Separation from the bed occurs if the water pressure exceeds the lowest normal stress, occurring at the lee sides of the bumps. When water pressure exceeds separation pressure, a cavity starts forming in the glacier and the whole ice mass is pushed downslope. If water pressure is lower than separation pressure, no cavities form.

The separation pressure decreases with increasing basal shear stress and increases with increasing roughness or obstacle size.

http://people.ee.ethz.ch/~luethim/pdf/script/pdg/pdg.pdf