Walker - 1967 - A geography of Italy

PART I I I : REGIONAL GEOGRAPHY
on the same nappe system as the Apennines themselves and the lithology of each
sector has much in common with that o f the main range parallel to it.
Between the Sub-Apennines and the coast are the Anti-Apennines, a term
which has little structural significance since they too are underlain by the
Apennine nappe system. T hey exhibit a rich variety o f lithology and landform;
the Apuan Alps, ruggedly carved in Permian and Triassic limestones, are undeniably
mountainous; most o f Tuscany between the Am o and the Ombrone is
hill and plateau coimtry developed on Tertiary sands and clays; from M.
Amiata to the Alban hills the landscape is chiefly the work o f Quaternary
volcanic eraptions; and between the Pontine Marshes and the Sacco-Liri trench
bold Cretaceous limestone blocks form the Lepini, Ausoni and Aurunci mountains.
Along the coast discontinuous lowlands accumulated behind sandy spits
and intermpted by rocky headlands, penetrate up the larger valleys, especially
those o f the Arno and Tiber.
In no respect is the contrast between the eastern and western flanks o f Central
Italy more obvious than in the drainage. The relatively simple comb-like
arrangement o f rejuvenated consequent streams draining the Emilian-Adriatic
slope has already been noted; on the Tyrrhenian side over two-thirds o f the
surface is drained to the Tiber and Arno whose courses are anything but simple;
sometimes they flow longitudinally through intermontane basins, sometimes
transversely through narrow gaps or gorges. These two systems owe their complexity,
at least in part, to the long history o f crastal instability to which the
‘inside’ o f the Apennine system has been subject since the Eocene. Thus during
the Pliocene only the higher parts o f what are now the Anti-Apennine uplands
remained above sea level forming an archipelago off the main western coastline,
which ran to the east o f the Chiana-Tiber line from just north of Chiusi to a few
miles west o f Tivoli. The intermontane basins o f the Sub-Apennine zone are all
o f tectonic origin and during the Pliocene some o f them were occupied by lakes;
the biggest stretched down the Val Tiberina from Sansepolcro through Todi to
T em i, with a branch from near Perugia to Spoleto. At the end of the Pliocene the
whole peninsula underwent an uplift which eventually raised the Sub-Apennine
zone some hundreds o f metres and converted the former Anti-Apennine archipelago
into a land mass much o f which was still penetrated by arms o f the sea
(e.g. the lower Valdarno); some areas,’notably the Val di Chiana, failed to establish
a satisfactory drainage system and remained swampy into historical times.
The uplift was far from uniform and it was accompanied by local collapse and
warping. Thus the Tiber lake, which persisted somewhat diminished into the
Quaternary, was obliged by an upwarping to abandon its former course (at that
time to the sea) through what is now the Nestone valley and cut a gorge westwards
from Todi. Similar movements modified the drainage elsewhere; the
Tiber and T em i basins were separated and henceforth the latter was drained
exclusively by the Nera through the Narni gorge. It seems likely that the drainage
o f the Salto into the Liri was reversed by earth-movements to flow into the
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