[osr Challenge] The Runestone

The Runestone: A Get Ready, Get Set, Go! Labyrinth Lord adventure set in the Ashford Valley.

Get Ready:
A magical runestone transports the PCs to another time and place! Can they get back to the now?

Get Set:
The PCs are in the woods, looking at runestones. Perhaps they're on an expedition for a sage, mapping runestone  locations and making stone rubbings off of them onto big sheets of  paper. They all manage to touch the same stone at the same time and find themselves somewhere else.

Go!:
Their hands all are on the runestone at the same time and they are transported to another place (a cruel GM may require each PC to make 3 Intellegence rolls to have most of their stuff - one roll each for weapons, armor and backpack containing food and specialised tools, including spellbooks and thieves tools. Failure means most of the PCs equipment was left at the other runestone).

The PCs find themselves in an extremely large cavern and beneath their hands is a broken runestone. The cavern is deep below the earth, has a large body of water on one side and no obvious exits. Can they figure out how to repair it to get home?

An elf or wizard can tell, 1-3 chance on 1d6, that the PCs have been transported into the future. The closer the PC  got to a roll of 1, the closer they determine that it has been at least 100,000 years into the future. Any priest in the party automatically looses connection to his patron diety (and is immediately aware of this) and only has the few spells granted to him that he prayed for this morning. Wizards without their spellbooks cannot re-memorize spells. Elves are unaffected.

The broken runestone will not connect them back to the other runestone. It is possible to remove the runestone from it's hole and put in another one (and that other one is hidden somewhere in the extensive cavern - most likely in in the water, guarded by a sea serpent). Moving the runestone is their problem - hopefully someone was lucky enough to bring the rope.

To complicate matters, the cavern is home to a tribe of albino apes, the descendants of the morlocks that have been plaguing the Ashford Valley recently. The apes will attack the PCs a few at a time, while the rest throw rocks and make screeching noises at them.

Notable NPCs:
Albino Ape; No. Encountered: 1d4+4; Alignment: Neutral; Armor Class: 6; Hit Dice: 4d8, Hit Points: 30, 24,  24, 23, 22, 20, 18, 16; Attacks: 2 claws (1d4/1d4) or rock (1d6); Save: F2; morale: 7.

Sea Serpent; No. Encountered: 1; Alignment: Neutral; Armor Class: 5; Hit Dice: 6d8, Hit Points: 45; Attacks: bite (2d6); Save: F3; morale: 8; Special abilities: constrict (successful attack envelopes target and constricts for additional 2d4 damage, constriction continues on subsequent combat rounds).