent formations; practically everything described as Recent, for instance, is really Pleistocene.

One could wish for a serious study of the fossil vertebrates, especially the mammals, which would contribute greatly towards elucidating the age of the Tertiary and Quaternary sediments of continental facies.

On the basis of the literature and the results obtained from our own observations, we shall endeavour to give here a resumé of the different Quaternary formations which cover such extensive areas in Venezuela, much greater than are generally indicated in the geologic maps.

Marine formations.-As regards the Pleistocene, the Venezuelan coast must be divided into two regions of different geologic history; a western region, which begins at Cabo Codera and continues as far as Goajira; and an eastern region, which continues from Codera as far as the peninsula of Paria. In the western region, the part from Cabo Codera up to Morón (Edo. Carabobo) corresponds to the Cordillera de la Costa, a mountainfront coast with only narrow steps of raised beaches and limited Holocene beaches in the coastal re-entrants or in the alluvial fans of the rivers. To the W. of Morón, the coastal region is formed by extensive plains with very long beaches, which alternate with some small rocky zones in the places where the hills of Falcón extend to the sea. Common to all this section is the presence of raised beaches or terraces of the same type as those observed in the coast of Colombia (where the coralline limestones of La Popa, Cartagena, are Miocene, not Pleistocene in age); in the regions of steep slopes, these beaches pass into alluvial fans of the rivers, at the same level. In general, one observes along the seacoast two or three terraces of varying width, which at times, in the hilly regions, may be reduced respectively to a profile like an alligator's head. The terraces which we have been able to recognize thus far are, two Pleistocene and one Recent. The oldest is the terrace of 50-60 meters elevation above sea-level, which can be well seen in the Playa Grande real-estate subdivision of Maiquetía, and to the east of Arrecifes (D. F.), possibly also in the south and west of Paraguaná. The next oldest is that of 16-20 m. above sea-level, which is the best preserved, and which may be observed to the west of Cabo Codera (in Chirimena, etc.) in Cabo Blanco (level of the Maiquetía airport), and between La Vela de Coro and Punta Triana. Possibly the beds described as "Case Ventura beds" may represent this level. The lowest raised beach, 3-6 meters above sea-level, is Holocene and is recognized generally all along the Venezuelan coast, both in the east and in the west, characterized by shelly sands, coral beds and formation of spits and bars. In the beach of Quisandal at Puerto Cabello, we have found fragments of pre-columbian ceramics cemented into the coralliferous limestones of the seashore. The "keys" (cayos) or small islands, so numerous in the vicinity of Puerto Cabello, San Luis and Chichiriviche, are coralliferous bottoms which are emerging in the form of islands, and possibly correspond to the two lower raised beaches, especially the Recent one.

The coast to the east of Cape Codera as far as the promontory of Paria has no raised beaches except the most recent or Holocene one, with the same characteristics as in the western region; when one finds any terrace, as in Barlovento, in Puerto Píritu and in Cumaná, they are river-terraces. Within this oriental region in general, two topographically different zones may be distinguished; one which comprehends the region from Cabo Codera to Barcelona, in which predominate coastal plains with extensive beaches, interrupted only where the Serranía del Interior reaches the coast at Unare and Píritu; and another mountainous zone, which extends from Barcelona to the promontory of Paria. All the eastern coast shows the typical topography of a sunken coast; while the western region described above was emerging, with the consequent formation of raised beaches, the eastern region was sinking. The islands, so numerous east of Barcelona and which pass gradually into the mainland, reveal themselves as the tops of submerged hills; the coast festooned with gulfs and bays, the estuarine river-mouths, valleys invaded by the sea (for instance, the Gulf of Santa Fé, the bay of Tigrillo, the great gulf of Cariaco); the hanging riverterrace of Puerto Píritu and the Colorado hill in Cumaná, all tell the same story. However, in the Holocene, the same as in the western region, a raised beach at 3-6 meters above sea-level was formed, with its sandy nets carrying shells of marine mollusks, and the bars which close the lagoons of Tacarigua, Unare, Puerto Píritu and those to the west of Carúpano.

