With a surface of 23,861 km2 (9,213 sq mi), Lombardy is the fourth-largest region of Italy. It is bordered by Switzerland (north: Canton Ticino and Canton Graubünden) and by the Italian regions of Trentino-Alto Adige/Südtirol and Veneto (east), Emilia-Romagna (south), and Piedmont (west). Three distinct natural zones can be fairly easily distinguished in Lombardy: mountains, hills and plains—the latter being divided in Alta (high plains) and Bassa (low plains).
The orography of Lombardy is characterised by the presence of three distinct belts: a northern mountainous belt constituted by the Alpine relief, a central piedmont area of mostly pebbly soils of alluvial origin, and the Lombard section of the Padan plain in the southernmost part of the region.
The most important mountainous area is an Alpine zone including the Lepontine and Rhaetian Alps, (Piz Bernina, 4,020 m), the Bergamo Alps, the Ortler Alps and the Adamello massif; it is followed by an Alpine foothills zone Prealpi, which include the main peaks are the Grigna Group (2,410 m), Resegone (1,875 m) and Presolana (2,521 m).
The plains of Lombardy, formed by alluvial deposits, can be divided into the Alta—an upper, permeable ground zone in the north and a lower zone—and the Bassa—dotted by the so-called line of fontanili, spring waters rising from impermeable ground. Inconsistent with the three distinctions above made is the small subregion of Oltrepò Pavese, formed by the Apennine foothills beyond the Po River.