Glacial rebound is still occuring in Scandinavia. If the Scots moved to their present location from Scythia at or about the time of the founding of Rome and the Exodus, that would imply the glaciers were already clearing out of Scotland. But they could have still been there when the Druids built Seahenge and Stonehenge, and the land could still have been descending at the time.
The Druids built neither. Both structures pre-date the Druids.
http://www.encyclopedia.com/articles/03814.html
The megalithic monuments of France and Great Britain, notably those at Carnac and Stonehenge, were once ascribed to them, but these are now known to predate Celtic culture.
http://www.thehistorynet.com/britishheritage/articles/1997/0397_text.htm
There is no proof that Druidic ceremonies ever took place in stone circles, and it is now understood that henges, stone circles and circle henges long predate the Celtic Druids.
http://www.duke.edu/~kkb1/arch.html
A common belief that started around the sevententh century was that Stonehenge was built by the Druids. Today a modern sect of druids tries to hold rituals there and claim it for themselves, but the evidence from excavations and dating would actually prove that the Druids had nothing to do with Stonehenge, and in fact Stonehenge is much older.
http://www.bartleby.com/65/dr/druids.html
In the late 18th and 19th cent., interest in the druids was spurred by archaeological discoveries and by the romantic movement. The megalithic monuments of France and Great Britain, notably those at Carnac and Stonehenge, were once ascribed to them, but these are now known to predate Celtic culture.
http://www.comp-archaeology.org/15OrientationLocation.htm
Since megaliths predate the Celtic Druids, the Beltane Fires might be something handed down from the darkness of prehistory.
http://www.mm2000.nu/sphinxii.html
John Aubery was the first to suggest the Druids as makers of Stonehenge (~1660), which is certainly false because the Druids are much later -- contemporary with the writings of Julius Caesar in Britannia or dating from a few centuries before -- by which time the Henge was 2000 years old.