The Falls are, geologically speaking, probably a very recent formation. About a million years ago, the Zambezi's course is thought to have been down a wide valley over a plateau dating from the karoo period, until it met the Middle Zambezi rift – where the Matetsi River mouth is now.
Here it fell about 250m over an escarpment. However, that fast-falling water would have eroded the lip of the waterfall and gouged out a deeper channel within the basalt rock of the escarpment plateau – and so the original falls steadily retreated upstream. These channels tended to follow some existing fissure – a crack or weakness, formed when the lava first cooled at the end of the karoo period. At around the Batoka Gorge these fissures naturally run east–west in the rock, parallel to the course of the valley.
By around the Middle Pleistocene period, between 35,000 and 40,000 years ago, this process had formed the Batoka Gorge, carving it out to within about 90km of the present falls.
However, as water eroded away the lip of the falls, its valley gradually turned north, until it was almost at right-angles to the basalt fault lines which run east–west. Then the water began to erode the fissures and turn them into walls of rock stretching across the valley, perpendicular to it, over which gushed broad curtains of water.
Once such a wall had formed, the water would wear down the rock until it found a fault line behind the wall, along which the water would erode and cause the rock subsequently to collapse. Thus the new fault line would become the wall of the new falls, behind the old one. This process resulted in the eight gorges that now form the river's slalom course after it has passed over the present Falls. Each gorge was once a great waterfall.
Today, on the eastern side of the Devil's Cataract, you can see this pattern starting again. The water is eroding away the rock of another fault line, behind the line of the present falls, which geologists expect will form a new waterfall a few thousand years from now.