All along the Venezuelan coast, especially in the low-lying regions but even in the steep ones, there develop the dunes, which in general correspond to the Recent raised beach but in some cases may have begun even in the Pleistocene.

Very frequent along the coast, especially in the Recent beach, are the shell-mounds of human origin or "kitchen-middens" (known as sambaquis in Brazil, parederos in Uruguay and Argentina), in which, along with the shells of marine and sometimes of fresh-water mollusks (Ampullaria, Planorbis, etc.), there are found remains of vertebrates, fragments of charcoal, earthenware shards, stone axe-heads and other utensils, made not only of stone but also of heavy shells, both Recent and fossil species. Occasionally, these deposits are cemented by iron oxides and by calcareous incrustations, forming veritable breccias, as in the case at El Eneal, Boca de Arca (as Prof. Cruxent informed us and we were able to observe on the site). As typical examples of shell-mounds, in addition to the one mentioned, we may cite the one at Chipes, Sabaneta, Falcón; those of the dunes between Coro and Paraguaná, that at Aragüita, near that of El Eneal, and those between Barcelona and Píritu. It is interesting to observe that the Chipes shell-mound is today some 10 kilometers distant from the coast, and that near it there is another mound which contains only fresh-water and land mollusk shells, presumably more recent and formed when the coast had already withdrawn and it was no longer possible to collect seafood at the locality.

In various islands of the Caribbean Sea (Margarita and others), reference has been made to coralline formations and beaches raised above sea-level.

Glaciation.-At present, perpetual snows are limited to a small number of peaks in the Mérida Andes (the peaks of Bolivar, La Concha, Bonpland and Humboldt). These snows, as elsewhere in the world, are gradually retreating, and have disappeared very recently, within the present century, from peaks such as that of El Toro. The present snow-fields, with occasional small glacial tongues, are remnants of the neves of the Quaternary glaciers. The Pleistocene glaciations have left U-shaped valleys, roches moutonnées, lakes and all the characteristic features of glaciated topography.

We have been able to verify the presence of the remains of two Pleistocene glaciations, of which the second very complex, may be divided into two principal phases. These glaciations correspond exactly to those which existed in Colombia and in the rest of the Andes.

The first and older glaciation, was the most extensive, the glaciers having descended to 2.700-3.000 meters altitude, the ice having covered all the major elevations between Barbacoas (in Lara) as far as a little to the south of the parallel of Pregonero in Táchira. In the valleys of the rivers Motatán, Chama and Santo Domingo, the terminal moraines, as we have been able to observe, are found below Chachopo and Mucuchíes, and above Santo Domingo, respectively. The moraines have been eroded and at times masked by the fluvio-glacial terrace of the second glaciation; there are remnants of the frontal or terminal moraines (cut through by the present rivers), of the lateral ones and of the ground-moraine, as well as accumulations of glacial erratics.

The moraines of the second glaciation are better preserved, at times with a remarkable and impressive perfection, as occurs in the valley of Santo Domingo, especially the moraine which has been utilized for the dam of La Victoria. The lowest or first terminal moraines reach approximately 3,300 meters altitude, and the higher ones, about 3,650 meters. This is the glaciation recognized by Jahn (1925), who was the first to identify it. In the Motatán, it is well represented in the main valley, from La Venta on, and in the tributary valleys above Chachopo. In the Chama, in the prihcipal valley, with the terminal moraines of the Casa del Gobierno and of the hamlet of Apartaderos, and in the tributary valleys, especially in the Mifafi valley and in the Yaques, whose terminal moraines are in the same confluence. In the Santo Domingo valley, there is the moraine of La Victoria which descends from the lake of Patos and that of the Mucubají lake in the principal valley, and in the numerous effluents which descend from the massif of Santo Domingo (such as that of Zerpas) and of El Aguila; here, there are some hanging glaciers of Pyrenean type.

In the pass of La Negra we have able to show its existence; here the evidence of the first glaciation is masked by the steepness of the slope, so precipitous that the glacial deposits could not find a foothold; here the hanging glaciers predominate.

