Summary: The Petroglyphs of Dissignac, Brittany, cover a square meter in a passage grave dating from c.4500 BC, that is known to have been used until c.2500 BC. These petroglyphs look like a pile of eleven iceaxes, mostly vertical, sometimes overlying one another. They are actually "route pictograms", very stylized maps, based on the design of an ancient instrument for measurement of latitudes called a "Jacob's Staff". These glyphs were carved over the lifetime of the monument, to encode, in the tangent angles of the glyphs, the latitude data of megalithic explorations. They could not cross the ocean to the north, but they discovered the islands of Ascension and St. Helena in the south (c.2900 BC). Then they followed known trade routes to the east, and eventually the west coast of America was found (c.2600 BC).