According to J.T. Sibley in her remarkable book The Divine Thunderbolt. Missile of the Gods (2009), a work which I highly recommend, there is evidence for worship of the Thunder God going back to before 2000 BCE in the Neolithic Denmark where an 'Axe God' in the form of a flint celt mounted on a wooden shaft has been found in Follenslev lake/bog. The shaft of the axe was originally buried in the ground to its horizontal line. "It is unclear whether the spherical 'head' above the axe might have been carved to resemble a human face."
This artifact is evidence for a very early knowledge of the single-bladed axe being a divine thunderweapon and precedes the Bronze Age rock carvings which depict God-like figures waving single-bladed axes in the air. Miss Sibley posits the theory that the axe was worshiped as a divine representation of the Thunder God and I am inclined to agree with her. Later on in her book she draws our attention to the fact that in the Baltic lands huge iron hammers were "worshiped in the ancient cult sites." These iron hammers were erected in a vertical position in either a grove or a temple as a "proxy for the humanoid idol of the god."
The Scythians likewise venerated Ares via a mighty iron sword:
"The impressive personification of the sword matches well with that of the hammer, and to my way of thinking each confirms the other. Both idea and name of two of the greatest gods pass over into the instrument by which they display their might."Herodotis 4, 62 informs us, that the Scythians worshipped Ares under the semblance or symbol of an ancient iron sword, which was elevated on an enormous stack of brushwood ['three furlongs in length and breadth, but less in height'] (Asgard and the Gods, Wilhelm Waegner, 1886).