Morphogenesis of the Ourense plains (NW of Spain)

Cadernos Lab. Xeolóxico de Laxe Coruña. 2003. Vol. 28, pp. 263-283 ISSN: 0213-4497 Morphogenesis of the Ourense Plains (NW of Spain) Morfogénesis de las superficies de Orense (NW de España) YEPES TEMIÑO J.1&. VIDAL ROMANÍ J. R.1 ABSTRACT We present a geomorphological analysis of Ourense Province (NW Spain) characterized by: a general narrowing of the fluvial network, highlands with smooth reliefs partially eroded and lowlands with residual reliefs, several extensive plains of erosion frequently limited by fractures -among which Tertiary grabens are inserted-, some “Hollow Surface”-type morphology, absence of sedimentary deposits outside the grabens, and a generalized outcrop of the Hercynian Massif substratum. Traditionally, this “piano’s keyboard morphology” has been interpreted as expression of block tectonics in tensile regimen; instead we suggest the existence of: an isostatic upheaval simultaneous to a sequence of tectonic pulses of compressive regimen with activity in favour of transcurrent faults, a General Surface (R600), several plains that present a “Hollow Surface”-type morphology (R1600 R1400 R1000), a generalized alteration that correspond to a same process of decomposition associated to fluctuating conditions of redox equilibrium, a erosional terraces related principaly to the palaeo-fluvial nets; moreover, we propose the existence of two morphoestructural lineament: the first one represented by the Fault of Vila Real (NE-SW) -a ramification of the “Basal Pyrenean Overthrust”-, that would have been active at an early moment of the tectonic sequence with a left transcurrent fault, secondly the lineament represented by the Fault of Maceda (NNW-SSE) that would be related to the “Fault System NW-SE” and would have produced a right transcurrent fault during a late tectonic pulse. Key words: regional geomorphology, planation surfaces, Hesperian Massif, Galicia NW Spain, Plate tectonics. (1) Instituto de Geología Isidro Parga Pondal, Universidade da Coruña, 15.071-La Coruña. E-mail: [email protected] INTRODUCTION The mapping area (figure 1) extends between the provinces of Lugo and Ourense (C.N.I.G. 1994; 1997). Geologically, the area belongs to Galicia-Tras-Os-Montes (BARRERA et al. 1989). Only in the NE part, the Asturoccidental-Leonesa area is represented. The main morphological characteristics of the territory are: 1) the deep narrowing of the rivers; 2) the abundance of little contrasted reliefs in the highlands and of residual ones in the lowlands; 3) the regional fractures separate extensive plains of erosion which limit small grabens located between 250-650 m of altitude; 4) outside the grabens the sedimentary deposits are scarce. 5) Geologically, it is an old territory (igneous outcrop, regolith, recent deposits in grabens); but its relief is rejuvenated (widespread incision, dismantlement of the plains, compartmentalization of the territory. On the whole, the territory is a mosaic of block mountains with inserted tectonic basins. In a traditional way, the previous studies (HERNÁNDEZ-PACHECO 1949; NONN 1966; VILLASANTE & PEDRAZA 1984; MARTIN SERRANO 1989; VERGNOLLE 1990; PÉREZ-ALBERTI 1993) accept the piano’s keyboard morphology that has been described (BIROT & SOLÉ 1954), interpreted as the expression of block tectonics in tensile regimen. In this paper, we present some observations which suggest the existence of block tectonics in compressive regimen, with displacements in favour of transcurrent faults. In view of the shortage of deposits; the reconstruction of the morphogeny has not been based on stratigraphic approaches, but on the analysis of: the topographical plains, the fluvial network, the fracture, and the alterations, as it has already been carried out in other areas of Galicia (VIDAL ROMANÍ 1996). THE