the origin of the sierra de aracena hollows in the sierra...

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. Nunez Granados a

aEcology (Physical Environment-Geomorphology), Campus de Rabanales, University of Cordoba, 14071-Cordoba, SpainbLehrstuhl Physische Geographie, Katholische Universitat Eichstatt, Ostenstra�e, 26, D-85072, Eichstatt, 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

The western sector of the Sierra Morena, the Sierra

of Aracena, is formed mainly of Precambrian and

Palaeozoic rocks typical of the Iberian Hercynian

massif (Fig. 1). This sector is characterised by large

morphological features such as planation surfaces and

Appalachian morphologies. The planation surfaces are

cut across plutonic rocks and schists, forming two

main levels at about 600–700 and 400–500 m above

sea level (Nunez and Recio, 1998); these are termed

surfaces I and II, respectively. The Appalachian mor-

phologies occur mainly on carbonate and metasedi-

mentary lithologies. These is also a series of Quater-

nary erosional river terraces developed on both the

planation surfaces and occurring mainly in narrow

valleys.

A series of enclosed hollows up to 3 km2 in area

and 150 m deep stand out in the landscape because of

their unusual geoecological characteristics. They

occur all over the western Sierra Morena (Fig. 2)

and are delimited by a different vegetation from the

surrounding areas, by their great depth and by the

eutrophic nature of the soils on the floors of the

hollows. We studied the morphology and genesis of

the hollows of the Sierra de Aracena, paying special

attention to palaeo-weathering features in them.

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

temperatures of 14–17 �C. These climatic factors

explain the establishment of umbraphile communities,

0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0169 -555X(01 )00154 -4

* Corresponding author.

E-mail address: [email protected] (J.M. Recio Espejo).

www.elsevier.com/locate/geomorph

Geomorphology 45 (2002) 197–209

such as gall oak and chestnut groves (with Quercus

faginea and Castanea sativa as the basic species) and

oak and cork oak groves (Q. suber and Q. rotundifo-

lia) as the most frequent communities all over the

Sierra Morena. The communities are part of one of the

typical cultural landscapes of grazing land with sparse

forest in the Andalucian region.

Under conditions of maximal rainfall and north-

ward exposure, this climatic regime would produce

acidic umbric soils rich in organic matter. However,

all the soils of the western Sierra Morena are poorly

developed Regosols, Leptosols and Cambisols

because of erosive processes accelerated by human

activities over the last two millennia. More strongly

developed soils, such as Luvisols and Acrisols are

relict from an earlier period (Cano and Recio, 1996).

Tropical conditions dominated the environment of

the Iberian basement during the Mesozoic (Rat, 1982).

Together with tectonic stability this allowed weath-

ering processes to dominate Mesozoic morphogenesis

(Molina, 1991; Martın Serrano, 1988). Some traces of

the resulting soils in the north-western sector of the

Iberian basement (River Duero basin) have been

described by Molina et al. (1990).

Hollows similar to these of the western Sierra

Morena have been described by Godard (1977), Twi-

dale (1982) and Ollier (1984) on plutonic rocks of the

French Central Massif, USA (Davil’s Marble) and the

Murrmungee Basin in Australia, respectively. For these

authors increased weathering compared with surround-

ing areas and fluvial removal of weathering products

were the main factors responsible for formation of the

enclosed hollows. Godard (1977) pointed out the

importance of different rock properties, and Twidale

(1982) related the genesis of hollows to differential

weathering in granitic landscapes. Ollier (1984) des-

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

J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209198

cribed hollows 100 m deep below a planation surface,

their floors covered with alluvial sediments.

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 classi-

fied using FAO (1977, 1989). Colours were defined

according to Munsell Colour (1990). pH in water was

determined by the method of Guitian and Carballas

(1976), carbonate by the method of Duchaufour

(1975), organic matter by the Sims and Haby (1971)

method, granulometry according to Soil Survey of

England and Wales (1982), and exchangeable ions by

Fig. 3. Geomorphology and bedrock geology of the hollow of

Dehesa de Valle Torres.

Fig. 2. Main hollows of the Aracena Massif and location of the studied soil profiles.

J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 199

Fig. 4. Topographic plans and sections of a plutonic hollow (Santa Eulalia) and an amphibolitic hollow (Calabazares).

