About half the river's course from its sources on the edge of the
Rangitoto Range is through deep valleys and gorges formed of Late
Jurassic to Early
Cretaceous Manaia Hill Group
greywacke (a form of
sandstone, with little or no
bedding, fine to medium grained,
interbedded with
siltstone and
conglomerate, and with many
quartz veins), which is buried in many places by
Quaternary ignimbrites. The main ignimbrite is the Ongatiti Formation, up to 150 m thick of compound, weakly to strongly welded,
vitrophyric, including
pumice-,
andesite and
rhyolite lavas. In several places the river runs past slopes covered in blocks of ignimbrite, where the underlying greywacke has eroded. For the remainder of its course, the river meanders over
alluvium and
colluvium to the Waipa. Initially these are mainly the Late Quaternary Piako Subgroup, which includes Late
Pleistocene alluvium, and minor fan deposits of unconsolidated to very soft, thinly to thickly bedded, yellow-grey to orange-brown, pumiceous mud, silt, sandy mud and gravel, with local muddy peat. Finally, the river flows mostly over the
Holocene floodplain, where the alluvium and colluvium consist of variously coloured, unconsolidated sand, silt, mud, clay, local gravel and thin intercalated (a form of
interbedding, where distinct deposits in close proximity migrate back and forth) peat beds. == Vegetation ==