PLAINS OF EROSION Eight plains of erosion have been distinguished (figure 1 and table 1), most of which have got associated to them intermediate surfaces of these types: degraded surface, glacis, or ramp. Some of them define gradual and uniform transitions between plains of erosion. The plains of erosion are topographically defined by two topographical heights: the highest one, denoted by the remains of a previous surface, and the lowest one, denoted by the joined degraded areas. The analysis of the spatial extension of the plains (table 2) reveals: a spatial distribution of different area; the main character of the surfaces R600 (24.7%) and R1000 (10.9%); the respectable extension of the grabens (11.9%); and an advanced state of degradation (slopes and valleys, 40.7%). The Surface of Serra de Queixa (R1600) This plain is well-preserved in the northern border of the Serra de Queixa-San Mamede (Cabeza de Manzaneda, 1781 m); where it is defined by a net steep (NWSE), at the N of which only some very degraded remains of the R1600 are identified. On the other hand, in the southern border of the mountain range, the plain is very degraded (Altos de Ganzedo, 1330 m). The western border is marked by a NE-SW structural alignment which has 264 Yepes Temiño & Vidal Romaní CAD. LAB. XEOL. LAXE 28 (2003) CAD. LAB. XEOL. LAXE 28 (2003) Morphogenesis of the Ourense Plains 265 Figure 1. Schematic topography of the SE of Galicia and N of Portugal. Legend: (1) 0-400 m; (2) 400800 m; (3) 800-1200 m; (4) 1200-1600 m; (5) 1600-2000 m; (6) 2000-2400 m; (7) River; (8) Town; (Al) Allariz; (B) O Barco; (Ba) Bande; (Br) Braga; (C) Carballiño; (Ce) Celanova; (Ch) Chantada; (Co) Cualedro; (Cv) Chaves; (G) La Gudiña; (L) Lobios; (Lu) Luintra; (M) Maceda; (Me) Melgaço; (ML) Monforte de Lemos; (Mo) Monçao; (Ou) Ourense; (Po) Ponte da Barca; (R) La Rúa; (Ri) Ribdavia; (P) Ponferrada; (PB) Puebla de Brollón; (PS) Puebla de Sanabria; (Q) Quiroga; (VB) Viana do Bolo; (Ve) Verín; (Vf) Villafranca del Bierzo; (XL) Xinzo da Limia. 266 Yepes Temiño & Vidal Romaní CAD. LAB. XEOL. LAXE 28 (2003) Table 1. Record of plains identified in the studied area. The rank of heights in which they are developed is included, as well as the calculated extension, both for the surfaces and for the grabens and slopes. The calculation of the extension was made from figure 2. Table 2. Erosion surfaces associated to the main basins of the Galician SE, with indication of the maximum and minimum heights marked by the sedimentary deposits. Legend: (Ri) Surface-Plain; (H Max) Highest altitude; (H min) Lowest altitude. Figure 2 (next page). Geomorphological map from the SE Galicia where eight plains of erosion have been identified. Legend: (1) Steep of topographical plain. (2) erosional terrace; Residual remmant. (3) Residual relief: Hill of circumdenudation, Castle-kopje. (4) Degradated crest; structural relief on fold axis. (5) Fluvial steep; landslide. (6) Degradated glacis: depositional, erosive. (7) River; sedimentary deposit. (8) Topographical plain. (9) Town. (ML) Monforte de Lemos. (Ou) Ourense. (XL) Xinzo da Limia. CAD. LAB. XEOL. LAXE 28 (2003) Morphogenesis of the Ourense Plains 267 been used by the river Návea to get narrowed and to dismantle the plain. And the eastern border would be characterized by a sequence of reliefs, progressively degraded towards the SE, strongly impacted by the fluvial network (Montes do Invernadoiro, 1550 m; Brotiais, 1532 m). These reliefs are terraced, until connecting with the surface of La Gudiña-Viana do Bolo (R1000). On the whole, it can be supposed that the contour of the plain of erosion would be defined by two systems of fractures: NE-SW, the main one; and NWSE, subordinate to the first one. Both systems would have dislocated the plain in some moment of the Mesozoic or of the Cenozoic. This