Combine the flour, brown sugar, oats and cinnamon.  Cut in the margarine until the mixture resembles coarse crumbs.  Toss 1 cup of the crumb mixture with the 2 cans of yams (drained) and the cranberries.  Place in a 1 1/2 qt casserole dish.  Top with the remaining crumbs.  Bake at 350 degrees for 35 minutes, then sprinkle marshmallows on top.  Broil until lightly browned.