This cache is NOT available from the motorway - you must either walk or cycle to it along the purpose built pathway. I suggest you park at the suggested parking location, and walk under the bridge, and around and up to the start of the shared pathway.
Logging Requirements
To log this EarthCache, visit the Viewing Platform on the Parahaki Bridge and answer the following questions:
- Observe the gorge and the landslip on the right hand side as you are looking upriver (into the gorge).
- What geological process allowed the river to maintain it's course through the ranges?
- Describe the colour and texture of the exposed rock.
- Can you see signs of weathering or erosion, and can you explain why this is more visible on one side of the gorge than the other? Why is greywacke considered a “rotten rock” in this environment?
- Photo: Take a photo of yourself or your GPS device at the viewing platform looking upriver with the large Landslip in the background, and the edge rail visible (see photo below).
Send your answers via the Geocaching message center or email to the cache owner. You may log your find immediately, but logs without answers may be deleted, as will any photos obviously taken from a vehicle. You have to walk to this one!
Water always flows downhill.
The Manawatū Gorge is a rare geological feature known as a water gap, where the Manawatū River cuts directly through the Ruahine and Tararua Ranges. Water gaps are typically formed by rivers that existed before the uplift of the surrounding landmass. As the land rose, the river maintained its course, eroding the rock and creating the gap.
This is known as antecedent drainage. As the Manawatū River was already there when the land got lifted up by tectonic plate movement, eventually forming the Ruahine and Tararua ranges, with the river creating a "water gap" across the mountains.
Most rivers arise from an already-existing range of mountains or hills, but beginning about 3 million years ago the central North Island mountain ranges began to uplift across the Manawatū's current course. Because it drained a large catchment, the river had sufficient flow to keep pace with and erode the rising mountains, eventually forming the Manawatū Gorge; other rivers were unable to and were diverted into the Manawatū instead. As the ranges rose, the river maintained its course by eroding downward, carving the gorge over millions of years, becoming the only river in New Zealand that crosses a mountain range, with it's headwaters northwest of Norsewood in the Tararua District, on the eastern slopes of the Ruahine Range, and flowing out to the Tasman sea 180 km later at Foxton Beach.
The gorge is approximately 6 km long, with steep rock walls. The dominant rock type in the gorge is greywacke, a hard, dark sandstone formed from ancient marine sediments. Greywacke is prone to weathering and fracturing, especially in steep terrain like the gorge. This leads to frequent landslides and rockfalls, which have historically impacted transport routes through the area, and the landslide you can see from this point led to the closure of the old road, and the construction of this route to replace it.
These rocks date back to the Mesozoic era, around 150–200 million years ago. The greywacke has been fractured and uplifted by tectonic activity along the North Island Fault System, which runs through this region.
What does 'Rotten Rock' mean?
Because of the constant tectonic movement, the rock of the Manawatū Gorge is cracked and shattered in lots of different places. This means there are places where the rock is very unstable, and where water can seep through and break it down even more. The rock isn’t actually rotten in the way wood becomes rotten. Greywacke becomes ‘rotted’ when wind and rain opens up cracks in it. Water flows into the cracks and breaks down minerals in the rock, which makes it weaker. ‘Rotten rock’ is usually a brown colour due to the rusting of the iron that can be found in greywacke.
Why is this such a problem here?
There are a few reasons why this keeps happening in the Manawatū Gorge. The steep cuts made into the cliff face to help create the original road and the rail tracks mean that there isn’t as much support for the looser rock above, which makes it more likely to slip. The road side of the gorge gets a lot more sunlight, meaning the rock gets wet and dry, warm and cold, more frequently. This makes it break down faster.
Enjoy your visit to the viewing platform, and take your time to enjoy the view and read the information boards nearby, however you don't need the information contained on them to answer this earthcache.