This second glaciation, the same as in Colombia and the rest of the Andes, presents many indications of retreats and advances, so pronounced at times, that they might even be considered as different glacial periods, with at least two principal phases distinguishable; this is observable in the region of Apartaderos, where at first the glacier of Santo Domingo descended also into the Chama valley, forming the conspicuous moraine which reaches as far as La Pueblita (the first phase); on the left moraine of the Mucubaji Lake, one observes the typical formation of different steps or levels left by the retreating glacier, exactly like those seen in Colombia in the glaciers of Chisacá and of Boca Grande, south of Bogotá (second phase). The first phase corresponds to the glacial moraines which extend down to about 3,300 meters altitude (La Pueblita and the La Victoria dam), and the second, to the moraines at about 3.650 meters elevation (Casa del Gobierno, Mifafi, Yaques, Lake of Mucubaji, etc.).

The importance of these glaciations is indicated also by the great number of lakes which they originated. Examining the map of the Central Andes, by Alfonso Vinci (1953), one may distinguish on the northern or Lake Maracaibo flank of the Serranía de la Culata, more than 110 lakes; on the flanks toward the Chama and Motatán or the same range, 194 lakes; on the northern flank of the Bolivar-Cucuñaque range, 95 (85 corresponding to the Chama, 10 to the Santo Domingo valley); and in the south flank, 39 (8 in the Chama valley, 31 in that of the Orinoco), making a grand total of 328 lakes. The distribution of these gives reason to suppose that glaciation was more intense on the northern than on the southern slopes, no doubt due to climatic conditions, especially cloudiness and insolation, similar relations being observable also in the present snow-fields.

At present, is difficult to determine precisely the age and correlation of these glaciations with the classical stages of Europe and North America. If we consider them as representing a single ice-age with various recessions, as does Caldenius in Argentina, this might correspond to the European Würmian or the North American Wisconsin, within which there have been recognized four phases of glacial activity. If on the other hand, we consider the evidence as indicating two glacial epochs (as generally interpreted in the Andes), these might correspond with the Russian and Würmian in Europe, or the Illinoisian and Wisconsinian of North America.

Concerning the question of the presence of prehistoric man in relation to the glaciation, we have no evidence, but if we may judge by what we have observed in Colombia, especially in the Bogotá Sabana, we would consider that man did not reach those regions until after the last glaciation.

Although we have no observations in the other mountain ranges of Venezuela, we believe that almost certainly in the Cordillera of Perijá there must be evidence of the first giaciation. In the Cordillera de la Costa, the altitudes are lower, but nevertheless, in the collado of Las Piedras (boundary between the states of Sucre and Monagas) at the foot of the Turumiquare peak (2.596 m.), there are huge rounded blocks of sandstone and accumulations of earth that at first sight give the impression of being moraine deposits.

Fluvial deposits.-These are extensively developed, both in the mountain regions and in the plains, and their areal extension is much greater than usually indicated on geologic maps. One may distinguish the following principal types; alluvial fans, piedmont deposits, terraces at stream confluences, river terraces properly speaking, and fluvio-glacial deposits. In general, these beds consist of gravels, sands, sandy clays, clays and silts, of variable composition and color, which frequently become reddish-brown by laterization; their compactness varies also from loose and friable to very coherent, especially when they become true laterites, as occurs with the ferruginous concentrates called in Spanish "moco de hierro" or "canga".

In the plains, especially in the western Llanos; the river deposits form wide alluvial fans in which the rivers continually change their courses, producing an intricate network of variable and shifting channels; in dry seasons the wind also plays a part, forming dunes where the vegetation is sparse. In the region of Colonia Turén, the soil is formed by fine black alluvium, some five meters thick, below lateritic earth. In the low plains, the lateritie, more or less sandy, conglomerates, frequently stand out in the form of "reefs" (arrecifes), as they are popularly called.

The alluvial fans reach a great development in the Andes, where they may be over 100 meters thick. One of the largest which has given rise to a large terrace on the right of the Motatán river, is that of Carvajal-Las Mesetas opposite the city of Valera. The towns of Boconó, Ejido and Mucurubá are also located on similar fans.

