It's not surprising that the authors avoided sampling bedrock underlying the erratics.
In areas where ice has been frozen to the bed, concentrations of cosmogenic nuclides in bedrock often tell a garbled story (Bierman et al., 1999).
These ages are young enough that they are inconsistent with the East Antarctic Ice Sheet contributing significantly to late Pleistocene sea-level rise.
Similarly, the decrease in ice thickness that Mackintosh et al.
Yet, children born when the first paper using cosmogenic nuclides to date such erratics was published (Phillips et al., 1990) are still not old enough to vote. took a simple and oft-used approach for characterizing the vertical extent of now-vanished ice.