Much of the ruggedness and beauty of the mountains is due to the erosive action of many alpine glaciers that once existed on the higher summits, and which have left behind their evidences in valleys and amphitheatres with towering walls, polished rock-expanses, glacial lakes and meadows and tumbling waterfalls.
In the mountains, ruggedness combines with thin and scattered soil to make these districts of small agricultural value.
Contemporary igneous outbursts are extremely common in some of the ancient formations, and add, by their resistance to atmospheric erosion, to the extreme ruggedness of the scenery.
The unity and ruggedness of the highlands of Wales have proved sufficient to isolate the people from those of the rest of South Britain, and to preserve a purely Celtic race, still very largely of Celtic speech.
Along the flood-plains of the larger rivers are fertile " bottomlands," but the ruggedness of the plateau country as a whole has retarded the development of the state, much of which is still sparsely populated.