Tropical landforms
567
an old fortress, and are named castellated forms (castle kopjes = koppies - rock towers).
Some castle koppies are dominated by spheroidally-weathered corestones. On the other
hand, rounded boulders can occur in any climatic environment.
The origin of the boulders supposes a deep differential weathering, which deepens with
higher intensity in zones of intense fracturing. Afterwards, an environmental change
brings about the saprolite erosion. As a result, the weathering profile base outcrops,
characterized by the presence of boulders that originate the above-mentioned forms
(Figure 21.33) (Linton, 1955). This author referred to this sequence of processes as a twostage hypothesis. Boulder generation can be observed on roadcuts and lineal works. The
balance between weathering and denudation must be kept in mind. If weathering is more
intense than erosion, then neither the weathering front nor the associated blocks outcrop.
On the other hand, when erosion is more intense than weathering, then the profile is
dismantled and basal blocks appear (Vidal Romanf and Twidale, 1998). Outcropping
boulders are exposed to subaerial weathering. The regolith can be removed by fluvial
action and give rise to block channels more than 10 km long, such as the Homen River of
north Portugal. Saprolite exportation can also take place by marine, glacial and eolian
erosion (Vidal Romanf and Twidale, 1998).
In general, the development of many boulder inselbergs is related to the decay of blocks
located within the inselberg. The main processes are block-weathering, grus erosion and
land surface lowering, which can lead to the disappearance of the boulder inselberg
(Figure 21.34) (Thomas, 1978).
As indicated above, boulders have a wide distribution all over the world and, because
deep weathering is required, in many zones they are assumed to be a very old alteration
(a)
A
__
A
9 I'
(b) ^
(c) A
........
~176
9 ,
9 ,,
A
.......
. ........
~ ...........
~ ..............
~176176176 ........
Major Joints
Growan
Corestones
Figure 21.33. The evolution of the tors and granite landscapes based on the drawings of Linton (1955).
567
an old fortress, and are named castellated forms (castle kopjes = koppies - rock towers).
Some castle koppies are dominated by spheroidally-weathered corestones. On the other
hand, rounded boulders can occur in any climatic environment.
The origin of the boulders supposes a deep differential weathering, which deepens with
higher intensity in zones of intense fracturing. Afterwards, an environmental change
brings about the saprolite erosion. As a result, the weathering profile base outcrops,
characterized by the presence of boulders that originate the above-mentioned forms
(Figure 21.33) (Linton, 1955). This author referred to this sequence of processes as a twostage hypothesis. Boulder generation can be observed on roadcuts and lineal works. The
balance between weathering and denudation must be kept in mind. If weathering is more
intense than erosion, then neither the weathering front nor the associated blocks outcrop.
On the other hand, when erosion is more intense than weathering, then the profile is
dismantled and basal blocks appear (Vidal Romanf and Twidale, 1998). Outcropping
boulders are exposed to subaerial weathering. The regolith can be removed by fluvial
action and give rise to block channels more than 10 km long, such as the Homen River of
north Portugal. Saprolite exportation can also take place by marine, glacial and eolian
erosion (Vidal Romanf and Twidale, 1998).
In general, the development of many boulder inselbergs is related to the decay of blocks
located within the inselberg. The main processes are block-weathering, grus erosion and
land surface lowering, which can lead to the disappearance of the boulder inselberg
(Figure 21.34) (Thomas, 1978).
As indicated above, boulders have a wide distribution all over the world and, because
deep weathering is required, in many zones they are assumed to be a very old alteration
(a)
A
__
A
9 I'
(b) ^
(c) A
........
~176
9 ,
9 ,,
A
.......
. ........
~ ...........
~ ..............
~176176176 ........
Major Joints
Growan
Corestones
Figure 21.33. The evolution of the tors and granite landscapes based on the drawings of Linton (1955).