At https://phys.org/print469174991.html … there are 35,000 megalithic constructs in Europe. A new study claims they were spread by a sea going culture that emerged in NW France 7000 years ago. It is said that the idea of megaliths were taken from Brittany in all directions – north and south, even into the Mediterranean, over a period of one thousand years (eventually reaching Salisbury Plain rather late in the sequence). C14 dates lie at the heart of the study, taken from 2000 sites across Europe. Other theories have suggested megaliths spread from the Near East and through the Mediterranean, eventually reaching the Atlantic seaboard. The new research appears to throw cold water on this idea. It demonstrates, absolutely, that Brittany was the origin of the megaliths (according to Mike Parker Pearson of UCL).
Meanwhile, Gary has sent in a link to https://www.iflscience.com/environment/the-western-sahara-is-covered-wit… … which may or may not be described as megaliths. If so they are not quite the same kind of thing as occurs in Brittany and the Western Isles. They are however situated in NW Africa – not too distant from the Atlantic coast. Some of them are said to date as early as 10,000 years ago – but presumably most of them are much later. The University of East Anglia conducted the Western Sahara Project between 2002 and 2009 to document the archaeology. They seem to have been constructed by a number of different cultures over the centuries. Some are just rows of heaped up rocks whilst others are properly placed large stones arranged in a circular pattern. They are thought mostly to represent burial sites and the same region is full of caves and rock paintings of animals such as gazelles and antelopes, giraffes and elephants, and rhinoceros – from back in the day when the Sahara was much wetter. After the Sahara dried out in Mid Holocene the region investigated appears to have become a refugia – which seems to indicate water was still available.