The piedmont plateaux, resulting principally from the union of alluvial fans, forms a great topographic "step" at the foot of all the mountain ranges, so that at the same time they form an upper step-like border to the neighbouring plains, such as the Llanos, Sabanas de Monay, the Carora plain and the depression of Lake Maracaibo, and even in interior valleys such as that of the Yaracuy at the foot of the Sierra de San Felipe and that of the Caracas valley at the foot of the Avila. Part of the "mesas" of Monagas and of Anzoátegui, especially those near the Serranía del Interior, correspond to this class of deposits.

River terraces ocour in almost all the valleys, both in the mountains and in the plains. Only in the valleys of the northern flanks of the ranges of Anzoátegui and Sucre, such as the valleys of the Naricual and Neverí near Barcelona, and in that of the Manzanares near Cumaná, we have not observed any; and if terraces do appear in this northern slope, as at Puerto Píritu, Cerro Colorado near Cumaná and along the coast from Cumaná to Cariaco (as red gravels), they appear at the seashore, indicating that this was distant when they were formed. It is in the Andes that river terraces are most developed, for instance in areas as typical as those of Valera, La Grita and San Cristobal with five different levels, counting from the present level up to the oldest which is more than 100 meters above the actual stream course. The levels most frequently developed over all the country are two; one at 16-30 meters, and one of about 6 meters above the modern river courses. In the Andes such terraces are generally called "mesas", and the towns are frequently built on them (Valera, La Mesa Timotes, Mucuchíes, Santo Domingo, Mérida, Lagunillas, La Grita, San Cristobal, Santo Domingo del Táchira, Michelena, Colón, Seboruco, Pregonero, Barinitas, etc.). Typical terraces built at river confluences, are those of Mérida, La Mesera and Palmira. The terraces of Mucuchíes and of Santo Domingo correspond to the second and the first glaciation, respectively.

Fluvio-glacial deposits, which may also form terraces are frequent in the Andes, for instance those of the Motatán river downstream from Timotes; those of the valleys of Las Piedras and of Santo Domingo in the Mitisús; those of the headwaters of Río de Oro in the Llanete, and those of Sabana Grande and Llano Grande between La Grita and the pass of La Negra.

In addition to the Mérida Andes, there are also very typical terrace zones such as the valley of the Misoa river east of Mene Grande in the Maracaibo Lake basin, with five levels of terraces; the valley of Caracas with two or three levels in addition to the piedmont level; and several valleys in the Mesas of Monagas and Anzoátegui in which four levels are conspicuous.

All these terraces are Quaternary, principally Pleistocene, and in fact the highest, with its great elevation above the present stream levels, might ba as old as upper Pliocene; but in most cases there is not enough information to correlate them. The terrace at present in process of formation, must correspond to the Holocene (Recent), but one cannot assign the same age with certainty to the terrace of 5-6 meters elevation, because in some sites, as in the river Valle in Caracas, remains of vertebrates have been found which possibly indicate Pleistocene age.

In Nirgua and in some neighbouring drainage areas, one observes alluvial deposits which should be studied to determine whether they are Tertiary or Pleistocene. In Barquisimeto, there are Pleistocene alluvial deposits resting on others which show faulting, these latter might be either Pliocene or Pleistocene.

Delta deposits.-The most important ones are those which are found in the East of Venezuela, in the margins of the gulf of Paria and of the Atlantic, and which can be traced back within the interior of the continent over distances which vary from 50 to 100 kilometers. The central zone, and a large part of the southern, correspond to the Orinoco River; the northwest, to the rivers which descend from the peninsula of Paria, from the Serranía del Interior and from the region of the "mesas" of Monagas and Anzoátegui; and those of the southeast, to the rivers which originate in Guayana, such as the Amacuro and the Parima. In general, these sediments consist of siliceous clays, brownish-yellow to bluish gray in color, occasionally covered by blackish-gray muck. In the central part of the Orinoco delta, some wells have been drilled in search of petroleum, which have penetrated to a few thousand feet, called by Hedberg (1950), Paria formation, but this name lends itself to confusion. The age probably ranges from Pliocene to Holocene, and the sediments are still accumulating, since this is an area of subsidence.