hypothesis would be supported by the correlation which is showed by the systems of fractures with the long axis of the Surface of Xinzo and with the steeps of the Serras do Burgo and Paderne-Taboadela. As for the morphology of the plain, it seems to be related to the lithology. On granite, soft and rounded reliefs are defined. They mark culminating heights (Cabeza de Manzaneda, 1781 m, the Majadales, 1750 m) and they would be the residual of a previous surface, while, on quartzite and slate the relief is more contrasted and the heights are lower (Pereixada, 1501 m). Regarding the morphogeny, the general development of a regolith and the numerous residuals (castle-kopjes on granite and conical hills on quartzite and slate) allow to attribute to this surface an origin by chemical erosion. The preservation of the vasques (gnammas), in the castle-kopjes of the Alto de San Mamede and of the original regolith would indicate that, at this point of the massif, the glaciarism would not have been developed during the Quaternary. This fact was already observed in other granite areas of Galicia (VIDAL ROMANÍ et al. 1994). Nowadays, the lingering erosion of the river Návea would have dismantled the initial surface until the degree of emptying it, reducing it to the initial contour. So only a residual would have been preserved in the central area (Altos do Acebral, 1606 m). It is what we have called a hollow surface morphology The Surface of Chaguazoso (R1400) This plain can be interpreted as a lower step of the R1600. It is identified both in the area of Queixa (Llanos de Chaguazoso; Portela das Merendas, 1400 m; Serra do Fial das Corzas, 1400 m; y Altos do Gancedo, 1300 m) and in San Mamede (As Donas, 1279 m; Lombo dos Gavianes, 1360 m; y O Marco; 1400 m). Towards the E, it could be correlated with the western slope of the Serra do Eixe (Llanos de Lamalonga, 1445 m; y Serra do Cañizo, 1469 m). Towards the S, with the surface of summits of the Serra de GerezXurés (1556 m). And inside the narrowing of the rivers Návea and Camba, it would be correlated with extensive surface-terrace remnants that would denote an old fluvial network, of a radial geometry to the Serra de Queixa. Concerning the morphogeny, the partial preservation of the original regolith, would allow assign to the plain an origin by chemical erosion. On the contrary, the dismantlement of the regolith, would be related with a Pleistocene glaciarism, favoured by the preserved position of the 268 Yepes Temiño & Vidal Romaní CAD. LAB. XEOL. LAXE 28 (2003) plain regarding the dominant winds in two areas: Chaguazoso (HERNÁNDEZPACHECO 1958; PÉREZ ALBERTI 1993; VIDAL ROMANÍ 1994), and Dam of San Agustín del river Bibei (SÁENZ RIDRUEJO 1968). Regarding the hollow out of the plain in the area of Chaguazoso, it can be considered a hollow surface, like in the R1600, although in this case the eroding down would have been by the glaciar erosion. The Surface of the Serra do Burgo (R1200) In the studied area, the R1200 is represented by the level of summits of the Serra do Burgo, to the N of the Serra de QueixaSan Mamede. Its western border is defined by a NE-SW morphoestructural steep, and the oriental, by the narrowing of the river Návea. On this plain paleo-valleys of plain bottom are observed; to those ones, remnants of erosive terraces are embedded, partially recovered by an alteration layer. The direction of the paleo-valleys would be SW-NE, concordant with that of the river Návea. This plain of erosion could be correlated with the surfaces of the Serras de Gerêz-Xurés, Pisco, Pena y Larouco. In almost all the cases it defines narrow blocks, extended in NE-SW direction, among which wide valleys are inserted (rivers Salas, Cábado, Rábago and Porto de Rei), comparable to the plain R800. In the Serra de Gerez-Xurés, the situation of the