相对薄的冠岩覆盖在相对厚的上里阿斯统粘土之上，向下直到拉脱面，这样一个组合，允许弓形坡和河谷外鼓特征得到最充分发育。

• The combination of relatively thin capping rocks overburden a considerable thickness of Upper Lias clay, down to the surface ofd6collement, allow of the fullest development of the camber characteristics and valley bulge features.