The Origin of the Sierra De Aracena Hollows in the Sierra Morena, Huelva, Andalucia, Spain

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Geomorphology 45 (2002) 197–209 www.elsevier.com/locate/geomorph

The origin of the Sierra de Aracena Hollows in the Sierra Morena, Huelva, Andalucia, Spain

J.M. Recio Espejo a,*, D. Faust b, M.A. Nun˜ez Granados a

aEcology (Physical Environment-Geomorphology), Campus de Rabanales, University of Co´rdoba, 14071-Co´rdoba, Spain bLehrstuhl Physische Geographie, Katholische Universita¨t Eichsta¨tt, Ostenstra e, 26, D-85072, Eichsta¨tt, Germany Received 1 March 2001; received in revised form 10 September 2001; accepted 28 September 2001

Abstract

Hollows in the Sierra de Aracena, part of western sector of Sierra Morena region (Huelva, Spain), are geoecologically unusual macroforms. They are underlain by deeply weathered bedrock but have eutrophic soils with distinctive vegetation. Paleosols with very dark colours, a predominance of smectites and large amounts of total and free iron occur on the floors on the hollows. An evolutionary model is proposed for the hollows, involving differential weathering during the Mesozoic on plutonic and amphibolitic rocks, alpine tectonic activity followed by Quaternary erosion and exhumation leading to formation of erosional terraces. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Hollows macroforms; Deep weathering; Hercynian massif; Sierra Morena; Spain

1. Introduction nary erosional river terraces developed on both the planation surfaces and occurring mainly in narrow The western sector of the Sierra Morena, the Sierra valleys. of Aracena, is formed mainly of Precambrian and A series of enclosed hollows up to 3 km2 in area Palaeozoic rocks typical of the Iberian Hercynian and 150 m deep stand out in the landscape because of massif (Fig. 1). This sector is characterised by large their unusual geoecological characteristics. They morphological features such as planation surfaces and occur all over the western Sierra Morena (Fig. 2) Appalachian morphologies. The planation surfaces are and are delimited by a different vegetation from the cut across plutonic rocks and schists, forming two surrounding areas, by their great depth and by the main levels at about 600–700 and 400–500 m above eutrophic nature of the soils on the floors of the sea level (Nu´n˜ez and Recio, 1998); these are termed hollows. We studied the morphology and genesis of surfaces I and II, respectively. The Appalachian mor- the hollows of the Sierra de Aracena, paying special phologies occur mainly on carbonate and metasedi- attention to palaeo-weathering features in them. mentary lithologies. These is also a series of Quater- The western Sierra Morena lies between 300 and 900 m above sea level, has a relatively high precip- itation of 800–1000 mm/year and average annual * Corresponding author. temperatures of 14–17 C. These climatic factors E-mail address: [email protected] (J.M. Recio Espejo). explain the establishment of umbraphile communities,

Fig. 1. Main lithological zones of the western Sierra Morena.

such as gall oak and chestnut groves (with Quercus Together with tectonic stability this allowed weath- faginea and Castanea sativa as the basic species) and ering processes to dominate Mesozoic morphogenesis oak and cork oak groves (Q. suber and Q. rotundifo- (Molina, 1991; Martı´n Serrano, 1988). Some traces of lia) as the most frequent communities all over the the resulting soils in the north-western sector of the Sierra Morena. The communities are part of one of the Iberian basement (River Duero basin) have been typical cultural landscapes of grazing land with sparse described by Molina et al. (1990). forest in the Andalucian region. Hollows similar to these of the western Sierra Under conditions of maximal rainfall and north- Morena have been described by Godard (1977), Twi- ward exposure, this climatic regime would produce dale (1982) and Ollier (1984) on plutonic rocks of the acidic umbric soils rich in organic matter. However, French Central Massif, USA (Davil’s Marble) and the all the soils of the western Sierra Morena are poorly Murrmungee Basin in Australia, respectively. For these developed Regosols, Leptosols and Cambisols authors increased weathering compared with surround- because of erosive processes accelerated by human ing areas and fluvial removal of weathering products activities over the last two millennia. More strongly were the main factors responsible for formation of the developed soils, such as Luvisols and Acrisols are enclosed hollows. Godard (1977) pointed out the relict from an earlier period (Cano and Recio, 1996). importance of different rock properties, and Twidale Tropical conditions dominated the environment of (1982) related the genesis of hollows to differential the Iberian basement during the Mesozoic (Rat, 1982). weathering in granitic landscapes. Ollier (1984) des- J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 199

