The Alcanadre River has its head in the Pyrenean External Ranges and reaches the Sariñena depression with N-S orientation. At the confluence with the Flumen River it turns 90º towards the east (Fig. 1) to finally merge with the Cinca River. The Alcanadre River migrated towards the south along the Middle/Upper Pleistocene and constitutes a limit between the smooth relief of the large Quaternary deposits of Sariñena to the north and the abrupt and geomorphologically diverse reliefs of Jubierre towards the south. Calle et al. (2013) identified eight fluvial terraces on the Alcanadre River in the Sariñena depression. Among them, Qt5 is the largest and was dated to 274 − 222 ky BP with ISRL (Rodríguez-Ochoa et al. 2019).
The ephemeral streams coming from the upper limestone reliefs south of the Jubierre descend 260 − 250 m to merge with the Alcanadre River (Fig. 2). These show an average gradient of 5–6%. Their strong erosive power has led to the uphead erosion of the basin (Fig. 3a). There are several levels of Quaternary pediments in the south margin of the Alcanadre River (Fig. 2). These form narrow morphologies dividing the valleys where the main streams flow. These pediments are residual landforms of a large detritic cover and form slightly tilted mesas and testimonial buttes (Fig. 2). The escarpment rim is mainly formed by accumulations of angular limestone fragments coming from the cornices of the Sierra de Pallaruelo-Monte de Sora Unit. These accumulations were formed in several stages and represent periods when high loads were carried by the streams from Jubierre that formed large detritic mantles along the piedmont. At present, incision processes affecting the entire piedmont are dominant and expose the erodible subjacent marls and clays (Fig. 2, 3c). It is sometimes possible to find interbedded resistant layers (sandstones and limestones) in the form of narrow steps with segmented scarps and buttes (Figs. 3d, 3e). Where the resistant layers are eroded, there are only residual hills following a long evolutionary process. The result is a complex labyrinthine landscape that is almost empty of human activity and with a high scenographic value.
4.1. Erosive processes in the hills
Many residual hills (tozales) are currently very unstable due to erosion caused by several factors. Firstly, their morphology is very abrupt, with slope gradients over 60º. The lack of hard layers on the top of the morphologies, as well as the scarce vegetation cover, offer a wetting front exposed to rain splash (Figs. 3c, 3d, 3e). In addition, the hills develop a steep hydraulic gradient with respect to the drainage base level of the surrounding streams and this leads to high flows with considerable carrying capacity (Benito et al. 1991). Secondly, the lithologies are prone to erosion as they are composed of lutites and marls with a high content of calcium and sodium sulphates. These are like other lithologies from the Ebro depression where erosion measurements were made by several authors (Gutiérrez et al. 1988, 1997; Benito et al. 1991, 1993; Sirvent et al. 1997). Lastly, the present semiarid climate is characterised by scattered concentrated rains, especially convective storms, during the summer and this magnifies the factors previously mentioned. The variability of this continental Mediterranean semiarid climate is prone to the development of many of the already mentioned processes (Bryan and Yair 1982; Bryan 1987).
The salts contained on marls and clays contribute to haloclasty and hydroclasty – and this increases the weathering processes of the surfaces, together with the dispersive properties of the soils (Jones 1971). The rill subparallel networks are developed from the upper middle sector of the hills (Fig. 3d) with popcorn interills in the lower sections of the hillslope. When intermediate steps are present, it is possible to find conical forms (locally known as “elephant legs”) with gullies and pipes (Figs. 3f, 3g). These surfaces tend to be covered by popcorn morphologies and pedestals (Fig. 3g), and sometimes with large amounts of saline efflorescence. Much of the water flows through piping to reach flat surfaces, such as the structural landings on the base of the hills. As indicated by Jones (1981) and Harvey (1982), piping is the triggering process for the development of these types of rills and gullies. Sediment movement reaches a high level of efficiency by developing silty alluvial fans in the piping outlets as well as surface runoff (Fig. 3f). These environments change quickly and give place to various stages of stepped micro-alluvial fans formed with the silty sediments that are moved through incision cycles. It is usual to find high levels of polygonal cracking density over these silty-clay accumulations. These cracks facilitate rain penetration until they are sealed by sediments or biological soil crusts (Fig. 3h). Biological crusts are formed by lichens and mosses and can temporally slow erosion. These morphologies are shown on the map of Los Pedregales when they are large enough to be represented on our working scale.
