Here's why:

* Slide: In a slide, a coherent mass of material moves downslope along a clearly defined plane or surface. This plane can be curved, like in your example. The movement is typically relatively fast and can be triggered by factors like rainfall, earthquakes, or human activities.

Other types of mass movements include:

* Creep: Slow, gradual movement of soil or rock downslope.

* Flow: Movement of unconsolidated material like mud, debris, or rock fragments that behaves like a fluid.

* Fall: Freefall of rock or debris from a cliff or steep slope.

Since your description involves a coherent block moving along a curved surface, a slide is the most accurate classification.