plain regarding the prevailing winds, allowed the glaciarism development during part of the Quaternary (SCHMIDTTOME 1978; VIDAL ROMANÍ et al., 1990; BRUM et al., 1992). The Surface of Castro Caldelas (R1000) This plain is well-represented in the studied area, being embedded to the Serra de Queixa-San Mamede and O Courel. The Llanos de Puebla de Trives and Llanos de Castro Caldelas are in the N area of Queixa-San Mamede; the S border of Plains of Castro Caldelas is denoted by the Serra do Burgo; the W by the Fault of Maceda; and the N and E, by the narrowing of the river Sil. On these plains, remains of terraces of a SW-NE paleo-network are recognized, nowadays captured by the river Sil. The fluvial erosion has begun to dismantle the plain, conferring it the aspect of a hollow surface similar to the R1600 and the R1400. The Llanos de Puebla de Trives constitute a remnant of the R1000, a bit more degraded, which is extended in the N slope of Cabeza de Manzaneda, at the E of the river Návea. The Llanos de La Gudiña-Viana do Bolo are in the E area of Queixa and they define a corridor of meridian direction between La Gudiña and La Rua, enough degraded towards the N; and flanked by Serras de Queixa y Eixe-Secundeira. This plain of erosion would denote an old base level of the rivers paleo-Camba and paleo-Bibei. This same level continues in Serras de Entirnos, Texeiras y Llanos de Carracedo da Serra. In the W area of San Mamede, remnants of the R1000 have been mapped, which are very degraded and broken into fragments by the fluvial incision. In the Graben of Maceda they would constitute the southern continuation of the steep of the Fault of Maceda; but more to the S, near Correchouso, Toro and Portocamba, they would correspond to surface-terraces CAD. LAB. XEOL. LAXE 28 (2003) Morphogenesis of the Ourense Plains 269 and paleo-courses of an old NW-SE network that later on would have been captured by a S-N network. In the Serra do Courel, the plain R1000 defines the eastern end of the graben of Monforte and the general level of summits of the Serras de Peña Redonda, Trapa y Llanos de Maceiras. At a regional range, the R1000 would be correlated, because of its altitude, with the Serras de Oribio, in Sarria; Faro, in Chantada; and Faro de Avión, in La Cañiza. The morphogeny of the R1000 is by fluvial erosion. There can be still distinguished on it several incision levels, associate deposits (BARRERA et al. 1989; VERGNOLLE 1990), and residuals of older surfaces: Monte Meda (1321 m), Monte Cerengo (1235 m), Monte Legua (1311 m) and Cabeza Grande (1249 m). Nevertheless, some of their borders correspond to morphoestructural steeps: Fault of Maceda, NNW-SSE; river Sil, WSW-ENE; and Serra da Pena Redonda, NW-SE. The Surface of Baldriz (R800) This surface is represented in Esgos (Altos da Virxe do Monte) and in the graben of Xinzo (by the borders of the graben, except the northern one, and the Serra da Medorra). More to the S of the studied area, it would be correlated with the extensive interfluve of the areas of: BaltarCualedro, Chaves-Bragança and Alcañices de Zamora (MARTIN SERRANO 1989). The morphogeny of the R800 is by fluvial erosion. Nowadays, there can be still distinguished on it, some stretches of a paleo-network NW-SE (Baldriz area) and several residuals: Monte Meda (1094 m), Monte Talariño (984 m) and Monte Penamá (927 m). Signs of fluvial touch have also been identified in the small remnants, comparable to the R800. This is the case of: 1) the tailpieces of Penelo (Quiroga) and Serra de Pena Redonda (Monforte); 2) the erosive terraces of the Sil (Parada y Teixeira), Lor, Támega, Riveiriña and Parada; and 3) the glacisterrace type ramps of the Sil (Castro Caldelas). Nevertheless, as it happened in the R1000, the borders of the R800 coincide with structural lines. The Surface of Xinzo