In a Masonic context the heads also represent a hammer or an axe, and may be referred to as an Axe Head Cross or Brick-axe Cross, representing bricklayers and other construction trades. (See Axe Cross.)
Although it is a hammer, it is still a cross shape. And for some unknown reason it is invariably inverted like St. Peter's Cross and sometimes referred to as the Wolf's Cross. (This was the name given to an amulet discovered in Iceland, estimated to be around 1,000 years old and which might be Pagan, Christian, or possibly a mixture of the two.) Another somewhat romantic name for this form is Viking Cross.
In Norse and German mythology, Thor is the powerful son of Odin. He rides around in a chariot pulled by two goats, curiously named Tanngrisnir (tooth grinder) and Tanngnjóstr (tooth gnasher). If he gets hungry, he eats one of the goats. It doesn't matter which one, because when he gets hungry he gets very hungry, so he eats both. On touching the carcasses, the goats are instantly restored to health and can be put back to work.
Thor wields a hammer, called a Mjöllnir and every time he throws this, thunder is heard in the area. (The Swedish for thunder is tordön, translated as "Thor's din".) The hammer has magical boomerang properties in that it flies back to its owner after hitting the target. We humans can hear this as the thunder ricochets from hill to hill.
It's a good idea not to annoy this god. When he comes storming after sinners, the sparks of his chariot wheels create the lightening we see. (See also the Taranis wheel)
The god was so powerful and popularly feared, that a day of the week was named after him: Thursday. And this god is seriously into recycling: With his ability to resurrect his goats, the echoing from his boomerang hammer, and his regular weekly appearances on our calendar, this god will no doubt be around for a long time.
The Romans followed the idea of having two animals pull their chariots and as you can read here, this may have had influence on modern day space travel. Who knows, one day the circle will be complete when an astronaut catches a glimpse of Thor.