We are going to focus on the evolutionary study of the slopes of two of the most geomorphologically interesting hills: Tozales of Los Pedregales North (LPN) and South (LPS) (Figs. 3b, 3c, 3d). Another two hills are also studied for a better documentation of the older slopes: Tozal Solitario 1 (Fig. 3e) and Tozal Solitario 2.
4.2. Slope evolution on Tozales de Los Pedregales N (LPN) and S (LPN)
These two hills are in the central sector of Jubierre. Both were formed on the detritic sediments of the Bujaraloz-Sariñena Fm of the Agenian-Aragonian age. They are composed of marls, clays, and siltstones (red, orange, and yellowish). Some narrow layers of lacustrine limestones and sandstones generate largely resistant steps among the soft sediments (Figs. 3c, 3d, 3e). There are also some greyish sandstone paleochannels on the lower section of the accumulations found at the stream incisions, as in the Barranco de la Torre (Figs. 2, 3c). Both hills are located over narrow platforms, the LPS is higher than the LPN (Fig. 4a). These platforms are surrounded by deep incisions produced by the fluvial network. The hills are N-S oriented and clearly visible on the landscape due to their topography and lack of vegetation (Figs. 3b, 3c, 3d).
4.2.1. Los Pedregales North (LPN)
The LPN hill has an elongated NNE-SSW shape with a central crest that lacks a resistant upper caprock, and so it can be classified as a residual hill. It is 25 m long and 12 m wide and composed of marls and clays with interbedded thin limestone layers. The water divide is formed by a sharp crest, with 255 m a.s.l. to the SSW and 252 m a.s.l. towards the NNE (Figs. 4a, 4b, 3c). The hilltop is 8 m above the structural platform over which it is located and 27 m above the Barranco de la Torre stream channel. In the geomorphological map (Fig. 5) it is possible to see the isolated position of the hill over the flat structural surface and the steps formed by narrow limestone and sandstone steps descending towards the stream channel. The permanence of the present hill depends on the hardness of these basal structural levels (Figs. 5, 6a, 6b). The main scarp is N-S aligned and its retreat has produced many large blocks that cover part of the slope.
The LPN hill is characterised by the strong erosion produced by microrills and micropiping created by runoff and epidermal flows (Fig. 5). In addition, their stony compositions mean that residual slopes have been preserved. These reliefs have subtriangular shapes, typical of talus flatirons, and feature a high sharpened apex and a smooth reverse (Fig. 5). The deposit is formed by angular clasts of limestone. Two sets of taluses from different ages were identified. They were classified according to their distance from the LPN hill and height, as well as the presence (or absence) of archaeological remains. After classification, considering all the recorded hills from Jubierre, the farthest and therefore oldest slopes from LPN were named S3 and their remains as taluses 6 and 9 in Fig. 5 (see also Figs. 6a). No archaeological remains were found on this stage. The next stage, S2, is younger, closer to the hill than previous stage (S3), and mainly preserved toward the east of the hill (taluses 1 to 5 in Fig. 5) (see also Figs. 6a, 6b). There is also a small relict of S2 on the border of the western scarp (talus 8 in Figs. 5, 6a), and another that is considerably eroded towards the north (talus 7 in Fig. 5).
Most of S2 taluses reveal ceramic Bronze Age potsherds, rolled by transport, among the limestone clasts (Fig. 6a, 6c, 6d). Eighteen ceramic fragments and several flints were recovered from talus 1 (Fig. 5). There are also fragments of mud bricks and mud pavement fragments on taluses 3 and 5, although the quantity was small and the chronological value low. The ceramic fragments recovered from talus 1 belong to small vessels with a fine paste. They were part of open carinated vessels that are typical of the intermediate and late phases of the Bronze Age (second millennia BC; Picazo 1993). A distal section of a flint laminate fits within the proposed chronology. This is also the case of the Genó archaeological sites belonging to the Final Bronze Age (1200 − 1000 cal BC) in the oriental Ebro basin (Maya et al. 1998).
These remains indicate that the hilltop was settled during the slope formation, although there is no in situ evidence of archaeological residual materials from the talus. In addition, the remains provide a relative age for stage S2 indicating that it was contemporary or slightly later then Bronze Age.
