Once in depth of snow and ice reaches up to more than 30-40 meters the force of friction begins to pull a glacier downhill?

Once the depth of snow and ice exceeds 30-40 meters, the weight of the ice causes it to deform under its own mass. This deformation creates a slippery base that allows glaciers to slide downhill due to the force of gravity. The friction between the glacier and the underlying bedrock also aids in the glacier's movement.