In some areas, such as the edge of the
Po Basin, the edge of the Alps is unambiguous, but where the Alps border on other mountainous or hilly regions, the border may be harder to place. These neighbouring ranges include the
Apennines, the
Massif Central, the
Jura, the
Black Forest, the
Bohemian Forest, the
Carpathians, and the mountains of the
Balkan Peninsula. The boundary between the Apennines and the Alps is usually taken to be the
Colle di Cadibona, at 435 m above sea level, above
Savona on the Italian coast. The
Rhône forms a clear boundary between the
tectonically-formed Alps and the largely
volcanically-formed Massif Central. Moving upstream, the Rhône turns to the east near
Lyon, and passes to the south of the Jura range before reaching
Lake Geneva. An area of flat ground reaches from there to
Lake Neuchâtel, continuing the border, with the Jura to the north-west and the Alps to the south east. From Lake Neuchâtel to its confluence with the
Rhine, the
Aare forms the border. The Black Forest is separated from the Alps by the Rhine and
Lake Constance, but exact delimitation is difficult in southern Germany, where the land gently slopes up to meet the mountains (known in German as the
Schwäbisch-Bayerisches Alpenvorland, the "Swabian-Bavarian pre-Alps"). In Austria, the
Danube runs to the north of the Alps, separating it from the majority of the Bohemian Forest, although some small areas, such as the
Dunkelsteiner Wald south of the
Wachau, belong geologically to the Bohemian Forest despite being south of the
Danube. The
Vienna Woods near
Vienna forms the north-eastern corner of the Alps, and here the Danube passes at its closest to the Alps (see
Viennese Basin). East of Vienna, only the
Marchfeld, a 30-km wide
flood plain separates the easternmost Alps from the
Lesser Carpathians. After Vienna, the
Pannonian Basin, a large area of
steppe, meets the edge of the Alps, clearly delimiting the eastern limit of the Alps. The south-easternmost extension of the Alps is to be found in
Slovenia, including
Pohorje, the
Kamnik Alps and the
Julian Alps (the last being shared with Italy). The town of
Idrija may be taken as marking the dividing line between the Alps to the north and the
karst plateau to the south, which then leads on to the mountains of the Balkan Peninsula. The remainder of the southern edge of the Alps is clearly delimited by the basin of the
Po. This delimitation of the Alps is, however, largely subjective and open to argument. In particular, some people restrict the use of the term "Alps" to the higher mountains in the centre of the range, relegating the surrounding hills and mountains to the status of "pre-Alps" or
foothills. This can sometimes lead to conflicting definitions, such as
Mont Ventoux being considered to lie outside the Alps (there are no comparably sized mountains around it, and it is at a considerable distance from the main chain of the Alps). It is not possible to define the Alps geologically, since the same
orogenous events that created the Alps also created neighbouring ranges such as the Carpathians (see also
geology of the Alps). The Alps are a distinct physiographic province of the larger Alpine System physiographic division, but the Alps are composed of three distinct physiographic sections, the Eastern, Western and Southern Alps. ==Subdivisions==