Walker - 1967 - A geography of Italy

STRUCTURE
as exemplified in the small section o f the Carso left to Italy, the Norcia plateau,
the Gran Sasso and the Sette Comuni, are rarer than one would expect. T o judge
from the dryness o f the caves in many areas (e.g. the Murge and the Sette
Comuni) subterranean erosion is not proceeding very actively at present;
similarly many o f the surface features, for example the dry valleys (gravine) o f
the Murge, the deep gorges o f the Astico and Cismon, and the dolines o f Cima
Dodici, must be attributed to conditions no longer obtaining - a fossil landscape
in fact. On the other hand the marls and chalks show clearly the scars o f contemporary
surface erosion.
With their meagre soils (see p. 88) the limestone massifs are usually o f little
agricultural value and wide expanses are abandoned to pasture and forest. The
profitable areas are the tectonic depressions which are frequently floored with
fertile upper Tertiary and Quaternary lacustrine deposits (Rieti, Aquila, Sulmona).
Water supplies are a major problem with an obvious bearing on the
distribution o f settlement. In fact, the main beneficiaries o f limestone zones are
often neighbouring areas at a lower level, which enjoy the water supplies stored
in the calcareous massifs; the basins o f the lower Tiber, Pescara and Volturno
are favoured in this way.
THE TERTIARY SEDIMENTS. Structurally (as noted above) there is a distinction
between the folded Tertiaries (Eocene, Oligocène and Miocene), which form
the Apennine foundation, and the Pliocene sediments (molasse) which frequently
overlie them and which were uplifted in the post-Pliocene without serious disturbance.
Resistant rocks do occur in the Tertiaries, the Oligocène sandstone o f
the northern Apennines for example, but as a group their main characteristics
are their instability and their susceptibihty to erosion, weaknesses which are the
more readily exploited because o f the torrential nature o f the rainfall and the
considerable altitudes at which the rocks occur. T o an English eye the ferocity
of erosion revealed in the landscape is often shocking. On quite moderate slopes
the Eocene and Pliocene clays and the argille scagliose slump down repeatedly
when saturated, leaving a shallow depression uphill and a lumpy mass o f debris
below; such landslips are known ssfrane. Where the slopes are less vigorous the
clays and marls tend to erode into a monotonously undulating surface, but
locally a chaotic relief o f fretted domes and mud-choked depressions may result;
this is the case in the badlands o f the Crete Senesi. Elsewhere the clays may be
scarred by a maze o f steep gullies (calancM) so that settlement and communications
must seek the temporary stability o f the crests. When weakly consolidated
sandstones cap a succession o f sands and clays, as is often the case in the Pliocene
sediments, the tabular smface is deeply etched by the dendritic drainage pattern
and the plateau edges fall away in steep, fretted scarps which give way to calanchi
as the torrents reach the clays; this is classically exemplified at Volterra (pi. xia).
In short, the Tertiaries, overwhelmingly soft and impermeable, are being
wasted rapidly by erosion and the demolition o f the uplands has its counterpart
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