Super erratics and erratic clusters

 I came across this on the fantastic "Glaciers Online" site, organized by Swiss Education.  This is the famous "super erratic" at Bremgarten in Switzerland.  Notice that it's in four large pieces -- so as in the case of the Big Rock at Okotoks in Canada, we have in effect a "giant erratic cluster."

http://www.swisseduc.ch/glaciers/index-en.html

If you look further on the Glaciers Online site there are many other spectacular photos of erratic clusters, like this one on Svalbard:

These are granite erratics that appear to have been transported as a group from a long way off -- reminiscent of the Darwin Boulders in Patagonia.

Here's an even more quirky occurrence from Broggerbreen in Svalbard -- a rectangulat piece of granite that looks for all the world as if a stonemason has been hard at work on it!

How are erratics like this entrained?  Well, there appear to be two (at least!) prerequisites:  high stresses on the glacier bed, creating fractures which weaken the coherence of the rock and permit quarrying to take place; and then freezing-on at the bed, with large stones picked up and carried off downglacier.  Here is a rock surface (again in Svalbard) that has been broken up by subglacial processes and "prepared" for exploitation:

Some blocks have already been dragged away.

I remain convinced that in the right circumstances extremely large masses of bedrock can be dragged away by overriding ice and transported for hundreds of kilometres within a glacier.....