Surface to 0.3 m: asphalt and soil
0.3 to 0.6 m: basalt scoria
0.6 to 8.9 m: hard, solid basalt
8.9 to 9.3 m: broken, fractured basalt
9.3 to 11.5 m: hard, solid basalt
11.5 to 14 m: basalt scoria
20.3 to 24 m: Tauranga group alluvium
24 to 309 m: Waitemata group alternating mudstone/sandstone
Alluvium: Clay, sand, silt, and gravel that was deposited by running water. As the speed of the water in a stream or river decreases, the capacity of the river to carry particles such as sand also decreases. If the speed slows enough, the water can no longer carry the particles and they will sink and be deposited on the river bed. Alluvium is also often deposited by floodwaters.
Basalt: Volcanic rock/old lava flows. The term "basalt" refers to the composition of the rock.
Fractured basalt: Rock with lots of cracks in it.
Lithification: The process by which sediments become rock, usually involving the loss of fluid and pore spaces due to pressure from overlying sediments, as well as the cementation of the grains to one another.
Mudstone: Mudstone is a sedimentary rock made when silt and clay (grain sizes < 0.0002 mm) are compacted and lithified. Mudstone is finer-grained than sandstone and siltstone.
Sandstone: Sandstone is a sedimentary rock made when sand (grain sizes between 0.02 mm to 2 mm) is compacted and lithified. It is more coarse-grained than either mudstone or siltstone.
Scoria: Rocks with lots of air bubbles in them. Scoria forms when air gets trapped in molten lava (such as during fountaining), forming a frothy mixture of lava and air. The lava with air bubbles freezes into rock quickly, resulting in a rock with holes in it.
Sedimentary Rock: Sedimentary rocks form when overlying pressure becomes great enough to cement particles together in the lithification process.
Siltstone: Siltstone is a sedimentary rock made when silt (grain sizes between 0.002 mm to 0.02 mm) is compacted and lithified. Siltstone is finer-grained than sandstone but more coarse-grained than mudstone.
Tauranga group: The Tauranga Group is comprised of mudstone, sand, gravel, and peat alluvium. These sediments underlie volcanic basalt flows underneath much of Auckland.
Waitemata group: This is the material that you see in the sea cliffs around the Auckland region, e.g. around Mission Bay. It consists of alternating beds of sandstone, siltstone and mudstone. These were created when sand, silt and mud sediments were deposited in layers and subjected to high pressures from above, transforming them into rocks.
* International Soil Science Society standard grain sizes used