The northern Apennines curve gently from west - northwest towards the east - southeast. Between the Giovi Pass (472 m) near Genova and the upper valleys of the Tiber and the Metauro, the Apennines show an asymmetrical profile. The northern slope running down to the Po valley is relatively gradual, composed of ridges running at right angles to the line of the chain. The southern (‘‘internal’’) slope is relatively abrupt, marked by broad valleys and basins, running in parallel to the mountain chain itself. The underlying geological structure here has a profound effect on the landscape. On this southern side of the Apennines, there is a series of intermontane basins, well sunk, by Pliocene tectonic action, between parallel ridges running with the main Apennine chain from the northwest to the southeast or from north to south. These basins are drained by the Magra, Serchio, Arno (Sieve, Chiana), and Tiber rivers. All were once lake basins, now turned into river valleys, leading to a broadly similar sequence of often heavy clay sediments. Lake Trasimene, the largest lake of the peninsula (128 sq km), is formed in a shallow (6 m) depression within the alluvial sediments at one end of the Chiana valley.
Much of the relief has been shaped by fluvial action, but given variation by the type of parent rock. The narrow V-shaped valleys of the Ligurian Apennines are cut out of the local marly limestones, sandstones, and shales. The broad alluvial valleys of Emilia to the north are formed from clays and marls. The sharper, narrower Romagna valleys to the northeast are cut from marly sandstones.
The internal area of Tuscany is composed of two zones. The northern area immediately to the south of the Arno has geological formations similar to the Apennines themselves (including limestones and conglomerates). The southern zone around Volterra and beyond has a high presence of marine Pliocene deposits (clays, sands, and gravels). This zone is much affected by dissection and erosion, particularly under the impact of modern agriculture, but this degradation is almost certainly a longer-standing problem. At the southernmost limit of this area lies Monte Amiata (1738 m), the most northerly and some of the most distantly active (9 million to 1 million years ago) evidence of vulcanism in the peninsula. The southern coastal part of this zone comprises the distinctive Maremma region (see below).
World History
