what natural boundary did the Allied forces want to cross before Christmas but couldn't.?

Question

and also why was hitler so determined not to let them cross?

Answer 1

The Rhine.
This was the most formidable barrier to allied forces getting to the industrial centre of the Ruhr.
The Rhine was eventually crossed at Remagen despite the bridge being badly damaged by the defenders.
A temporary bridge was erected to allow allied forces to cross in large numbers.
This was brought from England and was originally the temporary Waterloo bridge across the Thames in London when that bridge was being replaced just before the war.