The low ridge between the Waikato and Waipa rivers is made of Walton Subgroup (eQa – pumiceous fine-grained sand and silt with interbedded peat, pumiceous gravelly sand, diatomaceous mud, and non-welded ignimbrite and tephra), covered in places by Piako Subgroup (1Qa – Late Pleistocene, mainly locally derived, stream and coastal alluvium, and minor fans, with up to 20 meters of unconsolidated to very soft, thinly to thickly bedded, yellow-grey to orange-brown, pumiceous mud, silt, sandy mud and gravel, with muddy peat in some valleys).