Wairere Boulders. Basalt karst on boulder on hill slopes.
2.8 million years ago the land surface of this area was level with the top of the surrounding hills and a fluid basalt flow swept over it before coming to a halt and solidifying as a sheet of columnar basalt lava, several tens of metres thick. Eroded blocks of this hard basalt fill Wairere Valley and some have been partly dissolved to create karst shapes.
After the eruption, erosion started biting back through the basalt and into the soft underlying sedimentary rocks (Northland Allochthon). The underlying rocks were easily eroded and erosion undercut the hard basalt sheet until large blocks fell away breaking along the vertical cooling joints. Progressively the valley was deepened and cut further and further back into the hills at a rate of valley widening of about 1 m per 1000 years. Most of the eroded material broke up and has been carried away down the Hokianga River (for 80% of the time since 2.6 Myrs ago, sea level has been lower and the Hokianga Harbour was a river valley).
Today in Wairere Valley we mostly see the basalt boulders that have fallen off the lava flow bluffs (at the top of the hills) in the last few hundred thousand years. Throughout this time, up until the arrival of Europeans, the valley would have been dense native forest. Few of the boulders that broke off from the bluffs would have rolled all the way down to the valley floor in one go. Most it seems slowly slid (sometimes rolled), over thousands to tens of thousands of years, down the forested slopes. The underlying clay-rich sedimentary rocks become soft and slippery when saturated with water in winter and this allows some of the boulders (even today) to slide a few centimetres downhill each winter. As they slid many of the boulders maintained their same orientation (did not roll). Most boulders under forest become covered in vegetation and the roots of these shrubs and trees on the tops of the boulders accumulate humus, which slowly breaks down into weakly acidic humic acid in the rain water that resides in the damp humus. Over hundreds-thousands of years this humic acid appears to have slowly dissolved into the tops of the boulders forming solution basins and the humic acid that seeped down the sides slowly dissolved out fluted channels - some of which can be 30 cm or more deep and wide. You can see many of the best fluted boulders still on the valley slopes today and still oriented correctly.
Once the boulders reached the valley bottom they often rolled over or have rolled during huge floods that jostle the boulders about. Thus in the valley floor the fluting is often not as obvious or the fluting is no longer vertical and the solution basins may no longer be on the boulder tops.
The valley bottom is choked with large boulders of basalt - all of which have slid or rolled down the hillsides and collected together in the bottom. Even the largest floods have not been able to flush them out but undoubtedly they are moving downstream and decreasing in size extremely slowly.