Final answer:
The stripes of sediment that have fallen from valley walls end up in the middle of the glacier through a process called englacial or supraglacial sediment transport.
Step-by-step explanation:
The stripes of sediment that have fallen from valley walls end up in the middle of the glacier through a process called englacial or supraglacial sediment transport.
During englacial sediment transport, sediment is incorporated into the glacier as it flows, either through falling into crevasses or being deposited on the surface and then buried by subsequent snowfall. Over time, the sediment gets carried along as the glacier moves and eventually ends up in the middle of the glacier.
Supraglacial sediment transport occurs when sediment is deposited on the surface of the glacier, either as debris falling from valley walls or being carried by wind. This sediment then gets transported along with the glacier as it moves and can accumulate in the middle of the glacier.