The complexity of its geological history combined with the wide variety of its substratum rock types, often
dislocated by numerous fault-lines and folding of the rocky strata by orogenic forces, have contributed to Italy's
extremely diverse morphology. Less than a quarter (23%) of its total territory is formed by plains, while mountainous
areas occupy over a third of its surface (35%). Finally, over two-fifths (42%) consists of hill zones. Italy's
maximum height above sea level corresponds with the summit of Mt. Bianco, 4,810 m., on the border with France.
The far eastern section of the Po Plain has in contrast some zones slightly below sea level, which are generally
subject to subsidence phenomena. However, physically, the Italian territory can be considered to consist of the
following regional units, characterized by a certain morphological similarity and at times also climatic: the Alpine
system and Po-Venetian Plain in the continental section; the Apennine system and anti-Apennine reliefs in the peninsula
section; and the large islands of Sicily and Sardinia.
Almost the whole southern side of this great mountainous system belongs to Italy, covering as it does a length
of circa 110 km from the mouth of the Rhône to the mid-Danube plains and varying in width from circa 150
to 250 km. This southern side contains many longitudinal (Valle d'Aosta, Valtellina, Val Venosta and Val Pusteria) and transversal valleys (Val di Susa, Val d'Ossola, Val Camonica and
Valle dell'Adige). It can be divided in three sectors: western, central and eastern Alps. The first two of mainly
crystalline rocks and the third of sedimentary rocks. Their traditional groupings are still in use: western sector
of Ligurian, Maritime, Cottian and Graian Alps; central sector of Pennine, Lepontine and Rhaetian Alps; and eastern
sector of Adige, Carnic and Julian Alps. The first two groups contain the highest peaks, often exceeding 4,000
m. (Gran Paradiso, Mont Blanc, Cervino, Rosa and Bernina). The pre-Alpine belt is mainly formed of sedimentary
rocks. It stretches from the mouth of the Valle d'Aosta to the Valle dell'Isonzo and is particularly disjointed, especially in two zones: the Lombard pre-Alps, where the landscape
of valleys is enlivened by large glacially excavated lakes (Orta, Maggiore, Lugano, Como Iseo and Garda);
and the Venetian pre-Alps, which contain numerous plateaux (Lessini, Sette Comuni and Cansiglio).
The Po-Venetian Plain
This is the principal Italian plain, extending for circa 42 sq km to the south of the Alpine arc and having
its other border with the northern Apennines and the Adriatic where it merges into a coast that is low and sandy
on the Romagna shore and ringed by lagoons on the Venetian shore. The
Po River cuts across the centre of the plain and, over the past two thousand years, has created a huge delta on
the edge of the Adriatic Sea. In this it has been assisted by many Alpine and Apennine tributaries, as well as
by other watercourses descending directly to the sea from the Venetian pre-Alps (Adige, Brenta, Piave, Tagliamento
and Isonzo) and the northern Apennines (Reno, Lamone and Marecchia). The Po-Venetian Plain has a mean altitude
of circa 50 m, while in the marginal belt at the foot of the pre-Alps and the Alps it exceeds 200 m. This is the
point at which it is possible to distinguish a high (gravel and sand) from a low (mainly mud and clay) plain, separated
by a row of springs that have had an important influence in the development of the plain's agricultural economy
(cultivation of the rice fields, water etc.). This plain also has an extremely important economic and social role.
Though it forms only a seventh part of the national territory it contains about a third of the Italian population.
The Apennine range extends for over 1,200 km from the Colle di Cadibona (touching on the Ligurian Alps) to
the extreme south of Calabria and then includes all the north Sicilian
mountains. It forms the mountain backbone of the Italian peninsula, unfolding in an extensive concave chain that
opens towards the Tyrrhenian Sea. Sometimes its mountains run parallel and sometimes they seem detached in isolated
groups, usually separated by wide valley and basins (Valdarno, Val Tiberina, Valle del Volturno, Vallo di Diano,
Piana del Fucino, etc.). Furthermore, these alternate with numerous transversal valleys that often narrow into
gorges. As with the Alps so with the Apennines, three sectors can be distinguished: a northern one of largely sandstones,
marls and clays, covering Liguria, Tuscany and Emilia; a central one essentially of limestones, covering Umbria,
Marches, and Latium, Abruzzo and, finally, a southern one of mixed rock types, covering Campania Basilicata
Calabria. Along both edges of the peninsula extensive depressions separate the Apennine chains from isolated reliefs. These are usually
given the name Antiapennine: Tuscan Antiapennine, with the Monti del Chianti, Amiata and Colline Metallifere; Latio Campania
Antiapennine, with its volcanic belt running from Cimini Mounts to Roccamonfina and Vesuvio; and Puglia regions Apulian
Antiapennine, with the Gargano, Murge and Salentina Peninsula. In Sicily , the Iblei Mounts can be considered to fulfil
an Antiapennine position. Adjacent to the Antiapennine reliefs and generally opening on to the sea there are fairly extensive river plains. On the Tyrrhenian side of the Italian
peninsula these consist mainly of the lower Valdarno, the Ombrone section of the Maremma, the Pontine Marshes and
the Campanian plains of the Garigliano, Volturno and Sele. On the Adriatic side, the largest river plains are those of the Tavoliere in Puglia and the Piana di Sibari in Calabria. On the islands
there are the plain of Catania in Sicily and that of the Campidano in Sardinia.
Besides the reliefs already mentioned, Sicily also has Etna, Italy's
major active volcano, and a large and undulating inland plateau. The latter is mainly formed of chalk rocks and
rich sulphur deposits that with the heights of the Monti Erei connect the Iblei to the northern chains (Madonie,
Nebrodi, etc.). Sardinia in its turn is characterized by reliefs of no great height, mainly formed from crystalline (granites) and volcanic (trachytes and basalts) rocks. On the western
side extend large flat areas like the previously mentioned Campidano, limited by the gulfs of Cagliari and Oristano. The minor island groups are mainly present in the Tyrrhenian Sea,
such as: the Tuscan archipelago (290 sq km), dividing the Ligurian and north Tyrrhenian seas; the Campanian
archipelago (71 sq km) with the Pontine Isles; Ustica (8.6 sq km); Aeolian Isles (115 sq km); Egadi Isles (38 sq
km); Pantelleria (83 sq km) and the Pelagian Isles (25.5 sq km) in the Channel of Sicily.
In the Adriatic, besides the various low and sandy islands of the Po delta and Venetian lagoon, there emerges the
Tremiti archipelago (3 sq km) to the north of the Gargano. Finally, there are numerous islands along the coasts
of Sardinia (Asinara, La Maddalena, Caprera, San Pietro, Sant'Antioco, etc.,), mainly due to the sinking and subsequent submersion of the margins of this major Tyrrhenian island.
The complexity of the peninsula's relief is echoed in the diversity of its coastal profile. Along the low and
sandy Adriatic shores this is generally rectilinear, with the exceptions of the bulge of the Po delta and of the
two rocky promontories of the Conero and Gargano. The Ionian and Tyrrhenian shores are very different, their extensive
sandy curves, corresponding to the edges of the coastal plains, alternating with high rocky coasts or steep promontories
like those of Piombino, Argentario, Circeo, the Sorrento Peninsula, etc. The coasts of Sicily and Sardinia present a similar morphological picture, the latter having
frequent rias or deep inlets resulting from the sinking of long stretches of the eastern coast.