Fig. 2. Main hollows of the Aracena Massif and location of the studied soil profiles. cribed hollows 100 m deep below a planation surface, (1976), carbonate by the method of Duchaufour their floors covered with alluvial sediments. (1975), organic matter by the Sims and Haby (1971) method, granulometry according to Soil Survey of England and Wales (1982), and exchangeable ions by 2. Materials and methods

For a morphological study of the hollows we used topographic maps at scales of 1:50,000 (National Topographic Map) and 1:10,000 (Andalusian Carto- graphic Service). The up-dated goelogical maps at a scale 1:50,000 issued by the Spanish Geological and Mining Institute (IGME, 1982, 1983, 1984; ITGE, 1990) were used to identify the bedrock around and beneath the hollows, and air photos at a scale 1:30,000 for their detailed geomorphological characteristics. Soil profiles were described in the field and classified using FAO (1977, 1989). Colours were defined according to Munsell Colour (1990). pH in water was Fig. 3. Geomorphology and bedrock geology of the hollow of determined by the method of Guitia´n and Carballas Dehesa de Valle Torres. 200 ..RcoEpj ta./Goopooy4 20)197–209 (2002) 45 Geomorphology / al. et Espejo Recio J.M.

Fig. 4. Topographic plans and sections of a plutonic hollow (Santa Eulalia) and an amphibolitic hollow (Calabazares). J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 201

Fig. 5. Topographical relationship of hollows to planation surfaces levels NI and NII. the methods of Pinta (1971) and Guitia´n and Carballas The forms of iron were determined according to Mehra (1976). Clay minerals were quantified according to and Jackson (1960), Barro´n and Torrent (1986) and Montealegre (1976) and Brindley and Brown (1980). Torrente and Cabedo (1986). The mineralogy of sand

Fig. 6. Current generalised relationships between lithology, soils and vegetation in the hollows. 202 J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 fractions was determined by the methods of Partenoff slopes are usually of acid metasedimentary rocks et al. (1970). (phyllites and schists) (Figs. 3 and 4). This suggests that the main factor controlling the presence of hollows is differential weathering of the various bedrock 3. Results types. Weathering would have affected the amphibolitic and plutonic lithologies to a greater extent as 3.1. Morphological features they are richer in weatherable minerals and more permeable than the acid metasedimentary rocks, A total of 16 hollows was described within the which are composed mainly of quartz. In some 3250 km2 of our study area (Fig. 2). Most are near hollows, there is a clear relationship between the fault Sierra de Aracena, which is in the central sector of the pattern (determining changes in bedrock) and the western Sierra Morena. The hollows are 0.2–3 km2 in margin of the hollows. In other situations, the role area, with a circular or subcircular outline and steep of tectonics in the genesis of these forms is less clear. ( f 15) side slopes and depths of 100–150 m. All the hollows have been captured and excavated Amphibolitic and plutonic rocks (quartz–diorites by the present fluvial systems. Fluvial action seems to and diorites) occur on their floors, and their marginal account for the appearance of two different morpho-

Table 1 Macromorphological properties of profiles I–VI Profile Horizon Depth (cm) Colour (dry) Colour (moist) Structure Reaction HCl Boundary Umbric Leptosol (H: 680 m, slope: 32–46%, Par. mat.: slates, veg.: rockroses)

I A/C1 0–15 7.5YR5/4 7.5YR3/3 Granular Nil Abrupt C 15– > – – – Nil –

Eutric Regosol (H: 600 m, slope: 4–8%, Par. mat.: colluvium, veg.: grazing land)

