Here are some examples of what that might mean depending on the context:

In geology:

* Landslides: A large mass of soil, rock, or debris moving downhill.

* Glaciers: Slow-moving rivers of ice that carve out valleys and transport sediment.

* Lava flows: Molten rock that flows from a volcano.

In engineering:

* Conveyor belts: Used to move large quantities of materials like grain, coal, or gravel.

* Dredging: Removing large quantities of sediment from the bottom of bodies of water.

* Construction: Moving large amounts of earth or concrete during building projects.

In everyday life:

* Moving a large piece of furniture: You might use a dolly or other tools to move it as one unit.

* Traffic jams: A large number of vehicles stuck in a single, slow-moving mass.

It's important to consider the context to understand what "material moves in one large mass" means.

Can you tell me more about what you're interested in? I might be able to give you a more specific answer.