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 While smaller groups within the Alps may be easily defined by the passes on either side, defining larger units can be problematic. A traditional divide exists between the Western Alps and the Eastern Alps, which uses the Splügen Pass () on the Swiss-Italian border, together with the Rhine to the north and Lake Como in the south as the defining features. While the Splügen Pass is neither the lowest nor the most important pass in the Alps, it is approximately halfway along the main chain, and makes a convenient boundary. Eastern Alps The Eastern Alps are commonly subdivided according to the different lithology (rock composition) of the more central parts of the Alps and the groups at its northern and southern fringes: Flysch zone (up from the Vienna Woods to the Bregenz Forest). Northern Limestone Alps, peaks up to 3000 m; Central Eastern Alps (Austria, Swiss), peaks up to 4050 m; Southern Limestone Alps, peaks up to 3500 m. The border between the Central Eastern Alps and the Southern Limestone Alps is the Periadriatic Seam.