miles north-west of Leeds.
Covering 10 acres, the plateau is cut off from surrounding countryside by a 150 metre long wall with a man-made circular depression in the rock at one end, and a human skull in a rock cleft at the other. The complex is certainly prehistoric - and probably dates from at least the late Bronze Age.
The plateau seems to have been a sacred area covered with mysterious walls, enclosures and burial cairns. Unlike much of the surrounding countryside - including land on the far side of the wall that has the skull at one end - the plateau is grass-covered, an indication that it was probably never grazed or cultivated in antiquity. Most adjacent land of similar height is bare limestone, perhaps through over-use by farmers in prehistoric times.
Under the "skull wall," archaeologists found three ritually deposited pony limb bones, interred at regular intervals. Beneath the surface of the plateau itself, archaeologists believe there is a network of largely unexplored small caverns and natural passages, some of which may have been used for burials. One double cave in the side of the sacred plateau looks like a skull - and local place-name evidence strongly suggests it once contained human burials.
From the plateau, it is possible to see for about 20 miles, says Robert Watts, a Bradford University archaeologist, in charge of surveying the site for the university's department of archaeological science. "It's a peculiar place, with an atmosphere all of its own, bleak, remote and cut off from everywhere. It's like a sort of eyrie, high up and separate from the rest of the countryside."
The skull was found at the bottom of a 1.5m crevass at the wall's southern end. It was badly eroded and part of its cranium was covered in moss, but preliminary examination does suggest it did not become detached naturally.
The three equine bones, interred under the wall, were from different ponies, yet all appear to have been deposited at the same time. Vertically they are at the same level within the wall - put there when it was being refurbished, and presumably ritually rededicated.
The ponies are likely to have been religious sacrifices - and the human whose skull was found may well have performed a similar role. Alternatively, the skull could have been a tribal battle trophy. Its owner was a slightly-built male aged 25-50.
Offering human heads to the gods, whether those of human sacrifices or dead enemy warriors killed in battle, was an important feature of prehistoric Celtic ritual. Skulls of defeated enemies were sometimes kept in niches inside religious sanctuaries, or attached, trophy-like, to wooden poles. Some were even adapted for use as ritual drinking vessels endowed with magical properties.
Throughout the Celtic world, the human head - and brain - came to have increasing importance. There are references in Celtic mythology to brain tissue being preserved as trophies.
The sacred plateau's "skull wall" points due north, the direction of great importance to prehistoric tribespeople because, it is thought, the north is a "dead" area of the sky untouched by the sun and the moon in their apparent travels across the heavens.
The wall's orientation probably reflects the partially funerary and death-related nature of the sacred plateau itself.