Walker - 1967 - A geography of Italy

P A R T I I I ; R E G IO N A L G E O G R A P H Y
and thicken to over 2000m in the delta. The post-Pliocene uplift o f the syncline
was far from uniform; the sea bed seems to have emerged most steadily on the
Alpine hinter-zone (i.e. that part o f the syncline structurally associated with the
Alps) while in the Padane foretrench (associated structurally with the Apennines)
the uplift of the southern margin was partially offset by a severe sagging further
north. In fact, although the uplift increasingly revealed a strip o f Pliocene
deposits along the foot o f the Apennines, an area o f sea survived for a very long
time along the Padane foretrench so that here marine deposits often occur at
quite modest depths. This sagging movement is sometimes invoked to explain
the absence in Piedmont o f a calcareous Pre-Alpine zone which it is thought may
be buried below the Quaternary sediments.
Along the margins o f the plain, where the force o f the rivers was checked,
detritus cones and alluvial fans spread out into the Po depression until they
formed a continuous belt in which the material was naturally graded from large
pebbles upstream to very fine silt downstream. At any one place the character of
sedimentation rarely remained uniform for long; the erratic behaviour o f the
rivers, the unevenness o f the uplift and the fluctuating climatic conditions
favoured rapid changes in the natme o f successive deposits. Because o f their
more northerly situation, greater height and more abundant precipitation, the
Alps were much more severely glaciated than the Apennines; the latter carried
only a very few small glaciers, none o f them large enough to impinge on the
plain, but from the Stura to the Tagliamento several massive valley glaciers succeeded
in reaching it to form morainic arcs which now serve to raise the level of
the major Alpine lakes. T o the south o f these arcs the melt waters from the
glaciers spread vast outwash fans o f gravel, sand and silt which merge into a continuous
east-west belt. These surfaces were not left undisturbed for long; indeed,
they were radically modified as the behaviour o f the rivers varied with successive
glacial and interglacial phases and with the fluctuations in sea level and shoreline
associated with them. The net result has been the creation, southwards o f the
moraines, o f a series o f gently inclined, low fluvio-glacial platforms forming
the interfluves between the modern floodplains o f the Po’s left-bank tributaries.
The presence of these platforms and their associated moraines provides one of
the main contrasts between the Valle Padana north of the Po and that to the south
o f it. In the latter zone the surface deposits are o f post-glacial alluvitun; their
texture becomes increasingly fine northwards but really coarse sediments are
restricted to the detritus cones on the Apennine margin; elsewhere the fluvioglacial
platforms, so typical o f the trans-Padane sector o f the plain, are so poorly
developed as to be imperceptible.
Above Piacenza the Po hugs the edge o f the Apennines and their structural
continuation in the hills o f Monferrato; downstream it flows at an increasingly
greater distance from the Apennines. In part this pattern may reflect the fundamental
structure o f the syncline and in particular the persistence o f collapse in
the Padane foretrench, but the relative load-carrying capacities o f the Alpine and
120