Jon_B wrote:

"This is the largest shell I've seen but much smaller
ones are clearly visible in some of the limestone on
the Great Pyramid itself."

The limestone of which the pyramids are built and on
which they rest consist of highly fossiliferous marine
deposits that accumulated during the middle Eocene
Epoch. Fossils of this age are common throughout the
bedrock underlying the Giza Plateau.

Som basic references are:

Aigner T (1983) Facies and origin of nummulitic buildups:
an example from the Giza Pyramids Plateau (Middle Eocene,
Egypt). Neues Jahrbuch Geologie und Palaeontologie
Abhandlung. vol. 166, no. 3, pp. 347–36

Aigner, T., 1983, A Pliocene cliff-line around the Giza
Pyramids Plateau, Egypt. Palaeogeography, Palaeoclimatology,
Palaeoecology. vol. 42, pp. 313-322.

Aref, M. M., and Refai, E., 1987, Paleokarst processes
in the Eocene limestones of the Pyramids Plateau, Giza,
Egypt. Journal of African Earth Sciences. vol. 6, no. 3,
pp. 367-377.

Hamza, F. H., 1983, Post-Pliocene transgressive phase
along the northern part of the Nile Valley, Egypt. Neues
Jahrbuch Geologie und Palaeontologie Monatshefte.
vol. 1983, no. 6, 338-344.

Holail, H.H., 1990, Diagenesis of the middle Eocene
Nummulite Bank of Giza Pyramids Plateau, Egypt;
petrologic and (super 18) O/ (super 16) O evidence.
American Association of Petroleum Geologists
Bulletin, vol. 74, no. 5, pp.676-677.

Best Regards,

Paul
Baton Rouge, LA