Peter Stafford [Paul Tabori] – The Wild White Witch (New English Library, 1973: 1979)
It was a cry of agony torn from a human throat yet inhuman in its wordless despair. Whoever uttered it had abandoned all hope and relinquished all faith in rescue or mercy. The sound stopped so abruptly that it seemed as if the tongue of the sufferer had been torn out by the roots.
In terror and frantic haste Jeremy began to search the wall in front of him and found two primitive images drawn in faded charcoal: a crude, diamond-shaped representation of the female sex, with dull red lines radiating from it like the rays of the sun, and next to it, in the same schematic manner, a large pointed, dagger-like phallus.
With a slight grating noise a section of the wall swung in and down, disclosing a slablike threshold that bridged a deep rock chamber. From the black depths a faint, reddish light filtered upward. Jeremy could distinguish a sound, halfway between a chant and murmur.
It took a few moments before his eyes became accustomed to the dimness and he could focus on the spectacle below him. He could not see the entire expanse of the chamber, but what he could make out was sufficient to chill his blood and nearly stop his heart!
Thanks to Steve Goodwin for the cover scan