da Limia (R600) This plain is identified along the river Miño, and in the borders of the grabens of Monforte and Maceda. Comparable surface-terraces are also observed, in the river Sil: La Rasa (560 m), confluence of the Cabe; and Peña Grande de Quiroga (613 m). Outside the studied area, the R600 would be correlated with the basin of Rábade and Serras da Loba and Montouto (Dorsal Galaica or Galician Ridge). The river Miño defines a level of regional devastation among Rábade (Lugo) and Ribadavia (Ourense). In the stretch Rábade-Portomarín, it extends among the Dorsal Galaica and Serras del Mirador and Punazo; in total, a strip of 40 km parallel to the river Miño. In the stretch Portomarín-Os Peares, the Graben of Monforte defines the E border of the plain, diminishing the extension of the strip to 20 km. In this stretch, the course of the river Miño is N-S and its narrowing reaches 400 m. In the stretch Os PearesMonção, the Miño has dismantled great 270 Yepes Temiño & Vidal Romaní CAD. LAB. XEOL. LAXE 28 (2003) part of the R600 to substitute it for a R400. In the W riverbank only some remnants are identified in the head of the river Avia and in La Cañiza; and in the E riverbank, between the lineament Taboadela-Esgos and the Graben of Maceda. In the Graben of Monforte, two remnants of the R600 are observed in the southern border. Residuals of the R800 have been preserved on them. They coincide with axes of hercynian folds, of NW-SE direction (Barrera et al., 1989). They are the Serras do MarroxoMonte Vidual (680 m, 816 m) and the Serra de Auga Levada (887 m). These reliefs can be followed towards the S (Serra de Moncai, 531 m; and Serra das Penas, 787 m). The similarity of these residuals with those of the Sierra de la Culebra permits to compare them to the remains of the surface of phinimiocene erosion thet has been described (MARTIN SERRANO 1991), which would have been degraded, according to this author, before the Tertiary. Concerning the residuals identified on granite areas, they have been related to the phinimesozoic surface (MARTÍN SERRANO 1991): Monte Pozu (764 m), in La Peroxa; Monte Cabalo (732 m), in Luintra; Monte Boa (692 m), in Taboadela; and Alto del Portelo (834 m), in Xunqueira de Ambía. As to its morphogeny, it can be supposed prior to the beginning of the sedimentation in the basins which fossilize it (Xinzo, Monforte and Maceda). The concave profiles that these basins present would reinforce this hypothesis. In the case of Monforte, the scarce deformation of the deposits (DE GROOT 1974), suggests the existence of a paleo-relief in the basin. In the case of Xinzo, the slope of the R600 observed at the W of the graben is attributed to the fluvial erosion. This is suggested by the remains of surface-terraces of the river Arnoia found in Allariz and Xunqueira de Ambía. However, to a regional level the distribution of the R600 would be partially conditioned by structural factors (figure 3). It makes think so, the existence of steeps in some borders of the plain: 1) in the W border, the steep (N-S) with the Macizo Galaico Portugués; 2) in the E border, the steeps of the Fault of Maceda (NNW-SSE) with the Llanos de Castro Caldelas and the Serra de San Mamede; 3) At the W of Ourense, between Allariz and Esgos, the steep (NE-SW) with the R400; and 4) In Xinzo, the steep (NE-SW) that defines the W border of the graben, which is associated to a diabasis dike (BARRERA et al. 1989) fossilized by the basin deposits. The Surface of Chantada (R500) The remnants of this plain have a small extension. Their distribution is associated to the R600 in three points: Maceda, Chantada and Monforte (Puebla de Brollón, Sober and Ferreira de Pantón), and to the narrowing of six rivers: Arnoia (Xunqueira de Ambia, Allariz); Cabe (Lornís); Lor (confluence of the Sil); Miño (Monte