According to the Word Reference Dictionary, megalith is a "prehistoric monument of huge stones unworked". In the pic you can see Obelix carrying a menhir, a type of megalith.
In the province of La Coruña dolmens abound, especially in areas bordering Lugo and near the coast, near turn of the province of Pontevedra. The Galician megalithic is varied, with great buildings, but there are many small mounds or "mámoas".
According to the wiktionary, the word megalith comes from the Greek, formed from the word mega- (large) and the word -lithos (stone).
Other words with the prefix mega- are: megatherium, megaton or acromegaly. Other words with the root -lithos are: monolith, lithography or lithotripsy.