1. Introduction
To understand the forms and current evolution of the different mountains of the planet, it is necessary to consider what their past was like, which means knowing what elements have shaped their landscapes. Among them, the ice masses associated with cold phases have a fundamental role in explaining the current mountain landscape and their impact on these variations [
1,
2,
3].
Throughout history, the analysis of past glaciated landscapes has been approached from multiple perspectives. From a theoretical point of view, focused on the original glacial forms description [
4] to works that focus on modeling how the dominant ice masses have been and how they have changed over time based on parameters such as their extent or depth [
5], as well as the analysis of paleoenvironmental records from different perspectives and methods [
6,
7,
8]. Accordingly, the emergence of new methodologies associated with innovative techniques has been of great importance. Currently, it is possible to date glacial remnants with great precision and estimate, through modeling methods, the surface area occupied by ice masses at different moments or climatic phases.
There have been several phases of research regarding ice surface modeling, from the first moments in which a simple mapping of the shapes was carried out to others that used new techniques to obtain 2D models [
9,
10]. In the last decades, the number of researchers focused on improving these studies has grown considerably. Nowadays, 3D models can be used to know the possible location of ice masses and their relevance. Among the tools designed for this task, it is relevant to mention GlaRe, a software tool proposed by Pellitero et al. [
11], which was used during the development of this research.
The improvement of the techniques for obtaining 3D models and the greater precision of dating methods has led to the publication in recent decades of the results of many studies worldwide that analyze zones formerly occupied by glaciers and their evolution. In the case of the Iberian context, there are different publications in this regard [
1,
12], highlighting among them those dedicated to the Pyrenees area [
13,
14], the Cantabrian Mountains [
15,
16,
17,
18], or Sierra Nevada [
19,
20].
In Galicia’s case, the impact of glaciers has been crucial in the landscape’s evolution in many mountain areas and has been studied since the middle of the 20th century [
21,
22]. It is possible to highlight works at a global level [
23] or detailed studies focused on specific mountain ranges such as the Macizo do Xistral [
24], Manzaneda Massif [
25,
26,
27,
28], Serra de Ancares [
29,
30,
31,
32], Serra do Courel [
33,
34,
35] or in the Trevinca Massif [
6,
36,
37].
However, despite the abundance of publications, until now, a modern and comprehensive analysis has yet to be carried out on the extent and volume of glaciers in all areas affected by glaciers in Galicia. Some of the publications have focused on the reconstruction of paleotemperatures from periglacial data [
7], but this has not been the case with information on glacial evidence except for the paleoglaciers in the Leonese margin of the Trevinca Massif, specifically in the sector of the Tera Valley and Sanabria lake [
10,
38,
39]. More recently, a review of glacial geomorphology in the Galician Mountain context has been carried out on a global scale [
40,
41], which has contributed to identifying the different glacial phases developed in Trevinca [
42].
The main objective of this study was the detailed mapping and characterization of the glacial deposits and forms present in the Trevinca Massif. From them, it was possible to carry out the paleoglacial reconstruction, estimating the volume of the glacial masses, their extension, and the values of the ELA in the western sector, which, as mentioned above, is the most unknown, as there are already previous works on the eastern part [
10,
38,
39].
2. Study Area
The Trevinca Massif is located northwest of the Iberian Peninsula, between the autonomous communities of Galicia to the west and Castilla y León to the east (
Figure 1). Topographically, it has a rounded and massive appearance, resulting from the confluence of a group of mountain ranges at a central point. These are the Serra do Eixe, which stretches from SE to NW; the Serra Calva, from NE to SW; the Serra Segundeira, from N to S; and the Sierra de la Cabrera, which stretches from W to E. The highest elevations are found in the peaks of Pena Trevinca (2127 m) and Pena Survia (2116 m), both within the Serra do Eixe.
The relief of the massif must be related to the radial disposition of the rivers that flow in all directions and serve as a division between the different mountain ranges, as is the case, for example, of the Tera River that stretches from north to south and divides the Serra da Segundeira to the west from the Cabrera to the east. The fluvial network has fragmented the space generating significant topographic contrasts, as seen in
Figure 1. The central sector is characterized by its horizontality and staggered elevation, which contrasts with the edges of the massif in which numerous valleys are strongly embedded. This fact is fundamental to understanding the type of glaciation developed in this area and its magnitude, as shown in
Figure 2. Concerning the influence of the lithology and other factors in the glacial landforms evolution, in this sector, the structural control is more relevant than other differences such as the lithology, which is characterized by a great uniformity.
In the specific case of the western sector of the Trevinca Massif, on which the paleoglacial reconstruction was focused, the analysis of the territory was carried out about the main hydrographic basins, for the case of Xares, Canda, and Bibei-Barxacova. The first is associated with the Xares river, which has its sources west of the peaks of Pena Trevinca and Pena Survia. Along its path, it merges with the valleys of Meladas, Requeixo, and Morteira. The second, Canda, is the smallest of those analyzed, stretching E–W to the south of the Serra Calva, reaching the villages of Valdín and Seoane before joining the Xares valley. On the other hand, the Bibei-Barxacova basin is the widest of those analyzed. It is hierarchized by the Bibei river that begins to flow at its headwaters following an ENE–WSW direction to progressively turn towards an N–S direction as it is joined by different valleys modeled by streams and rivers until their confluence with those of the Barxacova in the surrounding of Pías (Zamora).
