Between a rock and a hard place.
There has always been a heated debate between contractors and clients about what constitutes rock. CESMM4 allows the engineer to define rock for each contract, but not so SMM7.
NRM2 now gives a little more guidance in Work section 5: Excavation and filling as follows;
Rock is any hard material that can only be removed by the use of wedges, rock hammers, special plant or explosives due to its size or location. A boulder not exceeding 5m3 in volume or one that can be lifted out in the bucket of an excavator will not constitute rock.
Duncan Cartlidge FRICS