J.M.Recio

Espejo

etal./Geomorphology45(2002)197–209

200

the methods of Pinta (1971) and Guitian and Carballas

(1976). Clay minerals were quantified according to

Montealegre (1976) and Brindley and Brown (1980).

The forms of iron were determined according to Mehra

and Jackson (1960), Barron and Torrent (1986) and

Torrente and Cabedo (1986). The mineralogy of sand

Fig. 5. Topographical relationship of hollows to planation surfaces levels NI and NII.

Fig. 6. Current generalised relationships between lithology, soils and vegetation in the hollows.


fractions was determined by the methods of Partenoff

et al. (1970).

3. Results

3.1. Morphological features

A total of 16 hollows was described within the

3250 km2 of our study area (Fig. 2). Most are near

Sierra de Aracena, which is in the central sector of the

western Sierra Morena. The hollows are 0.2–3 km2 in

area, with a circular or subcircular outline and steep

(f 15�) side slopes and depths of 100–150 m.

Amphibolitic and plutonic rocks (quartz–diorites

and diorites) occur on their floors, and their marginal

slopes are usually of acid metasedimentary rocks

(phyllites and schists) (Figs. 3 and 4). This suggests

that the main factor controlling the presence of hol-

lows is differential weathering of the various bedrock

types. Weathering would have affected the amphib-

olitic and plutonic lithologies to a greater extent as

they are richer in weatherable minerals and more

permeable than the acid metasedimentary rocks,

which are composed mainly of quartz. In some

hollows, there is a clear relationship between the fault

pattern (determining changes in bedrock) and the

margin of the hollows. In other situations, the role

of tectonics in the genesis of these forms is less clear.

All the hollows have been captured and excavated

by the present fluvial systems. Fluvial action seems to

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)

V Ap 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


logical forms of hollows: morphologies exhumed with

flat beds in plutonic hollows, and some others in

which the weak nature of amphibolites impedes the

conservation of this morphologies (Nunez and Recio,

1998) (Fig. 4).

The hollows show the same range of depths below

both planation surfaces NI and NII (Fig. 5). This

suggests that the differences in elevation resulted from

alpine faulting, as well as the larger Mesozoic mor-

phological structures like Appalachian morphologies

(Martın Serrano, 1988; Molina, 1991; Rodrıguez

Vidal and Diaz del Olmo, 1994).

3.2. Current soils

From an environmental point of view, the hollows

increase landscape diversity. This is mainly because of

the extensive horticultural croplands and better devel-

opment of grassland in the hollows. Both result

mainly from greater water availability in the hollows

and soil differences. The soils on the floors are either

regosols on colluvial materials accumulated on foot

slopes, or eutric cambisols under pasture in the central

sectors of the hollows. As shown in Fig. 6, we usually

find dehesa or grazing land with Q. rotundifolia on

the deeper soils in the hollows. The steep side slopes

of the hollows are occupied by shrub-like commun-

ities mainly of cistaceous and ericaceous plants or are

afforested with Eucalyptus sp.; here, the soils are

shallow and have leptic features because of erosion.

The physicochemical characteristics of the soil

profiles are shown in Tables 1–4. Profile I, a Lep-

tosol, is representative of the soils on steep slopes on

slaty materials. It has only an A/C1 horizon (15 cm)

thick, and is very weakly developed; it has a weakly

developed coarse granular structure and loamy tex-

ture. Its organic matter content is high (5.86%), its pH

low (4.6) (Table 2), and the exchange complex is

desaturated (Table 3). The features of the profile result

from the high rainfall, above 800 mm/year, and the

acid nature of the bedrock.

Profile II, located in the hollow of Valdelarco, is

classified as a eutric Regosol (FAO, 1989); it is a po-

orly developed soil with an Ap, C1 horizon sequence,

yellowish brown (10YR5/6 and 10YR4/4) colours and

pH of 5.6–6.2. The greater organic matter level in the

C1 horizon (4.07%) compared with the Ap (1.22%)

(Table 2) suggests continuous inputs of organic mate-

Table 2

Physico-chemical characteristics of profiles I–VI

Profile Horizon pH

(H2O)

Organic

matter (%)

Gravel

>2mm (%)

Sand

2–0.063 mm (%)

Silt

0.063–0.002 mm (%)

Clay

< 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 – – – – – –

V Ap 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.