Geologically, the Trevinca Massif is located in the so-called Ollo de Sapo area [
25] in the context of the central Iberian zone of the Iberian Massif (
Figure 1). Metamorphic rocks dominate this region with granular orthogneiss (“ollo de sapo”) and intercalations of gneiss, schist, and slate. These materials were intensely deformed and folded during the Variscan orogeny and cut by carboniferous igneous intrusions [
44]. In addition to the above and to understand the current landscape of the Trevinca sector, it is necessary to mention the deformation of the Iberian microplate during the Alpine orogeny and the uplift of the crystalline basement blocks [
45].
From a climatic point of view, nowadays, the conditions of the high mountains of the NW Iberian Peninsula are linked to elevation, giving rise to an oceanic mountain climate [
46] with slight continental characteristics. Rainfall is high, with an annual average of 1150 ± 115 mm, and is distributed irregularly throughout the year in the form of snow during the winter [
47]. For its part, the average temperature in the Trevinca environment is 8.2 ± 1.3 °C, with the average value of the maximum being 13.6 ± 1.3 °C and the minimum being 2.6 ± 1.4 °C (
http://agroclimap.aemet.es, accessed on: 8 January 2023).
5. Discussion
After an exhaustive review of previous works on the Trevinca Massif and the surrounding zones, it is necessary to mention that the glacial modeling in this area has been investigated for more than a century [
65], being carried out over time approaches from multiple perspectives [
4,
6,
21,
22,
23,
31,
36,
40,
43,
66,
67,
68]. A significant part of the works have focused on the eastern sector of the massif, in the surroundings of Lake Sanabria, so the analysis of the western part of Trevinca is of great interest to know the differences and similarities between both slopes.
As it occurred in other sectors recently, research has focused on the paleoenvironmental reconstruction linked to the pollen record of zones such as Sanabria Lake [
69,
70,
71,
72], in addition to the research on the extent of the glacial surface and inherited forms in the eastern area [
36,
38,
73,
74] and the establishment of numerical chronologies [
43,
62].
Cartography has been of great interest to geomorphology since its beginnings. Still, the last decades have seen an important advance thanks to the evolution of new technologies such as GIS and editing software. In this sense, several publications in the Iberian Peninsula have created representations of the glacial geomorphology at different scales [
12].
One of the critical elements in this mapping is the moraine ridges. These have served as the basis for the delimitation of paleoglacial basins, being used as limits of the ice expansion [
11]. The primary analysis of the moraine ridges considered the western Trevinca complex to adapt the study to the interest area. For this reason, the elevational categories thresholds varied slightly from other works in which different nearby mountain ranges were included [
40].
In the distribution of the moraine ridges, the importance of the Bibei Valley and its confluence with the Barxacova Valley in the extension of the ice was corroborated. This sector showed the highest number of identifiable moraines (194), while that of the other two did not exceed 60. It should be noted that in the Bibei-Barxacova, the moraines appeared preferentially on the valley sides at the ends of the analyzed area, highlighting the Cepedelo sector (
Figure 2a,b). On the other hand, in the case of the Canda glacial complex, due to the evolution of the glacial mass, most of the moraines identified were in the highest part of the valley (
Table 3). In contrast, in the case of Xares, the moraines were widely distributed due to the subdivision of the sector into several smaller valleys.
These moraine accumulations were preserved, especially on flat or gently sloping summits on the valleys’ flanks, indicating that the ice exceeded the valleys during their maximum advance, showing flow directions transversal to them. Large erratic blocks also corroborated this supraglacial transport, such as those located in the interfluve of the Bibei and Barxacova valleys linked to the maximum ice extension [
36].
Previous studies on the western Trevinca estimated the extent of paleoglaciers at 326.5 km
2 [
40] during the lLGM when Bibei’s glacier tongue reached 30 km; in this study, this value was 27.26 km (
Table 4). In the case of the eastern sector, centered in the Tera Valley and the Sanabria area, ice thicknesses were estimated quite similarly in the two existing studies (400–454 m in Rodriguez-Rodríguez et al. [
38] and 457 m in the work of Fernández-Fernández [
10].
These characteristics fitted perfectly with previous studies, where different references to the smoothed shape appeared, especially in the higher areas. This was related to the appearance, during the Pleistocene, of an ice field drained by numerous radial glacier outlets using the existing drainage network [
39]. During the maximum advance, the glaciers formed several groups of well-defined lateral moraines, although no deposits could be identified in the lower parts of the valleys, showing, however, the bottom and sides of the valleys’ glacial evidence (
Figure 2), such as polishing and the development of roche moutonnée [
6,
43].
