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30 March 2016

Drumlin hills, shaped like upturned boats, are an Ice Age land form, well known in geology. However, the use of satellite images and LiDAR (ground penetrating radar from aircraft flying over regions of interest) has shown up the surface of our planet in great detail – and is springing surprises. Drumlins are often found in clusters, sometimes in their hundreds and thousands (mainly in Canada although drumlins also occur in Europe, even as far south as Ireland). They are thought to be footprints of the Ice Ages.

The big question has always been – were they formed progressively over a long period of time, beneath the ice sheet, or were they rapidly formed. New technology shows drumlins are just 'islands' of sediments and they can form long ridges up to a kilometer in length. They suggest drumlins and related land forms are caused by erosion but the data shows they have accumulated on hard rock surfaces too. This indicates a slow moving ice sheet undermined by fast or flowing corridors or ice streams which move more quickly. Melt water is not mentioned – but presumably the ice sheet when in a melting stage could give rise to drumlin fields. These ice streams or corridors move debris towards the margin of the ice sheets and in that respect drumlin fields can be used to mark the boundaries of ice sheets – and in this instance we are talking about the ice sheet of the Late Glacial Maximum. Receding glaciers in the modern world have exposed underlying beds of gravel, stones, and general rocky debris so the theory has a basis in reality. It is observable in the natural world. However, the authors also say the ridges of material may date to the 'last gasp' of the ice sheet – and here we are getting at the nub of the new findings. What role does water from a melting ice sheet play?

This is not just a matter of dating when drumlins or other land forms came into being – during the LGM or towards the close of the LGM, but it also impinges on the extent of the Laurentide ice sheet (the Late Glacial Maximum ice extent) as drumlins and those ridges may have run off the ice sheet (formed by melt waters) and the extent of Laurentide might have been somewhat narrower than it is in current mainstream geology. It definitely reached further south than earlier ice sheets (or the more recent ice sheets as really old ice sheets have been rubbed out by subsequent geological processes) so that must remain. I am thinking of how far west did the Laurentide ice sheet extend? Somebody with the surname of Shaw contended many years ago that it was not much further than Ontario. Don't know if this was disproved – or what. It would cause a rethink on the LGM if the ice sheet was restricted more or less to NE N America. Anyway, the article is published in Sedimentary Geology (March 2016), not a journal that causes much of a stir or one that is brought to the attention of the likes of PhysOrg or Science Daily, but in this instance the University of Toronto is responsible for the press release). Alternative opinions might be worth investigating via Google Scholar. Anyone with that inclination can get back to me via the contact link on our web page.

Oh, I forgot, the link is http://phys.org/print378470982.html. Enjoy.

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