sentences:  The effect in this area was to cause folding and faulting of the existing rock sequence, most particularly within the Welsh Basin, a process which intensified northwards.Parts of the sequence were subjected to low grade metamorphism, the most significant of which, from an economic point of view, would be the Cambrian and Ordovician slates of North Wales.The northeast to southwest 'grain' of much of the country was imparted at this time with a series of major fault zones persisting from that time to the present, some of which, particularly in the northwest, are believed to represent terrane boundaries.