II Ap 0–30 10YR5/6 10YR3/4 Massive Nil Sharp C1 30–> 10YR4/4 10YR3/3 Massive Nil –

Eutric Cambisol (H: 540 m, slope: 4–8%, Par. mat.: quartz diorites, veg.: pasture)

III Ap 0–100 10YR5/3 10YR3/4 Granular Nil Abrupt 2Bw 100–115 10YR6/8 10YR5/8 Prismatic Nil Diffuse

2BwC1 115–> 10YR6/6 10YR4/6 Prismatic Nil Diffuse

Eutric Cambisol (H: 520 m, slope: 32–46%, Par. mat.: gneiss, veg.: Genista sp.)

IV A1 0–40 10YR5/4 10YR3/6 Massive Nil Abrupt 2Bw 40–60 10YR2/2 10YR2/1 Prismatic Nil Diffuse

2BC1 60–100 10YR2/2 10YR2/1 Prismatic Nil Diffuse R 100–> – – – Nil –

Eutric Cambisol (H: 280 m, Par. mat.: quartz diorites, veg.: pasture)

VAp 0–40 10YR6/8 7.5YR4/4 Granular Nil Abrupt 2Bw1 40–80 10YR5/4 10YR3/4 Prismatic Nil Diffuse 2Bw2 80–100 10YR5/6 10YR4/6 Prismatic Nil Diffuse C1 100–> 10YR5/6 10YR4/6 Single-grain Nil –

Eutric Regosol (over Palaeoacrisol) (H: 700 m, Par. mat.: quartz diorites, veg.: chestnut woodland)

VI A1 0–05 7.5YR6/4 7.5YR4/4 Granular Nil Sharp A1C1 05–35 7.5YR5/4 7.5YR3/4 Granular Nil Sharp 2C1 35–100 5YR6/6 5YR5/8 Single-grain Nil Diffuse 2C2 100–> 5YR7/6 5YR5/6 Single-grain Nil Diffuse 2C3 100–300 10YR7/8 10YR6/8 Single-grain Nil Diffuse J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 203 logical forms of hollows: morphologies exhumed with find dehesa or grazing land with Q. rotundifolia on flat beds in plutonic hollows, and some others in the deeper soils in the hollows. The steep side slopes which the weak nature of amphibolites impedes the of the hollows are occupied by shrub-like commun- conservation of this morphologies (Nu´n˜ez and Recio, ities mainly of cistaceous and ericaceous plants or are 1998) (Fig. 4). afforested with Eucalyptus sp.; here, the soils are The hollows show the same range of depths below shallow and have leptic features because of erosion. both planation surfaces NI and NII (Fig. 5). This The physicochemical characteristics of the soil suggests that the differences in elevation resulted from profiles are shown in Tables 1–4. Profile I, a Lep- alpine faulting, as well as the larger Mesozoic mor- tosol, is representative of the soils on steep slopes on phological structures like Appalachian morphologies slaty materials. It has only an A/C1 horizon (15 cm) (Martı´n Serrano, 1988; Molina, 1991; Rodrı´guez thick, and is very weakly developed; it has a weakly Vidal and Diaz del Olmo, 1994). developed coarse granular structure and loamy texture. Its organic matter content is high (5.86%), its pH 3.2. Current soils low (4.6) (Table 2), and the exchange complex is desaturated (Table 3). The features of the profile result From an environmental point of view, the hollows from the high rainfall, above 800 mm/year, and the increase landscape diversity. This is mainly because of acid nature of the bedrock. the extensive horticultural croplands and better devel- Profile II, located in the hollow of Valdelarco, is opment of grassland in the hollows. Both result classified as a eutric Regosol (FAO, 1989); it is a po- mainly from greater water availability in the hollows orly developed soil with an Ap, C1 horizon sequence, and soil differences. The soils on the floors are either yellowish brown (10YR5/6 and 10YR4/4) colours and regosols on colluvial materials accumulated on foot pH of 5.6–6.2. The greater organic matter level in the slopes, or eutric cambisols under pasture in the central C1 horizon (4.07%) compared with the Ap (1.22%) sectors of the hollows. As shown in Fig. 6, we usually (Table 2) suggests continuous inputs of organic mate-

