THe following is highly speculative and falls under the grey classification of prehistory.
The sphinx was shaped from insitu limestone.The lower body was excavated from softer limestone,which is easier to leach.Vertical erosion at this level can only be attributed to rainfall or chemical action.The later alternative is more feasible.Flood waters could not create the vertical erosion patterns.Since circa 3000BCE sand woul have frequently covered the lower body.This
provides protection against physical erosion.
The age of the sphinx is partially based on the syllabe "Khaf" on a circa 1400 BCE restoration plat."Khaf" is not enclosed in a cartouche to denote an Egyptian ruler,[Brested].It was used to identify Khafra[e] as the creator.
The sphinx would be about 200 years younger than the mud brick saqqura mastabas,which are more vunerable to erosion.They do not exhibit similar erosional patterns to the sphinx,[Scloch].
Elbas [2002] contends that a yardang in the western desert,that was shaped by aeolian forces to resemble the sphinx,may have provided inspiration for its construction.
By the latter half of the fifth millennium BCE proto-Egyptians had developed considerable skill as stone masons.This expertise would have enabled them to to hew the upper body from insitu-limestone and "quarry out" the lower section.
If it was created between 4000 and 3700 BCE,there was ample opportunity for rain to create the vertical erosion marks before sand covered the lower extremities.If the decline in rainfall was moderate a later date may have sufficed.
Prehistory has many examples of structural accomplishments.The inhabitants of the Andes Mountains moved massive stones at high altitudes.Their most awesome feat was the removal of a 2000 ton block from a mountain side at Sacsayhuaman,Peru, [Noorbergaen, 1983]
.It is conceivable that the sphinx was constructed in the first half of the fourth millennium BCE and the huge stone slabs were moved from the quarry.The use of a waterway could have facilitated their task,when the hyogea were been excavated from stone at Goza and Malta.A long Nile Valley stone working tradition was acknowleged on a plat at the base of the Step Pyramid at Djoser.It described Imhotep as the builder,sculptor,
and maker of stone vases.The latter skill can be traced to the second half of the sixth millennium BCE in the Nile Delta.
The dwellers of the Nile Valley and the Western Desert had the skils to create the sphinx pre-3500BCE.Whether they did is highly speculative,even on the first of April!!