This image shows the lower part of the Tasman Glacier. Here it is almost completely covered with rock debris. You can see the white ice in the far distance to the left, marking roughly the half way point along the glacier.
The rock debris tends to gather on the surface as the surrounding ice melts away in the warmer temperatures at the lower elevations. If the debris is a very thin layer (under about 10cm thick) the darker colour of the rock vs ice, absorbs the sun’s energy, increasing the melting of the ice just below. However if the debris is thicker, it acts as a protective insulator, preventing the sun from melting the ice so quickly.