Star maps on megaliths?

At www.stonepages.com/nes/archives/004126.html there is a bit of speculation that might be just imagination. A standing stone in S Wales which has 75 cup marks gouged out of its surface may represent a section of the night sky, it is claimed, but how this is arrived at is not disclosed. In addition, they say the particular part of the sky can be pinpointed and includes Cassiopeia, Orion, Sirius, and the North Star – but how do they know this? It once formed the capstone of a dolmen, structures built during the 4th millennium BC. However, between 2000 and 1700BC it was converted into a standing stone which indicates the dolmen might have collapsed – but why? Could an earthquake before 2000BC have been responsible?