Lacustrine formations.- In Venezuela there exist various types of lakes, from the Andean glacial lakes which are so numerous, to those formed in depressions like the Lake of Valencia, and the coastal lakes or lagoons like that of Tacarigua in the Barlovento region. The great lake of Maracaibo is in reality, a vast bay with a narrow communication (some 10 kilometers long) with the open sea.

The Andean lakes do not appear to present varved sediments, or at least such have not been reported.

The coastal lagoons are of Holocene age, and formerly had an extension much greater than at present; in large part they have been filled by terrigenous and even by marine deposits, which form the greater part of the coastal flats. The swamp regions, such as the vast swamps of the Catatumbo and Santa Ana rivers at the lower end of Lake Maracaibo are likewise Holocene, although it is possible that Pleistocene beds underlie them.

Lake Tacarigua (more generally known as the Lake of Valencia) has suffered great fluctuations in the level and extent of its waters. Little is known of its Pliocene and Pleistocene history; it needs to be investigated whether the Pleistocene terraces of the western plain of Carabobo are related to it. E. W. Berry (1939) believes that the sediments of the lake, ranging from Pliocene to Recent in age, reach a thickness of over 130 meters, with intercalations of diatomaceous earth and beds with Planorbis; he recognizes four terrace levels, in addition to the one at present forming, of which the oldest is situated at an elevation of 14 meters above lake level, the second at about 9.5 meters, the third at about 6.7, and the fourth at 3 meters elevation, approximately. From the archeologic and faunal remains (Canis familiaris, Bos taurus) found in the oldest terrace, we believe that all of them are post-Columbian, in other words that they represent the history of only the last four centuries.

In the Lake of Urao at Lagunillas in the state of Mérida, there are deposits of trona and of gay-lussite.

It is possible that some interior plains, such as those of Carora and the "sabanas" of Monay, are related to lake formations, as likewise the plain of the Guajira and certain regions of the Llanos.

Aeolian formations.- It is very probable that such formations are frequent in the Llanos (González de Juana has mentioned them in the Mesa formation), since dunes are formed in this region during the dry season; also in Guajira, whose flat surface is formed by very fine sediments; and even in the Andes themselves, derived from beds of periglacial and interglacial type.

Sand dunes are frequent along all the Caribbean coast, especially when this is flat or sandy; the areas around Cumaná, Barcelona to Píritu, San Juan de los Cayos, La Vela, Coro, the isthmus of Paraguaná and the Guajira peninsula, present excellent examples of them. Probably these correspond to the Holocene or Recent, but it might happen that some of them date back to the Upper Pleistocene. Frequently one finds shell-mounds of human origin among the dunes, as in the isthmus of Paraguaná and the dunes between Barcelona and Píritu.

Talus (scree) deposits and elluvium (residual deposits).- These are very frequent in all the mountainous regions of Venezuela, but especially in the Mérida Andes and probably in Perijá, as a result of periglacial formations; their study is extremely important for the construction of highways and public works in general, due to tne insecurity which such formations offer as foundations; they are the principal cause of the landslips and slides which interrupt traffic on the Andean highways. The age of these deposits, in many cases, may date back to the Pleistocene, especially the glacial stages.

Products of weathering and associated ones.- The alteration produced by the action of atmospheric agents and of percolating water in Venezuela, is intense, as corresponds to a tropical land. The desintegration of the rocks is frequent, and the products of weathering form soils which cover the rocks and mask the geology. In the Andes this phenomenon began already in the glacial ages, often with the formation of periglacial deposits.

One of the most important products of weathering is that of lateritic earth and laterite itself. The former are common all over the country, and the second, called locally "moco de hierro", is abundant in Guayana, frequently containing gold and diamonds; it is also common in regions in which ferruginous rocks are abundant, such as Tinaquillo.

Caliche, calcareous tufa or travertine formed by evaporation is not rare (Paraguaná, Tocópero, etc.). Sulphur and soluble sulphates of iron and alum, due to alteration of pyrite commonly appear in the regions of Cretaceous and Tertiary shales and graphitic schists.

Caves. - Caves, with their stalactite formations, are relatively frequent in the mountainous regions of calcareous rock (metamorphic and sedimentary), both in the Andes and in the coast. Some of these caves are extensive, complex and of great beauty, such as that of Los Guácharos in Caripe. Except for an occasional rare instance in the Andes, it does not appear that man has ever inhabited them. Their formation has doubtless been greatly facilitated by the long rainy periods of the Pleistocene, and the resulting percolation of waters.

