Mountain glaciers are like giant sandpaper blocks, scouring their valley homes as they advance and retreat. This scrubbing tends to purge any past record of a glacier's size, foiling scientists' efforts to model the extent of mountain ice during past climate shifts.

The good news is the sediments scoured from the land by glaciers eventually ends up dumped offshore, in the ocean. Scientists can extract the layers of mud and rock from ocean drilling cores to look at long-term changes in the size of glaciers, and how they relate to climate.