Turning east from Tsau, you'll soon start notice the road's gentle decline. It's entering the Gcwihaba Valley, a fossil river valley which may have been an ancient extension of the Okavango Delta. Within this, a group of six low hills protrude above the sand and rise to a maximum of 30m above the surrounding valley. (Compare this to the Male Hill at the Tsodilo, which reaches 410m.) About 40–50km northwest of these, the Aha Hills share a very similar geology and appearance.

All of these low hills are made almost exclusively of dolomite marble which, early in its formation, is thought to have been steeply folded causing many of its strata to stand vertically. It's estimated to be about 800–1,000 million years old.

This rock has been weathered into a very jagged, sharp surface and many loose blocks which aren't always easy underfoot. Underneath numerous faults and fractures split it. When weathered it appears a grey colour, but if you break it you'll find a pearly-white colour beneath. There are occasional bands of muscovite in the marble, and beds of more recent limestone and calcrete.

About 15–20km southwest of the Gcwihaba are the Koanaka Hills. These are also similar in geological origin, but essentially inaccessible to visitors. There has been some recent exploration of these (see Discovery and Exploration of Two New Caves in the Northwest District, in Further Reading), one of which – called the 'blue cave' – is Botswana's largest cave complex found to date.

Formation of the caves

Like many cave systems in the world, the Gcwihaba Caverns have been formed by the action of acidic groundwater flowing down through the faults in the rock. This gradually dissolves the alkaline limestone over the centuries. Some of this limestone has been redeposited as stalagmites and stalactites.

That said, this simple and quite standard explanation for the cave formation doesn't entirely explain why the cave system is more-or-less horizontal throughout. (It has northeast and southeast entrances, and appears to have had two levels at different times.) Because of this, Cooke (see Further Reading) notes that from the size of the caves and passages, it seems likely that very large volumes of groundwater must have moved within the rock here – perhaps a course of the old Gcwihaba River flowed through, underground. This would explain the sheer size of some of the caves here.

For that to have been the case, the area's water table must have been much higher for at least one period historically, and probably two. After these levels had subsided, and the caves emptied of water, then stalagmites and stalactites would have been formed gradually by a trickle-through of more moderate volumes of water (eg: rainfall) through fault-lines.