This is an interesting photo, taken near the edge of the Greenland Ice Sheet. We can see three different types of erratic train. On the right there are two moraine trails which can be traced back to mountain fronts which stand clear of the ice. These trails combine into one near the edge of the photo. Lower down, on the right side of the photo, there is a discernible moraine which can be traced back to a "rise" in the ice surface -- attributable to an isolated upland which is perhaps an accumulation centre although it is really a part of the ice sheet. There must be some erosion going on here, with debris entrained and carried away from the location where erosion is active.
The third and longest erratic train or moraine ridge spreads out at the bottom right of the photo where the ice is decelerating and spreading laterally. But notice how thin and well defined the trail is on the surface of the ice sheet, stretching far into the distance. The trail appears to pass between two upland areas, but it has some of the characteristics of a medial moraine. But we can see no obvious source for all this debris, which appears to be feeding into the glacier more or less continuously. But the source must be BENEATH the ice, and not above it -- and this illustrates the type of entrainment which I envisage for the bluestone erratics that have found their way to Stonehenge.
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