Table 3

Composition of the exchange complexes in soils of profiles I–VI

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 – – – – – – –

V Ap 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

etal./Geomorphology45(2002)197–209

204

rial to the profile. Its texture is loamy and there is

abundant gravel ( >2 mm) in both horizons. The

exchange complex is saturated (Table 3), and gives

the soil a eutric character.

Eutric Cambisols occur on the floors of hollows

developed directly on plutonic rocks. Profile III,

developed on quartz–diorites (I.T.G.E., 1990) is be-

neath some examples of dehesa surfaces with Q.

rotundifolia as their main plant cover. Profile III has

a remarkable lithological discontinuity resulting from

erosion followed by deposition; the surface horizon

(Ap) is rich in gravel (74.3%) and has 1.32% organic

matter. The 2Bw horizon shows a well developed

prismatic structure, brownish-yellow (10YR6/8 and

10YR5/8) colours and a clay–loam texture; pH values

are around 6.0 and the exchange complex is saturated

(Table 3).

3.3. Palaeoweathering

Profile IV is in the amphibolitic hollow of Calaba-

zares (Fig. 4), located more than 80 m above the

present floor of the hollow. It shows a clear litholog-

ical discontinuity between the sandy surface horizon

(A1) (dry colour 10YR5/4) and deeper horizons (2Bw

and 2BC1) with avery dark brown colour (10YR2/2

dry), a well developed prismatic structure and a clay

texture. The pH of these deeper horizons is 6.1, the

exchange complex is partially saturated, the organic

matter values are 3.15% and 2.22% and the clay

contents are 58% and 49% (Tables 2 and 3). The clay

minerals in the 2Bw horizon are predominantly smec-

tite (70%) and kaolinite (24%) with no illite; in the

2BC1 horizon the proportions of illite, kaolinite and

smectite are nearly equal (Table 4). These results

suggest that the 2Bw and 2BC1 horizons constitute

a truncated paleosol, because formation of smectite

and kaolinite has been previously associated with

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 –

V Ap 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

Table 5

Different forms of iron

Profile Horizon % Fed % Feo % Fet % Fed/Fet

III Ap 1.03 0.22 2.76 37.32

2Bw 2.02 0.15 6.34 31.86

2BwC1 2.43 0.19 8.59 28.29

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

Lithology

(gneiss)

– – 0.65 –

V Ap 1.62 0.15 4.60 35.22

2Bw1 1.18 0.16 4.91 24.03

2Bw2 1.22 0.09 5.62 21.71

C1 1.07 0.06 5.11 20.94

VI A1 2.22 0.18 5.38 42.19

A1C1 3.10 0.27 4.43 69.98

2C1 3.14 0.05 5.86 53.58

2C2 2.79 0.04 5.70 48.95

2C3 3.64 0.03 6.33 57.50

Lithology

(quartz–diorite)

4.06

Fed: dithionite iron, Feo: oxalate iron, Fet: total iron, and Fed/Fet.


poorly drained hollows under subtropical conditions

(Duchaufour, 1984; Pedro, 1984). Nunez et al.

(1998a) described similar palaeosols on plutonic bath-

oliths within the Sierra Morena region. Profile V

shows the same general features of the palaeosol: a

truncated character, well developed prismatic struc-

ture (Table 1), pH between 6.5 and 7 (Table 2),

saturated exchange complex (Table 3) and the occur-

rence of smectite and illite with subordinate kaolinite

in the clay fraction (Table 4).

In Profile IV the amounts of total iron (18.85% and

15.93%) and dithionite-extractable iron (5.02% and

5.26%) in the 2Bw and 2BC1 horizons, respectively

(Table 5) give a weathering index (Fed/Fet) of approx-

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.

The larger Fet content of Profile IV is partly explained

by the predominance of magnetite in the fine sand

fraction of the 2Bw and 2BC1 horizons (69.17% and

46.90%, respectively) (Table 6). In contrast, the fine

sand in Profile V consists mainly of light minerals,

especially quartz. These differences are related to the

nature of the parent materials, as amphibolites are

much richer in iron than quartz diorities.

