Choosing the correct stone

This fine garden entrance was built by Scottish Master Craftsman Jim Scott. Here you can see a good example of how wallers have to adapt their style to the available rock. At either side of the arch the adjoining wall has been built with round field stones which would not be suitable for the entrance. For the strength of the curve it must have been necessary to select a different type of rock and to dress these to produce the tapered effect. This is a situation becoming more common in the wallers world where they have to work as a mason but without mortar. Here in Scotland there are two seperate crafts which are overlapping . Quite a challenge for me perhaps I could get some lessons from you Jim?