The west side of Italy is given to a fault-block system, where the crust – extended by the lengthening mantle below – thinned, broke along roughly parallel fault lines, and the blocks alternatively sank into grabens or were raised by isostasy into horsts. This system prevails from Corsica eastward to the valley of the Tiber River, the last rift valley in that direction. It runs approximately across the direction of extension. In the fault-block system, the ridges are lower and are more steep-sided, since the walls are formed by faults. Geographically they are not considered part of the Apennines proper but are termed Sub-Apennines () or Anti-Apennines () or Pre-Apennines (). These mountains are found mainly in Tuscany, Lazio and Campania.