This is a question posed to me on Facebook. Here's my answer: Excellent question (puts on professor glasses. stretches fingers.) So.

In the Stone Age there were these people called the Battle Axe People. They had double headed axes. Knapped from stone. Tools, not massive weapons, and so not really that big. OK so picture those. Now fast forward to the Viking Age.

Viking Age people accidentally dug up the tool making areas, homes, discard piles, and graves of the Stone Age Battle Axe people while plowing their fields. Sometimes the stones would appear in the fields after the fields flood after a big rain storm. And all over their fields were these little double headed stone axes. Except they didn't know they were axes. The Viking Age people had steel. Their axes looked completely different. They called these things Thunderstones. Thunderstones blessed fields and made crops grow. Thor is the god of thunder. Thor carries a hammer... or is it an axe? The axe god, the sword god, and the spear god were known since ancient times-- that would be Thor, Tyr, and Odin.

The Viking Age people adopted this shape as the shape of Thor's Hammer. Viking Age people thought these Thunderstones were so holy they were buried with them. (Pity the poor archeologist who first had to figure out how that happened.) Viking Age people copied this shape in their own sacred art. This is the Thor's Hammer symbol that was worn as a symbol of heathen faith all over Northern Europe and on into the sea to Iceland and Greenland and so on. And it's the symbol of Asatru today, and is used by Asatruars and other heathens as a symbol of Thor.