Geoscience Reference
In-Depth Information
Badlands
KEY CONCEPTS
The visitor to the Mediterranean never forgets the spectacular 'badland' scenery which, though highly localized, leaves
an unforgettable impression; this is the landscape of the 'spaghetti' westerns, and the glossy photos of 'desertified'
areas in colour magazines. Badlands are named after the mauvaises terres seen by eighteenth-century French
travellers in the Rif mountains, Morroco, North Africa. In the Mediterranean region, badlands are found in Spain (Guadix,
Tabernas, Murcia, the Ebro valley), in Italy (Tuscany, Basilicata), Greece (Grevena, Corinth, Serres), in Asia Minor (the
Maeander valley in south-west Turkey) and in Israel (Negev). Badlands also occur in the Mediterranean parts of Australia
and California. The Mediterranean-type climate favours their formation, though they are not restricted to the
Mediterranean-type environment, and are found in continental climates (the Great Plains of the United States, the
Dinosaur and Big Muddy Badlands of Alberta, Canada) and even in warm-temperate and tropical locations (Nigeria;
Georgia and Alabama in the United States).
The dominant geomorphic process in these highly eroded and dissected landscapes is water incision to give gullying
(
Plate 25.8
).
A
gully
is a large, well defined channel, typically V-shaped, whereas a
rill
is clearly defined but smaller
and impermanent, often being destroyed by flaking of the surface by drying, by frost or by ploughing. Rills are more
common in ploughed fields and road cuts, but gullies can form on both cultivated and uncultivated land. A badland
area typically has an extensive network of channels and valleys with steep and bare slopes, a high drainage density
of over 50 km km
-2
and a large relative relief of up to 30 m. Overland flow carries out the bulk of the erosion, and
this explains why most badlands are restricted to soft rocks such as marls and clays of Tertiary and Quaternary age
with very low infiltration rates. Often these materials have a high sodium content and therefore disperse when exposed
to any surface wetting, to form crusts that seal the surface on drying. They are often colonized by lichens which
appear dead when dried out but swell back into life when wetted, forming a biological crust. Even light rain does not
infiltrate, but runs off to erode the sides and bottoms of gullies.
Secondary factors and processes are also important in explaining the distribution of badlands. Gullying often goes
hand in hand with piping, an underground network of pipes formed from cracks, joints and minor faults. Pipes enlarge
into tunnels, some large enough to walk up, then often collapse into gullies. Piping is more common in marls containing
salts and gypsum, both of which are soluble, and in 'cracking' clays such as montmorillonite which shrink and crack
and so are especially disposed to piping. Networks of pipes develop where large hydraulic gradients develop at the
base of steep slopes. Outflow of seepage water carries fine particles which start to excavate a tunnel from the
downflow end. Steep steps in the initial slope profile help to drive high hydraulic gradients, so piping is stimulated
by human actions such as road building, agricultural terracing and the construction of earth dams for reservoirs.
Slumping also contributes to the chaotic landscape of badlands. If the marls and clays are able to develop vertical
cracks, the channels will take on a rectangular cross-section, with vertical walls and a flat valley floor, which get larger
through the collapse of the walls of marl.
Badlands seem to have various causes. Those of the Great Plains, Utah and Alberta in North America are natural,
whereas those of California, Arizona and Georgia are attributed to the impact of European farming practices.
Badlands give the appearance of extremely rapid rates of erosion, with fresh gullies and actively eroding slopes, but
erosion rates are extremely variable. In the Ebro valley of north-east Spain erosion rates of 15-20 mm per year have
been measured. However, in the Guadix badlands of the Spanish Sierra Nevada, famous for its cliff houses or
troglodyte dwellings, erosion rates are very slow. From the evidence of 4 ka old archaeological structures, the badlands
here appear to have suffered little erosion during this time, with rates of only about 0·01 mm per year (Wise et al.
1982).
Some support for this view has come from the Tabernas badlands of south-east Spain, which have been the site of
long-term monitoring of run-off and erosion under the direction of Professor Puigdefábregas (Cantón et al. 2001). In
their instrumented micro-catchments most of the run-off is generated on bare and lichen-covered soil surfaces. Areas
protected by esparto grass (Stipa tenacissima), and other grasses and herbs, have low erosion rates. Erosion rates
in individual gullies can reach 2,800 g m
-2
, but in micro-catchments of about 2 ha they ranged between 100 g m
-2
