Strata are layers of rock, whether of sedimentary (e.g., sandstone or limestone) or of extrusive igneous (e.g., lava flow) origin. Sedimentary strata are formed when Earth's gravity acts upon particles being transported by wind, water, or ice and pulls them down to the earth's surface, where they form a layer. Sedimentary strata also may form from debris flows and viscous mud flows that move according to gravity. Extrusive igneous strata are formed when Earth's gravity acts upon particles within viscous molten rock and pulls them into a sheet-like or tabular mass called a lava flow. Extrusive igneous strata can also form when pyroclastic material is blown out of a volcano and falls to Earth, forming a layer of volcanic debris. All such layers obey the laws of superposition, original horizontality, and lateral continuity. Of these laws, original horizontality is most pertinent to...