Paleontology. - The studies so far effected on the fauna and flora of the Quaternary do not yet permit us to draw any important conclusions, but they offer a few hints of what a thorough investigation might bring forth. For example, in the few lists of marine fossils, which in general comprise Recent species (mollusks, reef corals, crab claws, etc.), one notes the occurrence of dwarfed forms (e. g. Melongena margaritana Richards, Tellina venezuelensis Richards), and of species which live in colder waters, such as Columbella recurva Sowerby, native to Río de la Plata. These details, which might appear insignificant, could indicate climatic changes which might permit the correlation of the deposits with the glacial stages, and thus help us to fix their age within the Quaternary.

Of the lacustrine organisms, we have no data except those by E. W. Berry on the Lake of Tacarigua (Valencia), in which he indicates principally diatoms and Planorbis, of Pleistocene and Recent forms, including among the latter, remains of the food of the Indians who lived in the region.

As regards the terrestrial vertebrates, about forty different fossil localities are known, some of them already noted by Humboldt; one may mention as an example the extremely rich locality of Carrizal near La Vela de Coro (Falcón), in which we have been able to identify some eight different species; also the regions so abundant in remains, between Barquisimeto and San Miguel in Lara. In most cases, the fossils collected have not been identified, and much less has there been any study of the stratigraphy and correlation of the deposits. The identifications given, such as "Megatherium", "Mylodon", "Glyptodon", "Mastodon" and "Cervus", should not be considered as precise generic determinations but at most, as indicating the family affinities; the specific identifications are generally based on insufficient remains, especially in the case of the extremely variable megatheriids. The fossil localities reported to date, correspond to the states of Sucre, Monagas, Anzoátegui, Distrito Federal (Caracas), Guárico, Trujillo, Lara (with some 20 localities), Falcón and Zulia.

On the basis of a rapid examination of the specimens found in the Liceo Cecilio Acosta in Coro, which were collected at Carrizal near La Vela, and of some specimens which we collected in the same locality, we may cite from that bed: a gigantic turtle (Podocnemis?), a mastodon (Stegomastodon cf. platense (Amegh.), a horse (Equus (Amerhippus) andium Branco?), a megatheriid (Eremotherium rusconii (Schaub), a mylodontid (Glossotherium cf. tropicorum (Hoffst) a glyptodontid (Propraopus cf. magnus (Wolf)), a toxodontid (Toxodon platensis Owen?), a macrauchenid (Macraucheniepsis sp.). It is very possible that the species reported to date from other localities may belong to these same species, but it would be necessary to verify this by a comparison of the specimens. Without more detail, the following forms have been reported; felids, Cervus? (a genus which did not reach South America) and a rodent (Amblyrhiza sp.).

As regards the age of these fossils, within the Pleistocene, little may be said. The find at Carrizal appears to belong to the Upper Pleistocene, and others which we have visited may be contemporaneous with it, or in some cases older; but until more detailed and serious studies are made, we cannot be sure of such points.

Movements and paleogeography.- Folds and faults are not frequent in the Pleistocene. Bucher (1952) notes indications of an anticline in the terrace alluviums along the Andean front in the Llanos, about 46 km. SW of Barinas and NW of Puerto Arturo in the Colombian frontier; he also indicates a fault in the moraine south of Mucuchíes. We have been able to observe in the trench of Santa Rosa in Barquisimeto, in the old highway to Caracas, some alluvial deposits, faulted within themselves and at the contact with the metamorphics, these alluvions being in turn covered by similar deposits posterior to the faulting.

The most frequent movements have been those of massive uplift or epirogenetic, possibly of isostatic origin, as the raised beaches and river terraces indicate. The coast line has varied, and whereas at the beginning it must have been quite sinuous, it has become gradually more regular, as the continent gained ground at the expense of the sea. As we have mentioned above, in the coasts west of Cabo Codera, the movements of emergence appear to have been continuous during the Quaternary, whereas in the eastern coasts there must have been submergence during the Pleistocene and emergence in the Holocene when the movement follows the same rhythm as in the west.

