Natural Barricade 2 : Natural features of the ground, such as hills, or timber of sufficient density that the surrounding exposures that require protection cannot be seen from the magazine when the trees are bare of leaves.
United States Mine Rescue Association USMRA (membership,mine rescue and safety training resources, and mine disasters and accidents), Mine Safety and Health Quizzes, Glossary of Blasting Terms, PA,USA 2016. http: //miningquiz.com/pdf/Glossaries/Glossary_of_Blasti ng_Terms.pdf, (Blasting - Amer) {997}