Here's why:

* Large sheet-like mass of rock: Rock glaciers are essentially frozen masses of rock debris, often mixed with ice, that flow slowly downhill like a glacier.

* Moved around one mile: While the movement of a rock glacier is slow (a few meters per year), over time they can travel significant distances, sometimes exceeding a mile.

Other possibilities:

While the most likely explanation is a rock glacier, there are other possibilities depending on the specific context:

* Rock slide: A large mass of rock that has broken loose and slid downhill. These can travel significant distances, but they typically move much faster than rock glaciers.

* Talus slope: A steep, unstable slope composed of broken rock fragments. These can move slowly over time due to gravity, but are unlikely to travel a mile unless there was a major event.

To determine the best answer, you'd need more information about the specific location and geological context.