Güimil); Támega (Laza); and Sil (La Teixeira). In general, the characteristics of the plain are related with the structure. In Maceda, the E border of the graben is an important steep of fault (30 km of longitude, 400 m of difference and NNW-SSE direction), to which quartz dikes are associated. In Chantada, the N border is a steep (NE-SW), in favour of CAD. LAB. XEOL. LAXE 28 (2003) Morphogenesis of the Ourense Plains 271 278 Yepes Temiño & Vidal Romaní CAD. LAB. XEOL. LAXE 28 (2003) Figure 4. Hypothesis about the possible work of blocks in the SE of Galicia during the Alpine Orogeny. The grabens-tectonic activity relationship is not evident, in view of the advanced degradation of the morphostructural steeps; however, two significant structural lineaments are distinguished: the first one (NE-SW) would be associated to the “Basal Pyrenean Overthrust” and would have worked as a transcurrent fault with left movement in a first phase (Phase I), defining a Pull-Apart basin in Xinzo; the second lineament (NW-SE) would be related to the “Fault System NWSE” and would have worked in a second phase (Phase II) as a transcurrent fault with right movement, defining the E border from the Graben of Maceda. The old age of the tectonic activity would come supported by the absence of significant slope deposits and the levelling down of the steep of the Fault of Maceda by the plains R800 y R600; the preceding of the NE-SW fracture in relation to the NW-SE direction comes pointed by the fact that the N border of the Graben of Xinzo is cut short by the Graben of Maceda. Legend: (look at figure 2). CAD. LAB. XEOL. LAXE 28 (2003) Morphogenesis of the Ourense Plains 279 Figure 5. Tectonic scheme from the Fault of Maceda in the Graben of Verín; simplified from the Map nº 3 in (ROEL & TOYOS 1993). This fault, of NNW-SSE to NW-SE direction, would be related to the “Fault System NW-SE” which is observed round La Coruña and would have worked as a transcurrent fault with right movement defining the E border from the Graben of Maceda during the Alpine Orogeny; the supposed displacement in favour of this structural lineament would come supported by the observation of a combined fault network (NW-SE / NE-SW), associated to hydrothermal expressions (SOUTO 1996), which would have affected with right transcurrent fault (ROEL & TOYOS 1993) the structures of the Hercynian Phase III. Legend: (1) Stratigraphic unit. (2) Anticlinal of Fase I. (3) Synclinal of Fase I. (4) Antiform of Fase III. (5) Synform of Fase III. (6) Fault. (7) Direction of fault displacement. (8) The Rodicio’s Cizalla Ductile Zone. Cabalo and Llanos de Monteverde), and in Esgos by the R800 (Serra da Virxen do Monte and Llanos de Cortacadela). All this would indicate the precedence of the steep of Maceda, in relation to the surfaces R800 and R600. CONCLUSIONS a) Eight different plains have been recognized between the heights 1800 m and 100 m. Its distribution has neither equivalent areas, nor proportion to the height. The most extensive plain would be the R600, a General Surface in the sense used by Martin Serrano (1989). Two types of surfaces have been distinguished for its origin: those of fluvial erosion (R1400, R1200, R1000 and R600) and the etching or of chemical erosion (R1600, R800 and R400). Two types of surfaces have been distinguished for their morphology: the Ramp Surfaces (Llanos de Puebla de Trives and the R400); and the Hollow Surfaces (Llanos de Chaguazoso, Llanos de Castro Caldelas and the Serra de Queixa-S. Mamede). These morphologies would express different stages in the dismantlement of the plains of erosion. The Ramp Surfaces would show the initial stage, with prevalence of the areolar erosion; and the Hollow Surfaces would show a more advanced stage, with prevalence of the linear erosion by an organized network. The importance of