Chapter 10

From the text:

Those lands had changed much since the days when dwarves dwelt in the Mountain, days which most people now remembered only as a very shadowy tradition. They had changed even in recent years, and since the last news that Gandalf had had of them. Great floods and rains had swollen the waters that flowed east; and there had been an earthquake or two (which some were inclined to attribute to the dragon-alluding to him chiefly with a curse and an ominous nod in the direction of the Mountain). The marshes and bogs had spread wider and wider on either side. Paths had vanished, and many a rider and wanderer too, if they had tried to find the lost ways across. The elf-road through the wood which the dwarves had followed on the advice of Beorn now came to a doubtful and little used end at the eastern edge of the forest; only the river offered any longer a safe way from the skirts of Mirkwood in the North to the mountain-shadowed plains beyond, and the river was guarded by the Wood-elves' king.

What is now the only safe path to the plains beyond mirkwood

What do we learn about Gandalf's plans

At a minimum, you can still refer to the study of this issue and other works of Tolkien