The phases described in this analysis (
Table 2 and
Table 3) through the grouping of the moraine ridges fitted clearly with previous studies in this area. This could be related to the Pías sector in the Bibei Valley, where the rear part of the terminal moraine was dated by OSL of three layers of fluvioglacial deposits, obtaining ages of 27–33 ka for the maximum glacial advance [
43]. These values also fitted with the studies elaborated at Sanabria Lake and San Martín de Castañeda [
38,
73,
74]. Different studies have placed the minimum elevation for the glacial fronts of the Tera and Bibei valleys between 900 and 950 m, with several glacial cirques remaining in the Trevinca Massif in the Holocene as vestiges of the importance of ice in this sector previously [
1]. The subsequent phases of glacial recession have also left their mark on the landscape, and the presence of till and glaciotectonic processes in areas above the maximum advance can be verified [
37] (
Figure 2f). Moreover, these values are corroborated by studies carried out in the eastern part of the massif [
62]. In this evolution, there is a generalized retreat of the ice, with some phase of readvancement detected in the surroundings of Sanabria [
73] but which ends with the enclosure of the glaciers in the highest elevations of the massif [
7,
64,
73].
Despite the previous explanations, it would not seem correct to extrapolate these values to the entire massif. In previous works, it could be seen how the northern valleys could work both as independent valley glaciers or as outlet glaciers channeled through the preglacial valleys, as was the case for the Seoane Glacier [
36]. The relationship between the dynamics of these forms and the main ice mass remains to be studied. Likewise, their relationship with the eastern (Tera Valley and Sierra de la Cabrera) and northern (Val de Casaio) sectors needs to be clarified. With these premises, extrapolating the results achieved in this sector to the whole mountain range would be risky.
The long period of studies in this sector has favored that the explanation of the geoforms and the definition of the parameters that estimate the behavior of this area have varied. This is perceived in the ELA estimation, where Stickel [
21] defined it at 1500 m, while Schmitz [
22] placed it at 1600 m. In this case, we estimated values that differed between the three western sectors and varied between 1427 and 1839 m depending on the calculation method.
The values obtained were similar to those of recent studies, which placed the ELA in the Tera Valley between 1520 and 1900 m [
73], or the work of Fernández-Fernández et al. [
10] for the eastern sector of Sanabria, where the ELA elevation varied between 1637 and 1796 m. These values were also similar to Serra da Estrela (1643 m) [
75], with a comparable location and characteristics, or for some approximation in the Galician case, such as that of Hernández-Pacheco [
76] for Manzaneda, where he placed the ELA at 1428 m. The variations in the ELA values in the context of the northern Iberian Peninsula were also previously analyzed concerning the climatic variations [
60], a comparison that was extended to other sectors of the European continent, as in the case of Ribolini et al. [
77] for the analysis of glaciers in southern Europe.
Regarding new technologies and studies on a glacier’s influence and its reconstruction, the possibilities offered by the GlaRe tool should be highlighted; it allows a very quick estimation with basic processing knowledge. This tool has prompted multiple works in which it was applied to obtain comparable results to those obtained with other methods [
54,
55,
78]. In sectors with glaciers no longer active, the estimation can be more complex, so it is of great interest to expand the analyzing zones using this tool for a better calibration, mainly through improving processing techniques.
As mentioned in the main GlaRe tool paper [
11], that method underestimates the ice thickness and volume with the different possible approximations. This could be perceived in the case of the limits of the glacier extension, where the ice thickness should have been more significant to form the moraine ridges that were identified, such as those around Cepedelo (
Figure 7) or in the lower part of the Xares glacial complex.
As with GlaRe, the approach of Pellitero et al. [
59] for estimating the ELA is of great relevance in this study since it allowed us to quickly obtain the central values that affected or had affected these sectors. As mentioned previously, the values were comparable to other studies where the authors applied different methodologies.
6. Conclusions
The values obtained, as well as the distribution of glacial evidence, allowed us to distinguish the maximum glacial advance of this area, which fitted with that proposed for other massifs of the northwest of the Iberian Peninsula [
79] and for other mountainous sectors of the Iberian Peninsula, especially the southern slopes of the Pyrenees, which are usually used as reference [
80,
81]. The phases described by grouping the moraine ridges corresponded clearly with those of previous studies in which different dating methods were used.
This work estimated the maximum ice thicknesses, which reached 527.03 m in the case of the Bibei-Barxacova paleoglacier, and its minimum value in the Xares paleoglacier, with 292.51 m. This dynamic was similar for the average thickness, with 218.16 m and 101.97 m, respectively. Regarding the ice extent, the smallest paleoglacier was the Canda one, with 14.85 km2 of ice, while in the Bibei-Barxacova paleoglacier, the ice mass extent reached 143.74 km2.
Regarding the ELA, the highest values corresponded to Bibei-Barxacova, exceeding 1800 m in some methods. The maximum differences between the three analysis sectors were around 200 m.
It was also possible to demonstrate the facilities offered by new technologies for reconstructing these environments and their simplicity of processing for estimating the ice thickness using the GlaRe tool and obtaining the ELA by different methods.
This study showed a detailed characterization of the glacial forms in the Trevinca Massif, especially in their western part. From this analysis, it is possible to carry out different samplings to find the correspondence of the geoforms with each of the considered phases.