The location and orientation of the apex enables reconstructing the position of the hilltop and the hill morphology by extending the hillside profile for each slope stage (Fig. 6b). In addition, it was possible to observe severely weathered sandstone blocks, some of which are large, sparse, and isolated. These had to be part of the hill caprock in some intermediate phase of its evolution. One of these blocks remains in an inclined position lying over the slope deposits. Because of its proximity to the hill, this block is part of the only remains of the S1 stage. It is represented on Fig. 6a (number 10) and on the profile of Fig. 6b.
4.2.3. Los Pedregales South (LPS)
The LPS is located over a structural platform above the base of LPN. The hill is 37 m long and 12 m wide, reaching 14 m height at 262 m a.s.l. and is located 34 m above the Barranco de la Torre channel (Figs. 4a, 4b). It is separated from LPN by a pass (and through which much water drains towards the NE) (Fig. 7a). It is geomorphologically more complex than LPN, and on the southern sector it still includes part of a 30 cm thick protective sandstone caprock (Fig. 3d). After a small pass, the hill extends towards the NNE as a descending crest and narrow remains of the S2 stage (talus 8 in Figs. 7a, 7b, 7c). The slope has a steep gradient because it is close to the Barranco de la Torre. It has a stepped appearance because it developed over the sandstone and limestone scarps descending towards the stream channel. It is possible to find clay and marls micro-modelling around the steps produced by rills and piping (Fig. 7a). There are cones, popcorns, vertical mood covers, and micro-pedestals among the piping outlets.
As in LPN, clay layers without a hard protection are exposed in the LPS. The layers are affected by a dense network of microrills and piping (Fig. 7a) with the formation of silty cones at its foot (Fig. 3f). They also show step formations and surficial processes of cracking and biological soil crusts, especially towards the south.
There are remains of older slopes with talus flatiron morphologies. Three evolutionary stages were identified. Two triangular shapes with the apex oriented towards the hill are from the older (S3) stage (taluses 1 and 7 in Figs. 7a, 7c, 8a). These have 0.3–0.4 m thick deposit on the apexes that decrease along the backslope and are composed of angular limestones from the Jubierre – without any material with chronological meaning. The position and orientation of these slopes enables tracing theoretical profiles to locate the paleo escarpment of a higher and larger old hill (Fig. 7c).
Stage S2 is represented by four very eroded taluses, located on the eastern side of the LPS hill (Figs. 7a, 7b, 7c, 8a) with small remains toward the SW (Fig. 7a) on the side of the main scarp (Fig. 7c). There were probably others of the same stage towards the west, but these eroded due to their proximity to the Barranco de la Torre. One of the taluses (talus 5 in Fig. 7a) is oriented in the opposite direction to the rest of the arrangement, and probably belonged to another relief located towards the SE. However, most eastern slopes from stage S2 (taluses 2, 3, 5, and 10 in Figs. 7a, 8a) have ceramic Bronze Age potsherds (the same stage as in LPN), as well as limestone detritic deposits. Thus, they were contemporary or slightly later than the archaeological remains whose source may be in a settlement on the top of the hill that was larger than today.
The most outstanding S2 talus is located toward the north of the hill crest (talus 8 in Figs. 7a, 8a) and it is exceptionally well preserved in the middle section of the butte. This is because there are many sandstone blocks on its apex protecting an S2 slope that also has a thick and compact deposit. The development of a biological crust also improved its resistance. The most recent slope (S1) is located on the eastern foot of the hill (talus 9 in Figs. 7a, 7c, 8a) and partially lays on an escarpment wall that also contributes to its conservation.
Considering the abundance of ceramic fragments, bones, and charcoals of the S2 (8) relict, a 1 m wide segment of the middle section of the outcrop was cleaned and described (Fig. 8b). The clayey sediments with blocks covering the S2 escarpment together with the head of the S1 slope were removed to access the S2 deposit. Although it is not a primary archaeological site, this is the best approach to learn about the human occupation of the hill over the ages.
