This type of contact between rocks is known as an angular unconformity. An unconformity represents a contact between two rock units where there is a substantial age difference between the rocks (usually caused by a break in the geological record). An angular unconformity is a type of unconformity seen when horizontal strata are deposited on older tilted (also known as dipping) layers. Originally, Ordovician (485 – 444 million years ago) mud and sand would have been deposited in deep water. The origin of this sediment was likely to be in turbidites (a deposit formed by a turbidity current, which is rapidly moving the water that moves down a slope under the sea). This mud and sand would have been uplifted before being folded into mountains and then eroded to form a flat surface. The onset of marine transgression lead to the flooding of the land by a subtropical sea leading to deposition of limestone horizontally 330 million years ago. When the sea regressed, further uplift and erosion of the valley by glaciers occurred. ~SA Image: http://bit.ly/1MfVqUt Copyright Val Vannet and licensed for reuse under this Creative Commons Licence More on turbidites: http://bit.ly/1GkIsEM and http://bit.ly/1GuhJaL