The picture and text below is 'illuminating':

"Then I become aware of ... a presence - a faint, ghostly glimmering, like moonglow, that has appeared on the solstice stone. I don't know how long it lasts, a second or two only I would guess, but while it is there it seems less like a projection - which I know it to be - than something immanent within the stone itself. And it seems to function as a herald for it fades almost as soon as it has appeared and in its place the full effect snaps on - instantaneously. It wasn't there, and then it's there.

As Chris had described, the effect does curiously resemble a poleaxe, or a flag on a pole, and consists of a 'shaft', narrow at the base but widening a little towards the top, running up the left hand side of the solstice stone, surmounted by a right-facing 'head' or 'flag'. An instant later an almond-shaped spot of light, like an eye, appears a few centimeters to the right of the 'flag' and the effect is complete.

Weirdly - I do not claim it has any significance - this flag-on-a-pole symbol is the ancient Egyptian hieroglyph neter, meaning 'god', or 'a god' - and not to be understood at all in the Judaeo-Christian usage of that word but rather as a reference to one of the supernatural powers or principles that guide and balance the universe. Manifested here, in this strange Stone Age temple, it glows, as though lit by inner fire." (Hancock 2)

The X form - in slightly different shape, with a central rhomb - occurs also at the top of the famous trilithon on Tongatapu:

According to tradition it (Ha'amonga-'a-Maui - Burden of Maui) was 'built by IIth Tui Tonga about 1200 AD as a gateway to the royal compound. Height 16 ft, width 19 ft, weight approx 35 tons'.

"During his coronation year king Taufa'ahau Tupou IV advanced the theory that the Ha'amonga stones must have served a greater purpose than the more obvious function as a gateway. This proved the case when closer investigation revealed a secret mark on the lintel (= threshold) stone ... On June 21st 1967 at dawn his majesty was present at this place and it was a thrilling moment when the sun rose at the exact point indicated by his interpretation of the lines etched on the great stone." (Text on a  wooden sign at the side of the trilithon. Ref. Danielsson).