Walker - 1967 - A geography of Italy

THE ALPS
the frontier in the Lugano area. With the exception o f the Splügen, the summits
of all the main passes lie within Swiss territory; on the other hand two very small
sectors o f Italy are drained northwards, one (the Lei valley) to the Hinter Rhein,
the other (the Spöl valley) to the Inn.
From the Northern Plain to the crests o f the Bernina and Lepontine Alps, the
mountain zone varies in width from 6o to lOO km. It is penetrated by a number o f
deep low-level valleys, both transverse and longitudinal, which favoured the
spread of the Italian type o f settlement as far as the watershed. The occurrence of
longitudinal valleys, notably the Valtellina, Val Brega, Val Vigezzo and the
middle Ticino (between Locarno and Bellinzona), is in marked contrast to the
western Alps. The numerous transverse valleys, o f which the Val d’Ossola, Val
Levantina, Val Mesocco, the Liro-M era valley and the Val Camonica are the
most important, offer a wide choice o f trans-Alpine passes. All the major valleys,
most o f them following fault zones, have been overdeepened by glaciers which
were thick enough in places to have scored the valley sides to a height o f 1500m.
Lakes Maggiore (Verbano), Como (Lario) and Iseo have been gouged out to
reach depths o f 372, 410 and 251m respectively; this means that at their deepest
points their beds are well below sea level. The higher valleys, many o f which are
notched with a post-glacial V , are impeded with moraines and occasionally graded
into valley steps (Val Mesocco). The lower valleys on the other hand, although
steep-sided, are floored by a ribbon o f alluvium with here and there the shallow
remnants o f once more extensive lakes. Well-drained detritus cones provide useful
cultivable land and favourite sites for settlement.
Structurally one o f the main contrasts between the central and the western
Alps is the presence in the latter o f a Pre-Alpine zone. As in all the Pre-Alps this
sector is predominantly calcareous, as opposed to the crystalline Alpine core.
The jimction between the two zones lies somewhat southwards o f the major fault
zone, known as the Tonale or Insubrica line, which runs from Domodossola
±rough Locarno, Sondrio, and the Tonale col and so along the Val di Sole to
Cles. Throughout much o f its length this line of weakness has been exploited by
the Adda, Oglio and Noce to provide an easy longitudinal routeway (fig. 22),
Between Lake Maggiore and the Lecco arm o f Como the Pre-Alpine zone is
much dissected by steep glaciated valleys, some o f them occupied by deep ribbon
lakes, and the dominance o f the Jurassic limestones is broken by the exposure
of Permian porphyries south o f Lugano. From Como to the Oglio valley (Val
Camonica) the Pre-Alps constitute a massive block through which there is no
satisfactory pass to the Valtellina. Here within a short distance o f the plain the
rounded, rather soft Cretaceous and Jurassic limestones are replaced by Triassic
rocks, mainly tough limestones and dolomites with some marls. The dolomites
produce the highest and most rugged relief in the Bergamasque Pre-Alps, for
example le Grigne, Pizzo Arera and Pizzo di Presolana, all o f them over 2400m.
The Brembo and the Serio, with their tributaries, cut through the Triassic
limestones in deep steep-sided valleys, but where the marls occm the relief is less
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