Question

What do megaliths imply about the northern European civilizations that build them?

Answer 1

There are many things one can infer from these civilizations just by knowing that they were able to build such massive structures. One was that they had a running civilization which was able to provide for its citizens in adeqaute amount - this allowed them to be able to undertake such feats. 

Another thing is that we know they had the necessary knowledge to actually build such structures - this tells us that they were relatively advanced for their age and time. 

Another thing we can know from this is that they probably valued some sort of rituals and had people who were at a higher level in society who also commanded for these structures to be built.