Table 2 Physico-chemical characteristics of profiles I–VI Profile Horizon pH Organic Gravel Sand Silt Clay

(H2O) matter (%) >2mm (%) 2–0.063 mm (%) 0.063–0.002 mm (%) < 0.002 mm (%)

I A/C1 4.6 5.86 38.9 34.5 42.9 22.6 C–– – – – –

II Ap 5.6 1.22 26.0 42.8 30.5 26.6 C1 6.2 4.07 55.5 48.4 28.1 23.6 III Ap 5.7 1.32 74.3 48.6 31.9 19.5 2Bw 6.0 0.68 00.0 9.0 41.8 49.2

2BwC1 6.1 0.23 00.0 19.9 49.4 30.6 IV A1 6.8 1.17 35.7 45.7 26.5 27.8 2Bw 6.1 3.15 00.0 16.9 25.5 57.7

2BC1 6.1 2.22 00.0 27.5 23.4 49.2 R–– – – – –

VAp 6.6 0.31 12.0 42.1 33.5 24.4 2Bw1 6.6 0.70 6.7 38.8 26.4 34.8 2Bw2 7.0 n.d. 0.0 44.2 16.7 39.1 C1 6.5 n.d. 1.5 68.5 18.5 13.6 VI A1 5.6 3.19 37.3 34.2 35.5 30.2 A1C1 5.4 2.59 54.8 20.5 42.7 36.8 2C1 5.4 0.13 1.5 4.4 59.0 36.6 2C2 5.2 n.d. 1.5 7.9 61.1 30.9 2C3 5.4 n.d. 1.5 8.8 66.6 24.6 n.d. = not detected. 204

Table 3 Composition of the exchange complexes in soils of profiles I–VI ..RcoEpj ta./Goopooy4 20)197–209 (2002) 45 Geomorphology / al. et Espejo Recio J.M. Profile Horizon Na + (cmol(+)/kg soil) K + (cmol(+)/kg soil) Ca ++ (cmol(+)/kg soil) Mg ++ (cmol(+)/kg soil) T (cmol(+)/kg soil) S (cmol(+)/kg soil) V (%)

I A/C1 0.70 0.34 1.86 0.83 22.08 3.73 16.89 C– – – – –– – II Ap 0.50 0.47 8.87 6.92 16.88 16.76 99.29 C1 0.68 0.65 8.01 6.71 16.05 16.05 100 III Ap 0.80 0.34 4.37 4.96 10.47 10.47 100 2Bw 0.84 0.20 4.02 10.86 15.92 15.92 100 2BwC1 ––– – ––– IV A1 0.48 0.19 7.55 6.06 19.32 14.28 73.91 2Bw 0.49 2.60 6.96 11.12 26.69 21.17 79.32 2BC1 ––– – ––– R– – – – – – – VAp 0.61 0.15 5.92 3.61 20.71 10.29 49.69 2Bw1 0.58 0.14 6.30 5.45 12.47 12.47 100 2Bw2 ––– – ––– C1 ––– – ––– VI A1 0.51 0.29 3.15 0.94 17.23 4.89 28.38 A1C1 ––– – ––– 2C1 0.36 0.07 1.23 2.22 9.81 3.88 39.55 2C2 ––– – ––– 2C3 0.45 0.09 0.86 1.17 9.39 2.57 27.37 T: exchange capacity, S: exchangeable cations, V: base saturation of exchange complex. J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 205

Table 4 Semiquantitative mineralogical analysis of clay fractions ( < 2 mm) separated from selected profiles Profile Horizon Illite (%) Kaolinite (%) Smectite (%) Vermiculite (%) Interstratified 13 A˚

