There are also large boulders that block pathways and shortcuts through the mountain area. These rocks will be destroyed by the wild bears as you repair the tunnel between the towns. You do not have to befriend the bears to break the boulders. The bears hibernate during Winter and are not available to destroy the boulders during that season.

To activate a bear event, walk into the area with the boulder between 9:00 am and 6:00 pm on a sunny day after you've completed a tunnel repair. The boulders are smashed in a specific order:

Konohana Mid-Mountain: Opens the lower path between Konohana Mid-Mountain and Konohana High-Mountain. Requires Tunnel Repair #1 to be completed.Konohana Mid-Mountain: Opens the path in the northwest corner to Konohana High-Mountain. Requires Tunnel Repair #2 to be completed.Konohana Mid-Mountain: Unlocks the jump mushroom shortcut back up to the path. Requires Tunnel Repair #3 to be completed (basically, the tunnel has been reconnected).Bluebell High-Mountain: Unlocks the jump mushroom to reach the area below the bridge. Requires Tunnel Repair #4 to be completed (the lower tunnel pathway).

source: http://fogu.com/hm10/the_two_towns/mountain.php