the hollow out would indicate the advance of the dismantlement. b) The general alteration would be a consequence of an arenization process, associated to fluctuating redox conditions. The variations observed would be due to the substratum: the granite would be kaolinized and the metamorphic lithologies would show an argilic covering of motley colours. The preservation of the basins and remains of the regolith in the R1600 would indicate that at these points of the massif the glaciar erosion was not very strong. c) The remains of erosive terraces would correspond to old networks, nowadays hung and dismantled, which would be associated to the elaboration and/or the dismantlement of the plains; and in some cases, they would be related to areas of structural weakness, more changeable and therefore more subject to erosion. d) Two significant morphoestructural directions are observed: NE-SW and NNW-SSE. The NE-SW direction would be a ramification of the Pyrenean Basal Thrust, represented by the Fault of Vila Real. The NNW-SSE direction would be represented by the Fault of Maceda, a dextral directional fault. The sequence of tectonic pulses would be related, in a first moment, to the NE-SW fracture and later on to the NNW-SSE one. The NE-SW direction would have conditioned the orientation of the river Návea and the tectonic regimen in the Graben of Xinzo. This is why the pattern of pull-apart type basin is proposed, with a left directional activity and certain inverse component. During the activity of the NE-SW fractures a solidary behaviour is supposed to the block formed by the Serra of QueixaSan Mamede, Surface of Chaguazoso and Serra do Burgo. e) The Graben of Xinzo is supposed to be prior to that of Maceda, because its borders are truncated by those of Maceda. Concerning the activity of the Fault of Maceda, it is synchronous or 280 Yepes Temiño & Vidal Romaní CAD. LAB. XEOL. LAXE 28 (2003) prior to the plains R600 and R800, which even the Fault steep in two points (Esgos and Luintra). ACKNOWLEDGEMENTS We are much indebted to our colleagues who have generously given us their opinions: J de Pedraza, J.M. Vilaplana, J. de D. Centeno, A. Martín Serrano and E. de Uña. Recibido: 10-II-03 Aceptado: 21-IV-03 CAD. LAB. XEOL. LAXE 28 (2003) Morphogenesis of the Ourense Plains 281 REFERENCES BARRERA MORATE J. L.; FARIAS ARQUER P.; GONZÁLEZ LODEIRO F.; MARQUÍNEZ GARCÍA J.; MARTÍN PARRA L. M.; MARTÍNEZ CATALÁN J. R.; DEL OLMO SANZ A.; DE PABLO MACIÁ J.G.; GALLASTEGUI G.; BEA F. & VILLASANTE PINTO R. (1989). Mapa Geológico de España. Scale 1:200.000, nº. 17/27, Ourense/Verín. Madrid, Instituto Tecnológico Geominero de España, 284 pp. 1 fold. Map. BIROT P. & SOLÉ SABARÍS L. (1954). Recherches morphologiques dans le NW de la Pèninsule Ibèrique. París, ed. Mémoires et Documents du C.N.R.S., 4: 7-61. BRUM A. DE; VIDAL ROMANÍ J. R.; VILAPLANA, J. M.; RODRÍGUES M. L.; ZEZERE J. L. & MONGE C. (1992). Formas e depósitos glaciários da Srra. do Geres-Xurés. PortugalGalicia. Levantamiento cartográfico. Cadernos do Laboratorio Xeoloxico de Laxe, 17: 121-135. C.N.I.G. (1994). Mapa Geográfico de Lugo. Scale 1:200.000. Madrid, ed. Centro Nacional de Información Geográfica del Ministerio de Fomento, 1 fold. Map. C.N.I.G. (1997). Mapa Geográfico de Ourense. Scale 1:200.000. Madrid, ed. Centro Nacional de Información Geográfica del Ministerio de Fomento, 1 fold. Map. FERRÚS B. (1994). Estructura de la cuenca de As Pontes (La Coruña). Cadernos do Laboratorio Xeolóxico de Laxe, 19: 73-91. GROOT R. DE. (1974). Quantitative analyses of pediments and fluvial terraces applied to the basin of Monforte de Lemos. Amsterdam, ed. Bodemkunding Laboratorium van de Universiteit van Amsterdam, 22: 127 pp. HERNÁNDEZ-PACHECO F. (1949). Geomorfología de la cuenca media del Sil. Memorias de la Real Academia de Ciencias Exactas