How to get more Yerba Santa?

After you have completed the question 'The Other Side of the River' you'll need to collect more Yerba Santa. Where you cross the river you should find some more. Start clearing a path to get to it. The game actually wants you to use your dynamite at this point. After crossing the river, place the dynamite on the floor on the path and set it off, you'll get a load of resources and can carry on with your other quests.