Titan's Tunnel
Once you have the RUBY, you can give it to the Titan in the Titan's Tunnel to gain passage to the west end of the peninsula. If you've survived the Earth Cave, the enemies in the Titan's Tunnel should pose no threat to you.
On your way out, don't miss the treasure room to the south. You'll find some Gold, a Silver Helmet (which is a good upgrade for your FIGHTER), and a Great Axe. The Great Axe is an interesting weapon, as it seems completely mundane yet you can't buy one anywhere. Like other axes, it sacrifices Hit Rate for Damage, which is not a particularly good tradeoff. In this case you'd be sacrificing 10% of Hit Rate compared to the various family swords you've found. Don't bother with this weapon.
Sarda's Cave
Speak to Sarda and he will give you the ROD, which will allow you to remove the stone plate blocking the way to Earth Cave B4 and the true source of the earth's rot. There isn't anything else of interest here, so head on back through the Titan's Tunnel. You will likely want to head to Melmond to rest up and buy new spells before returning to the Earth Cave to hunt down LICH.