The waterways of Wales owe their directions to a complex series of events which it is not our province to discuss in this place, but the minor features of Snowdonia are largely determined by planes of weakness which were produced in the rocks of the region during the occurrence of the marked earth-movements at the close of Silurian and Carboniferous times. The Post-Silurian earth-movements gave rise to planes of weakness running in a general northeast and south-west direction, and in a direction at right angles to this.