570
Climatic Geomorphology
Iregua River, and elsewhere, which form part of the marginal Tertiary units of the Ebro
Depression in Spain.
Bornhardt slopes are commonly steep and with a very variable size, from some metres
to several hundreds of metres (Figure 21.36). Their plan form presents circular or elliptical
morphologies, commonly affected by fracturing (King, 1948; Twidale and Bourne, 1978).
Where elongated and with low height, they are called "whalebacks" or "ruwares." Some
bornhardts appear isolated, while others cluster to form hemispheric groups, known in
Brazil as half-oranges (Birot, 1958). Contacts between the rock monolith and the
bornhardt foot can be sharp, with a clear knickpoint, or transition with no evident slope
modification. Several causes exist for explaining a sudden slope change: contact between
rocks of different mechanical properties, contrast between fresh rock and saprolite (Ollier,
1960; Thomas, 1965) and intense weathering at the slope base (Ruxton, 1958; Twidale,
1962; Mabbut, 1966; Twidale and Bourne, 1978).
Bornhardts can occur in different climatic environments (Kesel, 1973), from equatorial
rainforest zones, or intermediate latitudes (Figure 21.37 and Figure 21.38), to the north of
Norway (Btidel, 1978). For the Hercynian Massif of the Iberian Peninsula, numerous
works exist about granite bornhardts (Birot, 1949b; Birot and Sol& 1954a,b; Brum
Ferreira, 1978; Coud4-Gaussen, 1981; Sanz, 1988; Pedraza et al., 1989; Vidal Romanf and
Twidale, 1998, among others). They have also been recognized in granites of the Catalan
coastal ranges (Roqu4 and Pallf, 1998).
A set of factors exists that influences the location of domed inselbergs. Most studies
focused on checking bornhardt location in relation to rock type have concluded the
absence of lithological variations in their piedmont. Some studies, however, highlight a
Figure 21.36. Domed inselberg several hundreds of metres high. Abuja, central Nigeria. Photo: J. Ldpez
Martfnez.
Climatic Geomorphology
Iregua River, and elsewhere, which form part of the marginal Tertiary units of the Ebro
Depression in Spain.
Bornhardt slopes are commonly steep and with a very variable size, from some metres
to several hundreds of metres (Figure 21.36). Their plan form presents circular or elliptical
morphologies, commonly affected by fracturing (King, 1948; Twidale and Bourne, 1978).
Where elongated and with low height, they are called "whalebacks" or "ruwares." Some
bornhardts appear isolated, while others cluster to form hemispheric groups, known in
Brazil as half-oranges (Birot, 1958). Contacts between the rock monolith and the
bornhardt foot can be sharp, with a clear knickpoint, or transition with no evident slope
modification. Several causes exist for explaining a sudden slope change: contact between
rocks of different mechanical properties, contrast between fresh rock and saprolite (Ollier,
1960; Thomas, 1965) and intense weathering at the slope base (Ruxton, 1958; Twidale,
1962; Mabbut, 1966; Twidale and Bourne, 1978).
Bornhardts can occur in different climatic environments (Kesel, 1973), from equatorial
rainforest zones, or intermediate latitudes (Figure 21.37 and Figure 21.38), to the north of
Norway (Btidel, 1978). For the Hercynian Massif of the Iberian Peninsula, numerous
works exist about granite bornhardts (Birot, 1949b; Birot and Sol& 1954a,b; Brum
Ferreira, 1978; Coud4-Gaussen, 1981; Sanz, 1988; Pedraza et al., 1989; Vidal Romanf and
Twidale, 1998, among others). They have also been recognized in granites of the Catalan
coastal ranges (Roqu4 and Pallf, 1998).
A set of factors exists that influences the location of domed inselbergs. Most studies
focused on checking bornhardt location in relation to rock type have concluded the
absence of lithological variations in their piedmont. Some studies, however, highlight a
Figure 21.36. Domed inselberg several hundreds of metres high. Abuja, central Nigeria. Photo: J. Ldpez
Martfnez.