What behaviour does an ice sheet exhibit in crossing a cliff?
This depends on a few things.

If the cliff is small but widespread or the glacier is a huge, the glacier flows uphill until the cliff is gone. 

If the cliff is large the glacier flows around it, is diverted by it, or piles up behind it depending on the land around it.  

Glaciers are pushed along by back pressure, like toothpaste out of a tube. As long as there is enough accumulation the toe of a glacier can be driven uphill, but like anything else glaciers take the path of least resistance so if it can be diverted by the cliff it will. 

Note that the forces on the cliff will be huge, and exactly the forces rock is weak against, so the cliff is over topped it will be stripped away very quickly.