 These are younger rocks, mostly Carboniferous limestone, and the mountains are characterised by shallower slopes covered with moorland vegetation, culminating in Cross Fell at .South of this, the main line of the Pennines (often called "the backbone of England") continues into the Yorkshire Dales, a limestone-dominated area of broad valleys and moorland.The Yorkshire Three Peaks are some of the highest summits in the area, which became a national park in 1954.