III Ap 61 33 – Trace – 2Bw 41 37 22 – –

2BwC1 40 40 20 – – IV A1 –44 56 – – 2Bw – 24 70 – 6

2BC1 43 31 26 – VAp 44 23 33 – – 2Bw1 60 30 10 – – 2Bw2 40 30 30 – – C1 43 17 40 – – VI A1 63 37 – – – A1C1 43 46 – 11 – 2C1 20 80 – – – 2C2 19 81 – – – 2C3 40 57 – – 3 rial to the profile. Its texture is loamy and there is contents are 58% and 49% (Tables 2 and 3). The clay abundant gravel ( >2 mm) in both horizons. The minerals in the 2Bw horizon are predominantly smec- exchange complex is saturated (Table 3), and gives tite (70%) and kaolinite (24%) with no illite; in the the soil a eutric character. 2BC1 horizon the proportions of illite, kaolinite and Eutric Cambisols occur on the floors of hollows smectite are nearly equal (Table 4). These results developed directly on plutonic rocks. Profile III, suggest that the 2Bw and 2BC1 horizons constitute developed on quartz–diorites (I.T.G.E., 1990) is be- a truncated paleosol, because formation of smectite neath some examples of dehesa surfaces with Q. and kaolinite has been previously associated with rotundifolia as their main plant cover. Profile III has a remarkable lithological discontinuity resulting from erosion followed by deposition; the surface horizon Table 5 (Ap) is rich in gravel (74.3%) and has 1.32% organic Different forms of iron matter. The 2Bw horizon shows a well developed Profile Horizon % Fed %Feo %Fet %Fed/Fet prismatic structure, brownish-yellow (10YR6/8 and III Ap 1.03 0.22 2.76 37.32 10YR5/8) colours and a clay–loam texture; pH values 2Bw 2.02 0.15 6.34 31.86 are around 6.0 and the exchange complex is saturated 2BwC1 2.43 0.19 8.59 28.29 (Table 3). IV A1 1.51 0.30 3.64 41.48 2Bw 5.02 0.80 18.85 26.63

2BC1 5.26 0.78 15.93 33.02 3.3. Palaeoweathering Lithology – – 0.65 – (gneiss) Profile IV is in the amphibolitic hollow of Calaba- VAp 1.62 0.15 4.60 35.22 2B 1.18 0.16 4.91 24.03 zares (Fig. 4), located more than 80 m above the w1 2Bw2 1.22 0.09 5.62 21.71 present floor of the hollow. It shows a clear litholog- C1 1.07 0.06 5.11 20.94 ical discontinuity between the sandy surface horizon VI A1 2.22 0.18 5.38 42.19 (A1) (dry colour 10YR5/4) and deeper horizons (2Bw A1C1 3.10 0.27 4.43 69.98 and 2BC1) with avery dark brown colour (10YR2/2 2C1 3.14 0.05 5.86 53.58 2C 2.79 0.04 5.70 48.95 dry), a well developed prismatic structure and a clay 2 2C3 3.64 0.03 6.33 57.50 texture. The pH of these deeper horizons is 6.1, the Lithology 4.06 exchange complex is partially saturated, the organic (quartz–diorite) matter values are 3.15% and 2.22% and the clay Fed: dithionite iron, Feo: oxalate iron, Fet: total iron, and Fed/Fet. 206 J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209

Table 6 Mineralogical composition of heavy and light fractions of fine sand (0.5–0.063 mm) from selected profiles Profile Horizon Total heavy Opaque Mg Hm Gt Lc Total light Q Fd Mc Other fraction (%) min. (%) fraction (%)

