From Golden Sun Universe
The cave in this picture connects
Idejima to the mainland via the Suhalla Range, which gets torn apart by
Venus Lighthouse's ignition.
The
Suhalla Range is a large range of mountains in
Gondowan that essentially divides the continent into its two vertical halves, both in cultural terms and in gameplay terms. Branches of these mountains also surround the
Suhalla Desert. The
Suhalla Gate is a pass through them that more-or-less connects the desert to the area where
Venus Lighthouse and
Lalivero are. The town of
Suhalla just to the north of the desert shares the name of the mountains also, and the range separates
Idejima from the mainland of Gondowan through a small cave connected to the pass of Suhalla Gate.
The involved mountains' official name can be assumed to be the "Suhalla Range" based on how Alex calls them by that name in the scene at the beginning of Golden Sun: The Lost Age where an earthquake generated by Venus Lighthouse tears a rift through the mountain range, physically detaching Idejima from the base of the mountains (and causing Idejima to float out to sea).
