You will notice when driving here on Highway 57 that there are large sections where the land surface, a terrace, is often very flat, but in other places it has been deeply dissected. For example it has been eroded by the Tokomaru River, the Kahuterawa Stream and the Turitea Stream.
This terrace was first identified near the township of Tokomaru and is so named. However it extends much wider than the Tokomaru area, stretching along the western flanks of the Tararua Range from Otaki to the Manawatu Gorge. The Manawatu River has eroded this terrace along its course, but the Tokomaru Terrace continues to exist to the north and west of Palmerston North as well as in the Rangitikei and Taranaki regions.
The Tokomaru Terrace is an old sea bed formed when the sea level was much higher. Up to 125,000 years ago, in the last Interglacial period, prevailing currents washed sand into the Manawatu coastline which lay against the Tararua Ranges between Mt Taranaki and Kapiti Island. These sands formed a flat layer of sandstone. (Geologists can still identify remnants of sandhills butting up against the Tararua ranges!) A glacial period followed from about 120,000 years ago to 11,000 years ago. The climate was now cold and dry. Water was locked up as ice and the coastline receded exposing this extensive flat old sea bed. The cold, dry climate meant that the vegetation here was much different from today. There was no native bush south of Hamilton, instead sub-alpine shrubs, such as leatherwood. Dry silts in river beds were exposed to the prevailing winds and huge amounts of silt were blown over the Tokomaru Terrace and collected in depths more than 10m, especially on the eastern sides of the Manawatu River and the Rangitikiei River. This layer can be observed in the cutting at GZ. These wind-blown deposits are termed loess, they have a clay-like texture and have a distinctive yellow-grey colour. In places the loess has been overlaid as well with depositions of river gravels (conglomerates). A distinctive layer of volcanic ash lies within the layer of loess and has been identified in some road-side cuttings. Its origin was a rhyolitic eruption from the Taupo region 26,500 years ago and it provides a useful marker for dating the layers of the Tokomaru Terrace.
As well as changes in the coastline, uplift has occurred in New Zealand to leave this ancient sea bed well above the current coastline.
Acknowledgments:
Dr. Clel Wallace, Earth Science Section, Massey University, Palmerston North.
R.G. Heerdegen, Landforms of the Manawatu, Department of Geogaphy, Massey University, Palmerston North, 1972.
In order to claim this Earthcache please send the answers to these questions via Messsages or email.
Do not wait for a reply, you may go ahead and claim the find.
QUESTIONS
1. What is the elevation in metres at road level at GZ?
2. You are looking at a cutting in the upper level of the Tokomaru Terrace. Estimate the height of the terrace here from road level.
3. Describe the colour(s) of the soil in this cutting.
4. There are two contrasting layers of soil composition in this cutting. Describe the differences in appearance of the upper and lower layers.
5. Two different depositional processes have created these two layers. What are they? (Upper and lower?)
6. Looking around you, are these terrace soils resistant to erosion, or prone to erosion? Describe your observations.