III Ap 0.8 36.3 C C C F 99.2 A + R O 2Bw 7.3 14.0 A F C R 92.6 A + C C

2BwC1 0.8 49.0 F C A O 99.2 F + A C IV A1 3.6 15.0 A C C + 96.4 CCC À 2Bw 69.2 95.6 A O R À 30.8 AO ÀÀ 2BC1 46.9 97.9 A F O À 53.1 AOC À VAp 11.1 16.5 A C O O 88.9 AOOO 2Bw1 9.7 17.0 A R À R 90.2 AOOC 2Bw2 16.0 7.0 A O O À 84.0 C + C F C1 3.7 11.4 A C C À 96.3 C + C C VI A1 26.7 13.0 C C C O 73.3 A À + À A1C1 9.4 21.7 C C R R 90.6 A À + À 2C1 3.7 74.2 A R C R 96.3 A À ++ 2C2 3.3 91.6 C R A À 96.7 A À C+ 2C3 0.9 40.1 A F F À 99.1 C À C+ Mg = magnetite; Hm = hematite; Gt = goethite; Lc = leucoxene; Q = quartz; Fd = feldspar; Mc = muscovite. Abundant (A = > 52%), Common (C = 10.1–52%), Frequent (F = 5.1–10%), Occasional (O = 1.1–5.1%), Rare (R= 0.3–1%), Traces ( + = < 0.3%). poorly drained hollows under subtropical conditions de Aracena. Profile VI represents the soils they (Duchaufour, 1984; Pedro, 1984). Nuń˜ez et al. described. The paleohorizons have yellowish colours (1998a) described similar palaeosols on plutonic bath- values of pH 5–5.5, low cation exchange capacities, oliths within the Sierra Morena region. Profile V undersaturated exchange complexes and clay contents shows the same general features of the palaeosol: a of 25 37% (Tables 1–3). The Fed/Fet indices between truncated character, well developed prismatic struc- 42% and 70.% are approximately twice those of ture (Table 1), pH between 6.5 and 7 (Table 2), Profiles IV and V (Table 5). The fine sand fractions saturated exchange complex (Table 3) and the occur- of Profile VI (Table 6) consist mainly of quartz with rence of smectite and illite with subordinate kaolinite only small amounts of weatherable minerals (mica in the clay fraction (Table 4). and feldspar) and the clay fractions consist mainly of In Profile IV the amounts of total iron (18.85% and kaolinite with subordinate illite but no smectite (Table 15.93%) and dithionite-extractable iron (5.02% and 4). All these characteristics suggest that Profile VI and 5.26%) in the 2Bw and 2BC1 horizons, respectively similar soils are relict soils formed under subtropical (Table 5) give a weathering index (Fed/Fet) of approx- conditions. imately 25%. Similar Fed/Fet values occur in Profile V, though the actual values of Fet and Fed are less. These index values suggest quite strong weathering. 4. Discussion The larger Fet content of Profile IV is partly explained by the predominance of magnetite in the fine sand A subtropical climate in the western Mediterranean fraction of the 2Bw and 2BC1 horizons (69.17% and has been suggested for the Plio-Pleistocene from 46.90%, respectively) (Table 6). In contrast, the fine studies of paleosols and associated sediments (Espejo, sand in Profile V consists mainly of light minerals, 1985; Pendoń and Rodrı´guez Vidal, 1986; Martıń especially quartz. These differences are related to the Serrano, 1989), and from palynology (Suc, 1980; nature of the parent materials, as amphibolites are Suc et al. 1995). Nuń˜ez et al. (1998a,b) suggested much richer in iron than quartz diorities. that these kaolinitic soils without smectite were Nuń˜ez et al. (1998a) reported relict kaolinitic soil formed under subtropical environmental conditions horizons derived from quartz diorite preserved on contemporaneous to the smectitic–kaolinitic profiles planation surfaces near some hollows in the Sierra described earlier (Profiles III–V). J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 207

Fig. 7. Evolution of the hollows from Mesozoic to Quaternary. 208 J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209