Nunez et al. (1998a) reported relict kaolinitic soil

horizons derived from quartz diorite preserved on

planation surfaces near some hollows in the Sierra

de Aracena. Profile VI represents the soils they

described. The paleohorizons have yellowish colours

values of pH 5–5.5, low cation exchange capacities,

undersaturated exchange complexes and clay contents

of 25 37% (Tables 1–3). The Fed/Fet indices between

42% and 70.% are approximately twice those of

Profiles IV and V (Table 5). The fine sand fractions

of Profile VI (Table 6) consist mainly of quartz with

only small amounts of weatherable minerals (mica

and feldspar) and the clay fractions consist mainly of

kaolinite with subordinate illite but no smectite (Table

4). All these characteristics suggest that Profile VI and

similar soils are relict soils formed under subtropical

conditions.

4. Discussion

A subtropical climate in the western Mediterranean

has been suggested for the Plio-Pleistocene from

studies of paleosols and associated sediments (Espejo,

1985; Pendon and Rodrıguez Vidal, 1986; Martın

Serrano, 1989), and from palynology (Suc, 1980;

Suc et al. 1995). Nunez et al. (1998a,b) suggested

that these kaolinitic soils without smectite were

formed under subtropical environmental conditions

contemporaneous to the smectitic–kaolinitic profiles

described earlier (Profiles III–V).

Table 6

Mineralogical composition of heavy and light fractions of fine sand (0.5–0.063 mm) from selected profiles

Profile Horizon Total heavy

fraction (%)

Opaque

min. (%)

Mg Hm Gt Lc Total light

fraction (%)

Q Fd Mc Other

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 C C C �2Bw 69.2 95.6 A O R � 30.8 A O � �2BC1 46.9 97.9 A F O � 53.1 A O C �

V Ap 11.1 16.5 A C O O 88.9 A O O O

2Bw1 9.7 17.0 A R � R 90.2 A O O C

2Bw2 16.0 7.0 A O O � 84.0 C + C F

C1 3.7 11.4 A C C � 96.3 C + C C

VI A1 26.7 13.0 C C C O 73.3 A � + �A1C1 9.4 21.7 C C R R 90.6 A � + �2C1 3.7 74.2 A R C R 96.3 A � + +

2C2 3.3 91.6 C R A � 96.7 A � C +

2C3 0.9 40.1 A F F � 99.1 C � 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%).


Fig. 7. Evolution of the hollows from Mesozoic to Quaternary.


On the basis of the morphological and weathering

studies of the hollows in the Sierra Aracena, we

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

originated by differential weathering of bedrock types

with variable susceptibility to weathering because of

different mineralogical composition and permeability

(fracturing). Subtropical Tertiary or Plio-Pleistocene

conditions led to development of kaolinitic soils on

the upper planation surface and of smectitic and

kaolinitic soils in the hollows.

The different tectonic pulses which affected the

Iberian basement during the late Cenozoic produced

many fault-bounded blocks throughout the Hercynian

Massif (Rodrıguez Vidal and Dıaz del Olmo, 1994).

This tectonic reactivation, together with a change to a

cooler and drier climate provoked a change in fluvial

activity with greater erosion. This favoured connec-

tion of the weathering hollows to the fluvial network

and deepened them by evacuation of the thick weath-

ered regolith.

The general lowering of base level which charac-

terised the Quaternary fluvial evolution of the region

(Dıaz del Olmo and Rodrıguez Vidal, 1989) deepened

the hollows further by erosion of their floors. The

current morphology of the various hollows suggests

that the erosion was especially intense on amphibolitic

bedrock to create the hollows, whereas flat beds were

preserved on the plutonic rocks.

5. Conclusions

The hollows of the Sierra de Aracena (Sierra Mor-

ena region) have considerable environmental and eco-

logical significance. They are 0.2–3 km2 in area, up to

150 m deep and subcircular form. The main factors that

controlled their formation were the deeper weathering

in the Mesozoic of plutonic and amphibolitic rocks

compared with metasedimentary rocks, and fluvial

erosion and exhumation of the weathered material

during the Plio-Pleistocene.

The hollows are now 100–150 m deep below the

general planation surfaces and show strong relation-

ships between bedrock lithology, soils (Regosols and

Cambisols) and vegetation (grazing and pasture lands),

which are very different from those of the surrounding

areas. Their ecological characteristics (dehesa vegeta-

tion) are thus clearly related to their geology history.

Acknowledgements

We thank J.A. Catt for useful comments on a draft

of manuscript and the Andalusian Government for

financial support.

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the origin of the sierra de aracena hollows in the sierra...

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