A glacial erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. "Erratics" take their name from the Latin word errare, and are carried by glacial ice, often over distances of hundreds of kilometres. Erratics can range in size from pebbles to large boulders.
Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics are significant because:
- They can be transported by glaciers, and they are thereby one of a series of indicators which mark the path of prehistoric glacier movement. Their lithographic origin can be traced to the parent bedrock, allowing for confirmation of the ice flow route.
- They can be transported by ice rafting. This allows quantification of the extent of glacial flooding resulting from ice dam failure which release the waters stored in proglacial lakes. Erratics released by ice-rafts that were stranded and subsequently melt, dropping their load, allow characterization of the high-water marks for transient floods.
- Erratics dropped by icebergs melting in the ocean can be used to track Antarctic and Arctic-region glacial movements for periods prior to record retention. Also known as dropstones, these can be correlated with ocean temperatures and levels to better understand and calibrate models of the global climate.
The boulders were probably deposited by melting ice sheets at the end of the last ice age, around 12,000 years ago.
Weathering and erosion on the erratics
What is the difference between weathering and erosion?
Weathering is the process of decomposing, breaking up, or changing the colour of rocks. Weathering may be caused by the action of water, air, chemicals, plants or animals.
- Chemical weathering - involves chemical changes in the minerals of the rock, or on the surface of the rock that make the rock change its shape or color. Carbon dioxide, oxygen, water, and acids may all cause chemical weathering.
- Mechanical weathering - is the process of breaking a large rock into smaller pieces without changing the minerals in the rock. Mechanical weathering may be caused by frost, ice, plant roots, running water or heat from the sun..
Once the small pieces of rocks are changed or broken apart by weathering, they may start to be moved by wind, water, or ice. When the smaller rock pieces (now pebbles, sand or soil) are moved by these natural forces, it is called erosion.
So, if a rock is changed or broken but stays where it is, it is called weathering. If the pieces of weathered rock are moved away, it is called erosion.
Please take your time to examine and enjoy the boulders that can be found here and nearby. Visit the traditional cache nearby, N 53° 57.690 W 001° 40.002 to see some more boulders and visit the trig at point, see waymark below, for other great examples of glacial erratic rock formations.
In order to claim this Earthcache you will need to answer the following questions and e-mail them to the CO via the link at the top of the page. An optional photo of yourself or GPS device at GZ would be much appreciated. Please do not post any pictures of the stone at the posted co ordinates.
- Explain in your own words how the boulder at the posted co ordinates feels to the touch.
- How would you describe the grains that make up this boulder? This site may help http://geology.about.com/od/rocks/a/Rock-Tables.htm
- On top of the stone can be found 'pools' that fill with rainwater. How do you think they were formed, were they formed by chemical or mechanical weathering? Please describe in detail what you think has happened here to cause the ‘pools’?
- What would be the depth of the largest pool, on top of the stone be when full of water?
- What are the dimensions of the stone at the posted co ordinates?