On the basis of the morphological and weathering areas. Their ecological characteristics (dehesa vegeta- studies of the hollows in the Sierra Aracena, we tion) are thus clearly related to their geology history. propose an evolutionary model for the hollows (Fig. 7). The macroforms of the Hercynian Massif includ- ing the hollows present in the south-western sector Acknowledgements originated by differential weathering of bedrock types with variable susceptibility to weathering because of We thank J.A. Catt for useful comments on a draft different mineralogical composition and permeability of manuscript and the Andalusian Government for (fracturing). Subtropical Tertiary or Plio-Pleistocene financial support. conditions led to development of kaolinitic soils on the upper planation surface and of smectitic and kaolinitic soils in the hollows. References The different tectonic pulses which affected the Iberian basement during the late Cenozoic produced Barroń, V., Torrent, J., 1986. Use of the Kubelka–Munk theory to many fault-bounded blocks throughout the Hercynian study the influence of iron oxides on soil colour. Journal of Soil Science 37, 499–510. Massif (Rodrı´guez Vidal and Dıáz del Olmo, 1994). Brindley, C.W., Brown, C., 1980. Crystal Structures of Clay Min- This tectonic reactivation, together with a change to a erals and their X-ray Identification. Mineralogical Society, Lon- cooler and drier climate provoked a change in fluvial don. activity with greater erosion. This favoured connec- Cano, M.D., Recio, J.M., 1996. Formaciones terras-rossas sobre tion of the weathering hollows to the fluvial network calizas ca´mbricas en Sierra Morena Central (Hornachuelos, Co´r- doba). Cuaternario y Geomorfologıá 10, 79–88. and deepened them by evacuation of the thick weath- Dıáz del Olmo, F., Rodrı´guez Vidal, J., 1989. Macizo Hespe´rico ered regolith. Meridional. V. Bielza de Ory, Territorio and Sociedad en Espa- The general lowering of base level which charac- nã. Taurus, Madrid, pp. 70–80. terised the Quaternary fluvial evolution of the region Duchaufour, Ph., 1975. Edafologıá. Toray-Masson, Barcelona. (Dıáz del Olmo and Rodrı´guez Vidal, 1989) deepened Duchaufour, Ph., 1984. Edafologıà: 1. Edafogeńesis y clasificacioń. Masson, Barcelona. the hollows further by erosion of their floors. The Espejo, R., 1985. The ages and soils of two levels of Ranã surfaces current morphology of the various hollows suggests in Central Spain. Geoderma 35, 223–239. that the erosion was especially intense on amphibolitic FAO, 1977. Guıá para la descripcioń de perfiles de suelos. F.A.O, bedrock to create the hollows, whereas flat beds were Roma. preserved on the plutonic rocks. F.A.O., 1989. Carte mondiale des sols 1:5.000.000, Rome. Godard, A., 1977. Pays et paysajes du granite. Ed. PUF. Vendome. 232 pp. Guitiań, F., Carballas, T., 1976. Tećnicas de ana´lisis de suelos. Pico- 5. Conclusions Sacro, Santiago. IGME, 1982. Mapa geolo´gico a escala 1:50.000 de la Hoja no. 916 The hollows of the Sierra de Aracena (Sierra Mor- (Nerva), Madrid. IGME, 1983. Mapa geolo´gico a escala 1:50.000 de la Hoja no. 916 ena region) have considerable environmental and eco- 2 (Aroche), Madrid. logical significance. They are 0.2–3 km in area, up to IGME, 1984. Mapa geolo´gico a escala 1:50.000 de la Hoja no. 917 150 m deep and subcircular form. The main factors that (Aracena), Madrid. controlled their formation were the deeper weathering ITGE, 1990. Mapa geolo´gico a escala 1:50.000 de la Hoja no. 918 in the Mesozoic of plutonic and amphibolitic rocks (Santa Olalla del Cala), Madrid. Martıń Serrano, A., 1988. El relieve de la regioń occidental zamor- compared with metasedimentary rocks, and fluvial ona. La evolucioń geomorfolo´gica de un borde del Macizo Hes- erosion and exhumation of the weathered material pe´rico. Diputacioń de Zamora, Zamora. during the Plio-Pleistocene. Martıń Serrano, A., 1989. Caracterı´sticas, rango, significado and The hollows are now 100–150 m deep below the correlacioń de las Series Ocres del borde occidental de la Cuen- general planation surfaces and show strong relation- ca del Duero. Studia Geologica Salmanticensia 5, 239–252. Mehra, O.P., Jackson, M.L., 1960. Iron oxide removal from soils ships between bedrock lithology, soils (Regosols and and clays by a dithionite–citrate system buffered with sodium Cambisols) and vegetation (grazing and pasture lands), bicarbonate. 7th National Conference on Clays and Clay Min- which are very different from those of the surrounding eral, Washington, 317–327. J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 209

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