The S2 deposit lays over the clays and marls of the Miocene substrate (Fig. 8b). It reaches 120 cm in thickness, towards the left (S) diminishes downslope to 80 cm in thickness. The first unit (A) is composed of yellowish clays with limestone clasts chaotically distributed and whose major axes range between 2 and 10 cm. The unit also contains charcoal fragments. It is followed by Unit B that lays with an inclination of 26º and cuts through the previous unit. Unit B is about 20 cm thick and is composed of reddish clayey sediments with limestone clasts arranged along the inclination of the slope (Fig. 8b). It is a very continuous unit and below the base of the talus. Unit C is 20 cm thick, formed by yellowish silty sediments with the same inclination as the previous unit. It contains bone and charcoal but no ceramic fragments. A charcoal sample was taken (JUB-1) at 30 cm depth close to the contact with the upper unit (D) (Fig. 8b). A dating of 3252 ± 24 BP (1608 − 1446 cal BC, 2σ) was obtained (Table 1). Lastly, Unit D is formed of 20 cm of greyish silts that are completely grey on the top. The surface is covered by an eroded biological crust. Ceramic fragments are more abundant on this upper unit.
The archaeological survey and excavation made on slope 8 of the S2 stage provided ceramic fragments and lithics that resembled the S1 slope located at its foot (talus 9 in Figs. 7a, 8a). The ceramic fragments of S1 are the result of the erosion and sedimentation of the upper slope S2. In general, the ceramics are well preserved and show sharp edges, unaltered surfaces, and are rounded. In addition, a flint flake with thermal cones, a firing pin made over a fluvial quartzite, and bone and charcoal fragments were recovered.
The ceramic set is composed by eight fragments of hand-modelled Bronze Age vessels that were especially common in the middle and late phases. Chronologically they are located between 1700 and 1000 cal BC following comparison other regional studies (Picazo 2005). Among their features, we can highlight the open edges (Fig. 8c) corresponding to middle and small vessels with carinated or S-shaped profiles, with burnished finishes, and fairly good quality pastes. There are three main paste compositions: i) mostly chamotte; ii) mostly quartz; and iii) and mostly mica and quartz. The same materials were recovered from all the taluses showing chronological connections.
Other slopes from the S2 stage arranged as talus flatirons also contain ceramic potsherds as is the case of numbers 3 and 5 (Figs. 7a, 8a). These are smaller and rounded due to the distance from the main hill. They also have the same paste composition and typologies as those found on slopes 8–9 and the LPN taluses.
The younger S1 stage (talus 9 in Figs. 7a, 8a) is found at the foot of the remains of S2 and contains the materials eroded from the previous stage. The surface of S1 has a stony appearance due to the erosion of fine sediments, but the deposit contains abundant silt and clay together with ceramic fragments like S2. The preservation of the S1 slope may be related with the protection provided by the upper S2 remains. Although in the past it must have covered the entire middle slope and foot of the hill.
4.2.4. The slopes in other residual hills
There are many residual hills with talus slopes, although we were unable to locate others with archaeological remains. Another two hills with well-preserved slopes were found that included old stages (S3, S4) that were useful to complete the general evolutionary model. Both hills (Tozal Solitario 1 and Tozal Solitario 2) are very close to LPN and LPS. These hills have similar lithological and geomorphological characteristics, although their final morphologies differ.
Solitario 1 is N-S oriented and retains a sandstone layer in the south sector (Figs. 9a, 9b), while the middle to northern area is a narrow clayey crest falling towards the north. This hill presents old slopes from past stages (S4 and S3) that are especially large on the west side (Figs. 9a, 9b). As in LPN and LPS, among the limestone clasts of the slope S2 (Fig. 9c) a Bronze Age potsherd was located (Fig. 9d).
Solitario 2 is a clayey hill with a sandstone caprock and completely lacks vegetation. It is deeply affected by rills and piping located above a thick sandstone platform over which the cones of silt and gravels carried by the rills descending from the hill wall are deposited (Figs. 10a). Two S3 slopes are preserved while the S2 talus ring is completely preserved except towards the eastern hill face (Figs. 10a, 10b). Three large ceramic fragments, perhaps of the same vase were found at one of the talus flatiron of the S2 stage (Figs. 10a, 10c). These potsherds (Fig. 10d) show similar thickness, finish, and inclusions of those already described belonging to the Bronze Age, although they present rounded edges. Solitario 2 also retains a small talus closer to present hill from S1 stage (Fig. 10a, 10b).