Físicas y Naturales, 13: 112 pp. HERNÁNDEZ-PACHECO, F. (1958). El glaciarismo cuaternario de la Sierra de Queija. Boletín de la Real Sociedad Española de Historia Natural, 55: 27-74. MARTÍN-SERRANO A. (1989). Rasgos generales y problemática de las superficies de erosión en Galicia. Cadernos do Laboratorio Xeoloxico de Laxe, 14: 7-18. MARTÍN-SERRANO A. (1991). La definición y el encajamiento de la red fluvial actual sobre el Macizo Hespérico Peninsular en el marco de su geodinámica alpina. Revista de la Sociedad Geologica de España, 4(3-4): 337-351. MARTÍN-SERRANO A. (1994 a). Macizo Hespérico Septentrional. In: Gutierrez Elorza, M. (ed.), 1994. Geomorfología de España. Madrid, Editorial Rueda, 25-62. MARTÍN-SERRANO A. (1994 b). El relieve del Macizo Hespérico: génesis y cronología de los principales elementos morfológicos. Cadernos do Laboratorio Xeoloxico de Laxe, 19: 37-55. MONGE GANUZAS C. (1987). Estudio sedimentológico de la cuenca terciaria de Meirama. Un ejemplo de cuenca sedimentaria sobre una falla de salto en dirección. Cadernos do Laboratorio Xeolóxico de Laxe, 11: 51-68. NONN H. (1966). Les regions cotières de la Galicie, (Espagne): étude geomorphologique. Doctoral Thesis. University of Strasbourg. 591 pp. PÉREZ-ALBERTI A. (1993). La interacción entre procesos geomorfológicos en la génesis del relieve del SE de Galicia: el ejemplo del Macizo de Manzaneda y de la Depresión de Maceda. In: PÉREZ-ALBERTI, A., GUITIÁN RIVERA, L. & RAMIL REGO R. (eds.) 1993. La evolución del paisaje en las montañas del entorno de los Caminos Jacobeos, Santiago de Compostela, Consellería de Cultura de la Xunta de Galicia, 1-24. ROEL J & TOYOS J. M. (1993). Investigación de Andalucita en el área de Verín, Ourense. Madrid, Instituto Tecnológico Geominero de España, 20 pp. SAENZ RIDRUEJO C. (1968). Varves glaciares del alto Bibei. Revista de Obras Públicas, 5: 339-350. SANTANACH PRAT P. (1994). Las cuencas terciarias gallegas en la terminación occidental de los relieves pirenaicos. Cadernos do Laboratorio Xeoloxico de Laxe, 19: 57-71. SCHMIDT-TOMÉ P. (1978). Nuevos testigos de una glaciación wurmiense extensa y de altura muy baja en el NW de la Península Ibérica, Ourense-Portugal. Cuadernos do Seminario de Estudios Cerámicos de Sargadelos, 27: 221-243. SOUTO FIGUEROA M.G. (1996). As augas termais. In: Díaz Fierros, F. (ed.), 1996. As augas de Galicia Ponencia do Patrimonio Cultural, Santiago de Compostela, ed. Consello da Cultura Galega, 611 pp. 282 Yepes Temiño & Vidal Romaní CAD. LAB. XEOL. LAXE 28 (2003) VERGNOLLE C. (1990). Morphogenese des reliefs cotieres associes a la marga continentale nord-espagnole. L'exemple du Nord-Ouest de La Galice. La Coruña, Serie Nova Terra, 1, 314 pp. O Castro, Spain. VIDAL ROMANÍ J. R. & SANTOS L. (1994). La deglaciación finicuaternaria en el NW Peninsular. Serra de Queixa, Ourense. Datos geomorfológicos y paleobotánicos. Cuaternario y Geomorfología, 7: 21-44. VIDAL ROMANÍ J. R.; VILAPLANA J. M.; BRUM A. ; ZEZERE J. L.; RODRÍGUES L. & MONGE C. (1990). Los tills de la Srra. de Gerés-Xurés y la glaciación pleistocena. Portugal-Ourense. Cuaternario y Geomorfología, 4: 13-25. VIDAL ROMANÍ J.R. (1996). Geomorfología de Galicia. In: Historia de Galicia, Xeografía. La Coruña, ed. Hercules de Ediciones, 13: 7-67. VILLASANTE R. & PEDRAZA J. DE. (1984). Mapa geomorfológico de Ourense-Verín. escala 1:200.000. In: Mapa Geológico de España (Documentación Anexa). Scale 1:200.000, nº. 17/27, Ourense/Verín. Madrid, Instituto Tecnológico Geominero de España, 35 pp. 1 fold. Map. YEPES TEMIÑO J. (1998). Geomorfología de un sector comprendido entre las provincias de Lugo y Ourense (Galicia). Doctoral Thesis. Universidad Complutense de Madrid. Published in Serie Nova Terra, 22: 2002. 272 pp. O Castro, Spain. CAD. LAB. XEOL. LAXE 28 (2003) Morphogenesis of the Ourense Plains 283