An old message (2007) from Lynda Dredge:
Most of my research has been done in the north over more subdued Shield and limestone landscapes. Most of the erratics that I have traced as “trains” are smaller limestone-sourced blocks being transported over granitic terrain. (Most of my work on this was in Manitoba west of Hudson Bay and farther north, around Foxe Basin/Baffin Island.) The smaller size of the boulders would partly be due to the ease of comminution of the limestone, and splitting of the erratics along the limestone bedding planes. I have found that the limestone blocks (as well as carbonate matrix) have traveled more than 75 km, and the matrix material much farther, up to several hundred km. (see Dredge, 2000, Boreas: 144-156 for example). The dispersal trains in the Melville Peninsula and Manitoba examples (Can J of Earth Sciences, 1988) are quite broad, but within them are secondary dispersal plumes and erratic trains with exceedingly sharp lateral limits. The trains are relatively narrow – more like 300 m across. I have attributed these plumes to ice streams within a larger ice sheet.
This is a situation very close to that which Lionel Jackson and I have speculated about for the Irish Sea ice stream coming into Somerset.
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