Between the Sphinx's forepaws, the Dream Stela still stands before the badly eroded breast of the monument, under the much better preserved head.

Dr Schoch first saw the site, from the viewing platform for visitors to the Sphinx, in 1990. Work on site in winter 1992/3, alongside seismographic work by others that produced some of those readings suggestive of cavities in the bedrock, led him to believe that the Sphinx and its enclosure walls showed unmistakable signs of erosion by rainwater run-off, in much wetter times than today or during the Old the date of carving, so the Sphinx might be older than Dr Schoch's estimate of the time-span of its erosion. Between about 5000 BC and 10,000 BC, Egypt's climate fluctuated between relatively wetter and drier periods, so there were wetter times than today's in which rain erosion might have taken place if the Sphinx's body was already carved. It should also he noted that even in Old Kingdom times up to about 2300 BC, as is evidenced by plant and animal species depicted in the tomb paintings, the weather of Egypt was wetter than it has been since. But even now, winter rains pose something of a threat in Egypt. The ditch alongside Khafre's causeway evidently needed to be blocked in Old Kingdom times against the possibility that rainwater drainage could run down into the newly cut Sphinx enclosure, according to Professor Selim. But Dr Schoch concluded that only the heavier rains before 5000 BC could have produced the erosion pattern of the Sphinx and its enclosure walls.

Egyptologists, of course, can see no cultural context at all in which the Sphinx could have been carved before 5000 BC, or 3000 BC for that matter, and several good reasons for putting its carving at about 2500 BC. Before about 10,000 BC, conditions were cold and dry in Egypt, so erosion of a pre-existent Sphinx is hard to imagine - indeed, the pre-existence of the Sphinx before 10,000 BC is itself an idea all but impossible to entertain.

Dr Schoch made his ideas accessible to the world of Egyptology in an article in the summer 1992 issue of the American magazine KMT (which takes its title from one of the ancient Egyptian names for Egypt, Kemet, with inserted vowels, meaning the 'Black Land' - the fertile soil watered by the Nile). He began by observing that the Sphinx body and the walls of the enclosure exhibit a pattern of weathering commonly associated with exposure to rainwater run-off, showing a rolling and undulating vertical profile with many vertical and sloping channels where joints in the bedrock have been opened up.

He contrasted this situation at the Sphinx with the state of weathering seen at other rock-cut features of the Giza Plateau (such as various Old Kingdom tombs) where erosion by windblown sand has picked out areas of poorer quality rock to leave the ancient cut facades and doorways otherwise not markedly damaged, and certainly not displaying the rounded and undulating weathering of parts of the Sphinx.

These observations, as we shall see, naturally led on to arguments about precisely what levels of the limestone beds of Giza could be equated with what others in different parts of the necropolis: for the beds do not lie horizontally all over the whole site, but slope down from north to south and west to east; moreover, ancient quarrying has left them discontinuous over the area.

Thus it is not always easy to decide that, say, a certain Old Kingdom tomb away from the Sphinx is cut in the same rock as the body of the beast. When they are not of the same rock, comparison of their states of erosion is more complicated than simply saying that one is evidently eroded by the wind since Old Kingdom times and the other eroded by water since seven thousand or more years ago.

In his KMT article Dr Schoch took the view that the serious erosion going on at the Sphinx today is a comparatively recent phenomenon, of perhaps the past two hundred years, associated with changes in the water-table and with pollution. He noted that erosion by condensation of dew and subsequent evaporation, bringing crystallization in the pores of the rock and spalling off of surface flakes, is evidenced in the interiors of some of the rock-cut tombs of Giza, but he concluded that this process at the Sphinx today could not account for the highly rounded weathering on the body and the enclosure walls that indicate the action of rainwater over a long period.

The profile of the Sphinx with similar light on the cheek: the fundamental resemblance to Khafre's statue is clear.