Glacial erratic
Definition: An erratic is a boulder transported and deposited by a glacier having a lithology different than the bedrock upon which it is sitting on...
A glacial erratic is a piece of rock that differs from the size and type of rock native to the area on which it rests. "Erratics" take their name from the latin word "errere", and are carried by glacial ice, often over distances of hundreds of kilometres. Erratics can range in size from pebbles to large boulders such as Big Rock (16,500 tons) in Alberta, Canada.
Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics were once considered evidence of a massive flood approximately 10,000 years ago, similar to the legendary floods described in the texts of ancient civilizations throughout the world. Ancient legends of an epic flood come from many cultures including Mesoamerican, Sumerian (Epic of Gilgamesh), Hebrew (Old Testament) and Indian culture. In the 19th century, many scientists came to favor erratics as evidence for the end of the last glacial maximum (ice age) 13,000 years ago, rather than a flood.
Geologists have suggested that landslides or rockfalls initially dropped the rocks on top of glacial ice. The glaciers continued to move, carrying the rocks with them. When the ice melted, the erratics were left in their present locations. Erratics are useful indicators of patterns of former ice flow.
This cache
While searching for a regular cache in the beautiful Queenstown Gardens, I saw this huge rock nearby. Thanks to a friend geocacher, Rich68, who produced many EarthCaches, I am now more aware of geological phenomenon and, as the geocaching community in Queenstown didn’t register this one, here is my chance.
The glacial erratic in question is composed of 2 fairly irregular polygons, one being of very impressive size (30' long x 20' thick x 18' high) while the second one is smaller (15' long x 6' thick x 10' high), for an approximate total volume of some 6 500 cu ft and a weight of some 400 to 450 tonnes; as the sides are sharp, not eroded, this leads to think it was not under the glacier but inside or on top of it.
It is most likely that it was deposited by one ice field covering the region during the last glacial era which began its advance about 50,000 years ago, reached its maximum extent some 25,000 years ago, and had retreated from Europe and most of North America by 10,000 years ago. New Zealand was of low enough latitude for its high mountains to be heavily glaciated during this Ice Age. Great rivers of ice flowing from the Southern Alps merged on lower ground to form local ice sheets that deposited thick beds of moraine; glacial erratics are part of this moraine and were deposited when glaciers retreated.
In order to log this EarthCache, you must answer the following questions :
1 : This glacial erratic has split and a smaller block rests against the larger block. What is the angle of the fracture?
2 : Compare the type of rock in the glacial erratic with the bedrock on which it sits? How are they different?
3 : What type of mineral can we see as white veins on this glacial erratic?
4 : Optional: With your log, post a picture showing you with your GPS or your GPS, close to the glacial erratic.
Please e-mail me your answers, don’t forget to specify the EarthCache name and GCxxxx code and your email return address.
Thanks for visiting this cache.
Papou
![]()