Most of the region is mountainous or hilly, the main features being the Apennine chain along the internal boundary and an extensive system of hills descending towards the Adriatic. With the sole exception of Monte Vettore, high, the mountains do not exceed . The hilly area covers two-thirds of the region and is interrupted by wide gullies with numerous – albeit short – rivers and by alluvial plains perpendicular to the principal chain. The parallel mountain chains contain deep river gorges, the best known being those of the Furlo, the Rossa and the Frasassi.