This recipe will make enough "sedimentary rocks" for a class of about 20.
1/2 cup butter or margarine
4 cups puffed rice cereal
6 cups mini marshmallows
(You can also use chocolate chips, or different colored marshmallows for different "sediments"!)
In a large saucepan, melt butter or margarine on low heat. Add mini marshmallows, cook and stir until marshmallows are melted, remove from heat. Add puffed rice cereal and stir until well coated! Pour or scoop mixture into a lightly greased pan or onto wax paper. Let sit for 10 minutes and then cut into your sedimentary rock squares.