Answer :
Gran Colombia would encompass today's Colombia, Venezuela and Equador (and some other territories). The mountains that were an obstacle for this were the Andes.
I believe the answer is: the andes
Simon Bolivar aware that Crossing the andes would take out a lot of time and resources for simon bolivar's army.
But, The conviction that Simon's army would not be able to cross the andes also make the opposition lower their guard. On may 23rd, Simon Bolivar confided his plan to finally brought his army across the Andes, and surprise the enemy on the New Granadan side.
