, countryside near the
Po , eastern Piedmont , southern Lombardy The Po Valley is often regarded as a
syncline, or dip in the crust due to compression at the edges. Regardless of whether this concept accurately describes its geology, the valley is manifestly a sediment-filled trough, or virtual syncline, continuous with the deeps of the
Adriatic Sea. The surface terrain is therefore divided into two overall types of
landform: the plain, or flat surface of the fill, and the
anticline at the edges, taking the form of hilly country in which the
outcrops of the original rock are visible along with
alluvial fans formed from the outwash of the more severe anticlinal terrain; that is, the Apennines and the Alps. The valley is broadly divided into an upper, drier part, often not particularly suited for agriculture, and a lower, very fertile, and well-irrigated section, known in
Lombardy and western
Emilia as
la Bassa, 'the low (plain)'. The upper areas of the Po valley take local names which reflect in their meanings their being modestly suited for farming. So we have the Piedmontese
vaude and
baragge, the Lombard
brughiere and
Groane, or, exiting from the Po valley proper, the
Friulian
magredi, areas remote from easily reachable water tables and covered with dense woods or dry soils. This specific meaning for 'lower plain' derive from a geologic feature called the
fontanili line or zone, a band of springs around the Val Po, heaviest on the north, on the lowermost slopes of the anticline. It varies from a few kilometres to as much as wide. The fontanili line is the outcrop, or intersection, of the anticline's
water table with the surface at the edge of the
bassa. The rock above the line is porous. Surface water in the
intermittent streams of the mountains tends to disappear below ground only to spring out again in the
spring zone. The spring zone is often called "the middle valley."
Surface runoff water (the Po and its affluents) is not of much value to the valley's dense population for drinking and other immediate uses, being unreliable, often destructive, and heavily polluted by sewage and fertilizers. Its main anthropic value is for hydro-electric power, irrigation, and industrial transport. The cost of purifying it for human consumption makes that process less feasible. The fresh drinking water comes from hundreds of thousands of wells concentrated especially in the fontanili zone. The major settlements, therefore, are also in that zone, which has become the centre of economic development and
industry in Italy, and now is an almost continuous
megalopolis stretching from
Turin to
Trieste. The
bassa Padana was settled and farmed earliest, in Etruscan and Roman times. After the collapse of the
Roman Empire (5th century AD), lack of maintenance of the irrigation systems associated with a cooling climate phase (i.e. the so-called Migration Period or
The Dark Age Cold Period) led to the progressive waterlogging of the Po Valley and the natural depressions on the right side of the
Po turned in vast swamp basins. The waterlogging process of the area continued until the 10th-century influencing the human sustenance and settling practices. According to historical-archaeological data, indeed, the
wetlands were exploited for fishing as well as for transport by boat while the
early medieval sites settled on the fluvial ridges, in topographically higher and strategic position in the surrounding swampy meadows. The Po Valley has been completely turned to agriculture since the
Middle Ages, when efforts from monastic orders, feudal lords and free
communes converged. The older and smaller cities deriving from ancient times are still located there. According to historical maps and documents the land reclamation of the Po Valley reached its peak during the
Renaissance (15th–16th centuries) and continued in the
Modern Age (17th–18th centuries), with the last marsh areas only being reclaimed in the 20th century: channels and drainage system are still active and allow the Po Valley to be drained and be cultivatable. ==Tributaries==