Stage one 100 million years ago a deep ocean trench or geo- syncline was present in the ocean. This geo- syncline accumulated silt in the form of sand and mudstone. The sand and mudstone sediment was deposited due to fluvial transportation. With the pressure of the ocean above this sand and mudstone was compacted into sedimentary rock form. Due to the relative fluidity of the sediment at this time it stratified itself this is shown in the diagram by the different layers shown.
Stage 2 The earth’s mantle is extremely hot and this heat create’s convection currents. These convection currents move the earths continental plates as the float on the convection currents. This is known as continental drift. In this scenario we can see the indo-Australian plate colliding with the pacific plate. Due to this collision zone we get a squeezing motion on the sand and mudstone sediment which has been deposited in the syncline. When a sedimentary rock is put under extreme pressure it changes form to metamorphic in this case we see a change to the metamorphic rock of Greywacke. The squeezing movement gets concentrated on the Mohaka and Ruahine fault line.
Stage 2 Once the pressure builds up we begin to see a process called faulting. This faulting resulted in a rapid uplift of the sediment which was once in the geo-syncline. This rapid uplift formed the Ruahine range which is an example of a horst. You can see by the diagram that the horst is still stratified in mud and sandstone layers.
Stage 3 From 5 million years till present day we see the formed Ruahine range which is now under-going a number of surface process. The predominant weather system in New Zealand is westerly which gets blown of the Tasman Sea. This moisture rich weather hits the Ruahine range is forced to rise where it cools and condenses and brings about Orographic rain. This Orographic rain fuels the water supply to the Ruahine range and creates tributaries which eventually lead to another major feature of the Tukituki catchment that being the river itself.
Stage 3 The tributaries travel down stream carrying sediment in the form of Greywacke rocks eroding the Ruahine range. The transported sediment which comes from the Ruahine range is the reason for the alluvial fertile soils in which the central catchment region contains. The extreme high altitude of the Ruahine range also creates an area for freeze thaw this cracks rocks and erodes them. These two types of erosion lead to the scree or vertical erosion which is visible on the Ruahine range.
Stage 3 The continuing plate movement which created the horst has also lead to the fold mountains of the eastern hill country. The new rangitoto and elsthorpe anticline shoulder the atua synline where the Tukituki river flows. The Eastern hill country has been bought about due to the softness of the rock which allows it to be folded without snapping. The soil here is fertile but due to poor drainage doesn’t grow well.