Mt. Robert from Lake Rotoiti EarthCache
Mt. Robert from Lake Rotoiti
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An Earthcache on the shores of Lake Rotoiti examining some of the interesting geological features of the area.
Parking coordinates are provided which are located at Kerr Bay on the shores of Lake Rotoiti. You can then take a short walk along the Brunner Peninsula Walkway to the listed coordinates.
When you arrive at the listed Earthcache coordinates, you will be close to the shore of Lake Rotoiti and with a good view of the north face of Mount Robert to your immediate right and the St Arnaud Range to your left (facing the lake). An information board is placed here which is dedicated to Park Ranger, George Lyon, who was instrumental in starting the rehabilitation of Mount Robert.
During the last ice age (around 12,000 years ago) glaciers carved the ranges into the classic scenery of broad, steep-sided valleys with long lakes, sharp ridgelines and rounded alpine basins and tarns.
An interesting feature of Mount Robert is its North face which is comparatively bare of mature vegetation in comparison to its other sides and the other similar mountains nearby. This is because of a fire which swept the area in January 1887. It has been speculated as to whether this was an accidental fire or deliberately lit. Whichever is true, local farmers took advantage of the cleared area for grazing for the next 25 years. Given the fragile alpine eco-systems involved, it will take hundred of years for the mountain vegetation to fully regenerate. In the meantime, the mountain's terrain will be changed forever. The burning of Mount Robert has had the effect of turning the clock back in geological terms to conditions that would have been in place in the era shortly after glaciation, when vegetation was just starting to re-colonise the bare slopes.
The normal vegetation cover holds rocks and soil in place and protects the mountain from the erosive effects of wind and rain. Without a protective cover of vegetation, the mountain rock erodes from exposure to rain and to the winter, rock-splitting process of freeze and thaw.
Slowly with the help of managed plantings and time, regrowth is covering the mountain slopes. Over time the upper slopes will be changed by erosion but eventually rock slides will stabilise to a point where a succession of manuka, kanuka and beech can grow.
Some of the forces that cause erosion on mountains (following definitions / descriptions sourced from (visit link) Ward Cameron 2005) :
Freeze and Thaw -
Water is unique. As it freezes, it expands in volume. In the mountains, snow melts during the day, and the resulting water flows into cracks in rocks. Overnight, as temperatures drop, the water expands and freezes. This expansion provides an incredible wedging force and slowly chips away at the rock face. As water freezes, it expands 9%, providing the sheering force of dropping a shot put 3 m. onto the rock. Scree or loose rock slopes are often formed by the slow action of ice wedging.
Avalanches -
As snow accumulates high in the mountains, eventually the excess is released in the form of avalanches. Since these releases will follow the most efficient route, they normally follow the same path year after year. Some years may widen the path if snowfalls are large, other years may allow tree growth to slowly try to reclaim the path. The signs are always evident, and these paths provide opportunities for lush summer growth.
Running Water -
Water is the king of erosion. Nothing carves up a landscape like the action of water. Even glaciers are simply another form of this versatile material. As water begins to move, it picks up material in the form of gravel and debris. As these sediments are carried within the water, they provide the abrasive needed to begin carving river channels. As the water continues to move, it picks up increasing amounts of water and sediment, and the process continues.
Soil Creep -
Very subtle, creep can be almost undetectable. It is normally found on weathered mountain slopes. Under the force of gravity, the material slowly begins to slide downhill, and eventually the signs of creep become evident. Tree trunks curve upward as the creep had caused them to tilt downhill, and they in turn corrected for this.
Rock Slides -
Sometimes, as layers of rock are steeply uplifted, the bonding of one layer to another may be weakened by the action of water or other agents of erosion. As the force of gravity constantly pulls down on the surface of a steep slope, there may eventually be a failure. As a fracture occurs at the top, a layer may slide down suddenly creating a large rockslide.
To log this Earthcache:
For parts 1 - 3 - review the information available on the information board at these coordinates and email me the following:
1. Name the process described on the information board that occurs after heavy rain.
2. Mount Robert has the same rock type as which nearby feature.
3. Will the erosive effects leave Mount Robert:
a. Shorter and Sharper
b. Flatter and Rounder
4. At the edge of the lake within a few metres of these coordinates is a large rock which would have been deposited in the glaciation process. What is the approximate height of this rock? Also, with respect to the texture of the surface of the rock, does it have scrape marks on it from the glacier travel as if it had been plucked and dragged, or is it relatively smooth, such that it may have ridden on top of the ice after coming down onto the ice via rock slide or avalanche?
Additional Hints
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