The Waikato and King Country regions of New Zealand are built upon a basement of greywacke rocks, which form many of the hills.
Much of the land to the west of the Waikato River and in the King Country to the south has been covered by limestone and sandstone, forming bluffs and a karst landscape.
As with most of New Zealand, the basement rocks of the Waikato Region and King Country are composed of greywacke (indurated sandstone, siltstone and mudstone).
The Waipa Fault passes north–south through the Waikato-King Country region, from Taupiri, along the Waipā River, and south to near Ōhura.
Murihiku terrane greywacke lies beneath the Waikato–King Country region on the western side, and outcrops to the west of the Waipa Fault, from south of Waikato Heads to Awakino.
The Waipapa Morrinsville terrane greywacke lies beneath the Waikato–King Country region on the eastern side, south of the Hunua Ranges, and east of the Waipa Fault.
The land sunk and the sea transgressed, and calcareous sandstone, mudstone, and limestone were deposited in Oligocene times (34-24 Ma).
Over the last 2 million years, pyroclastic flows from the Taupō Volcanic Zone have deposited ignimbrites over the area east of Te Kūiti.
The rift valley is assumed to have developed about 2 Ma ago, due to the clockwise rotation of the Eastern North Island, that stretched the land between Auckland and East Cape.
Pumice from the Taupō Volcanic Zone has been deposited throughout the Waipa and Waikato Basins, and the Hauraki Plains, forming excellent soils and swampland.