Some of the most complex engravings are early e.g passage grave "art " , whilst much later in the Bronze Age you find funerary contexts with simple cup marks . Both styles would have been achievable to the engravers at any one time ,why one was chosen for a particular circumstance is the problem . It's worth mentioning that the more complex engravings tend to be found in less obvious /accessible situations e.g. passage graves , funerary contexts and when in the open air more likely to be on less conspicuous rocks e.g. flush with the ground bedrock , rather than prominent boulders .