The Barren Zone is the area of bare rock and ground left around the margins of a glacier when it is retreating. Typically this will be smooth rock with no soil covering, or fresh moraine lacking vegetation.
Glaciers that are advancing don't exhibit barren zones because the ice is moving forward. Only retreating glaciers exhibit them.
Photo of a glacier in Val Ferret, European Alps. The area surrounded by the red line is the Barren Zone. Most of this area was covered in glacial ice only twenty years ago. Vegetation has not yet managed to colonize the area, hence the description 'barren'.
Photograph of a glacier flowing into the Chamonix Valley, France (European Alps). The area surrounded by the red line is the Barren Zone.The height of the arete running along the left side of the glacier shows that the ice has not only retreated but has also thinned out. This is seen by many scientists as evidence supporting the existence of global warming.