A rock formation is a natural structure composed of rocks. CRAG is a steep, rugged rock face or cliff, often found in mountainous areas. SCAR is a jagged mark left on a rock surface by a glacier or other erosive force. An ARCH is a curved structure made of rock that spans an opening. A geode is a hollow rock with a crystal-lined interior. ORE is a mineral or rock from which a valuable metal can be extracted. Sial is the upper layer of the Earth's crust, composed mainly of silicate rocks. Terrane is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or 'SUTURED' to crust lying on another plate.