Purchaseless

The use of these projections […] is to apply upon the foetal scalp a pressure, counter to that made on the skull by the teeth of the crotchet, in order to ensure the penetration of the latter without the possibility of their slipping purchaseless along the smooth cranial bones, or of failure from any other cause whatsoever.

As he clawed at the gully's purchaseless walls, the laminar flow of snow debris rose to his waist, his chest, and then his shoulders, drawing him inexorably under, until snow topped his head and locked him into darkness.