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.
J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 201
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
J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209202
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.
J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 203
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.
J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 205
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%).
J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209206
Fig. 7. Evolution of the hollows from Mesozoic to Quaternary.
J.M. Recio Espejo et al. / Geomorphology 45 (2002) 197–209 207
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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