In the West we have, for instance, the case of the peninsula of Paraguaná, which probably was an island until the lower Pleistocene, when there began to form the tombolo which gradually joined it to the mainland; finally in the Holocene there emerged on the island the rocks which form the base of the dunes on which lived the pre-Colombian aborigines who have left there the remains of their food-gathering and of their handicrafts.

In the east, on the contrary, one receives the impression that there have been since the Tertiary, a series of periodic sinkings which brought about the break-up of the Cordillera de la Costa; perhaps the first to sink was the Serranía del Litoral, later that of the Interior, leaving as remains the mountainous zone of Píritu and forming the islands which border the coast, which appear exactly like the peaks of drowned ranges. One of these sinkings must have been later than the deposition of the alluvions of Puerto Píritu and Cumaná (Cerro Colorado, etc.); the sea must have invaded the low parts of the valleys. Finally, a recent uplift, common to all northern South America, which possibly is still continuing, formed the lagoons (already degenerated into swamps and coastal flats), with their elevated barrier beaches and the chains of sand-dunes.

Some time after the beginning of the Pleistocene, a coastal river, perhaps taking advantage of some fracture or fault, west of the present location of Puerto Píritu, began to capture the northern drainage of the Orinoco river, and with the normal headward erosion, penetrated rapidly into that territory, giving rise to the so-called depression of Unare, separated from the coast by the low mountainous zone of Guanape-Píritu-Caigua, a remnant of the submerged Serranía del Interior.

The delta of the Orinoco river, with its great thickness of deltaic sediments, is considered a zone of subsidence.. In the eastern shore of Lake Maracaibo, especially in Lagunillas, one finds partial sinkings which break the dikes and which have been attributed to the removal of petroleum.

Economic importance of the Pleistocene. - In addition to the wellknown importance of the Quaternary formations in agriculture, in the localization of towns, especially in the mountain regions, and in the clays, tile and even the glass industries, the Pleistocene has in Venezuela great importance in the field of mining.

Diamonds. - The alluvions of the Gran Sabana, of the lower Caroní, of the upper part of the basin of the Cuyuní and of the Camarán river (a tributary of the Mazaruni), are rich in diamonds, being especially exploited the Holocene alluvions and those of the lowest terraces. It is believed that there may be deposits in the region of Caura-Cuchivero (State of Bolívar) and in the upper Orinoco basin. In general, it may be stated that all alluvions of the rivers related to the basal conglomerate of the Roraima formation, are potentially diamond-bearing.

Gold. - Exploited in all the ancient and modern alluvions of Guayana. The auriferous area coincides in part with the geographic distribution of diamonds, but is more extensive.

Iron. - Laterite, particularly in the form known as "moco de hierro" or "canga", and pisolitic laterite, contains a large proportion of goethite and limonite, which in some cases might be a good source of iron-bearing regions.

Manganese. - These same laterites and the weathered zones in the region of Upata (Cerros Hacha, Jiménez, Guaruripia and Paja) also contain manganese oxides whose possible exploitation is being studied.

Bauxite. - Although we cannot state that all the deposits are Pleistocene, for lack of data, there exist deposits of bauxite and bauxitic earth in the hill of El Chorro, near Upata, in the State of Bolívar.

Trona (urao) and gaylussite. - These have been exploited in the lake of Urao, Lagunillas, in the State of Mérida, where they have been used since precolonial times in the preparation of tobacco called "chimó".

Guano and phosphatic formations. - These have been exploited in the islands of El Gran Roque, Los Monjes, Los Testigos and Aves.

Asphalt. -There are various asphalt lakes of importance, among these: in the east, that of Guanoco which is the largest in the world (ten times larger than that of La Brea in Trinidad) and those of Pedernales and Guanipa; in the west, that of Guasare to the west of Maracaibo.

The Quaternary formations, especially the alluvium and the moraine deposits, deserve special study because of the insecurity and permeability which characterize them, factors which render difficult the construction and conservation of important public works built on them, such as highways, dams, buildings, etc. The Andean regions are a good example of this problem.

José Royo y Gómez