Surface exposure dating with cosmogenic nuclides
Before sampling a rock, geologists must take detailed and careful measurements of the landsurface, and satisfy themselves that the rock is in a stable position, has not rolled, slipped downslope, been repeatedly buried and exhumed during periglacial rock cycling within the active layer frequently a problem with small bouldersand has not been covered with large amounts of soil, snow or vegetation.Signs of subglacial transport Scratches striations on a sandstone boulder show that it has undergone subglacial transport and erosion.Difficulties in cosmogenic nuclide dating Solifluction lobes on the Ulu Peninsula.
Ian Hey Cosmogenic nuclide production rates vary according to latitude and elevation.Alternatively, if the boulder has not undergone sufficient erosion to remove previously accumulated cosmogenic nuclides, it will have an older than expected age. This can be a particular problem in Antarctica, where cold-based ice may repeatedly cover a boulder, preventing the accumulation of cosmogenic nuclides, without eroding or even moving the rock.Rocks can therefore be left in a stable position or moved slightly, without having suffiicient erosion to remove cosmogenic nuclides from a previous exposure. This is typically characterised by spread of exposure ages across a single landform.Sampling and dating boulders in a transect down a mountain will rapidly establish how thick your ice sheet was and how quickly it thinned during deglaciation.Many mountains have trimlines on them, and are smoothed and eroded below the trimline, and more weathered with more evidence of periglaciation above the trimline.