Another interesting thing about Brittany is the number of prehistoric megaliths, many of which you can just stumble across on an ordinary hike. A dolmen is made of slabs of stone, some standing up vertically to form the walls, with others laid horizontally on top to make the roof. (Dol men means "stone table" in Breton.) They would often be covered by mounds of stone (a cairn), earth and stone (a tumulus) or just earth (a tertre - presumbaly the origin of the name of the Place du Tertre in Montmartre). Dolmens were burial sites and often have long, low access corridors leading to the main burial chamber.
There are also standing stones. The most famous are at Carnac, but we also discovered a smaller, but very well looked-after site at Monteneuf, where there's also a reconstruction of a neolithic village. My camera batteries had run out that day but you can see photos here.