Stop+1+Glacial+Erratic

[[image:reduced1.JPG width="235" height="162" align="left"]]GLACIAL ERRATIC
This large rock is called a glacier erratic (glacier "boulder" is a term the general public understand and it is presented on the plaque attached to the boulder). It was carried from tens or hundreds of kilometres away on top of, or within, the 1-1.5 km thick ice sheet that covered this area until approximately 10,000 years ago. When the ice melted it dumped the rock on campus (actually, the glacier was dropped in behind the Lady Beaverbrook residence from whence it was moved to the present site when the northern extension to the building was built).

The rock is an igneous rock (i.e. it crystallized from magma) and is completely different from the local sandstones found below the soil. The rock cooled deep in the Earth's crust before being eroded millions of years later by the glaciers.

EXERCISES: 1. Most of the crystals in the rock are (choose one): > Take care to ignore the black lichen that is growing on the rock - which forms tiny masses all over the rock. The area under the plaque is whiter because it is not covered with lichen!
 * A. 0.5 - 1.0mm in diameter (fine grained)
 * B. 1 - 3mm in diameter (medium grained)
 * C. 3 - 10mm in diameter (coarse grained)

2. Crystal size is related to the rate at which the rock cooled and crystallised. Larger crystals indicate slow cooling of the magma while smaller crystals suggest fast cooling. Do you think the magma cooled at a slow/medium or fast rate, based on your observation of grain size?

3. To show you have seen this rock, record the name of the two geologists recognised on the plaque attached to it.

To learn more about erratics and how they're deposited, take a look at the animation below (requires flash animation): media type="file" key="cannock01[1].swf"