The ice-sheet wore down from the hills and filled the valleys with debris until the surface has a billowy appearance.
The billowy plains still remain in places, but in the vicinity of streams the billows give way to deep ravines.
The pahoehoe has a smooth but billowy or hummooky surface, and is marked by lines which show that it cooled as it flowed.
They consist of slightly rounded domes or billowy snowfields of vast thickness.
The smooth surface of the viscous billowy lava is further diversified by long twisted " ropes," curving backwards and forwards up and down the undulations.