Breccia. Coarse sedimentary rock consisting of rock fragments; there is a layer of breccia between the Silurian graywacke and the Old Red Sandstone at Siccar Point.

Dike. A tabular (wall-shaped) intrusion of rock that cuts across the layering of preexisting rock.

Erosion. The grinding away and removal of the earth's surface materials by moving water, air, and ice.

Granite. A coarse-grained igneous rock. Werner argued that granite was the first rock precipitated from the receding universal ocean; thus it was the oldest type of rock on earth; Hutton argued that granite came from the subterranean regions of the earth and that it was often younger than sedimentary rocks (Hutton was right).

Ice Age. An interval of time in which the climate was colder than it is today and glaciers advanced to cover large areas of the continents. Mountain glaciers also grew.

Igneous Rock. Rock that forms when hot molten rock (magma or lava) cools and freezes solid.

Intrusion. Rock formed by the freezing of magma underground. The igneous veins found at Glen Tilt are intrusions.

Limestone. Sedimentary rock composed of calcite; most limestone consists of the shells of dead organisms like clams, corals, and plankton.

Lithification. The process that causes loose sediments to convert to sedimentary rocks. This transformation is caused by: (1) compaction from the weight of sediments above, and (2) cementation, caused when minerals precipitate out of water solutions passing through the sediment. Hutton was the first to understand the role of pressure, but he did not know about cementation.

Marl. A sedimentary rock that is essentially a mix of limestone and clay; it forms in coastal environments where rivers and streams flow into lakes, seas, or oceans. It is very common on the east side of England and Scotland.

Metamorphic Rock. Rock that forms when preexisting rock (either igneous or sedimentary) changes to new rock as a result of an increase in pressure and temperature. For example, marble is the metamorphic rock that results from the extreme pressuring and heating of limestone, which is a sedimentary rock. The existence of metamorphism was not understood in James Hut-ton's day, and many metamorphic rocks were confused with sedimentary, because both can have layering. Hutton's insistence that both subterranean heat and pressure were needed to form stratified rocks was correct for metamorphic rocks, but incorrect for sedimentary.

Plate Tectonics. The theory that the outer layer of the earth consists of separate lithosphere plates that move with respect to one another.

Precipitate. The solid crystals formed when atoms dissolved in a solution come together. Werner's system of the earth depended on precipitation for the formulation of all rocks; Werner's followers thought that rocks, such as granite, formed as the result of the receding and evaporation of the universal ocean. This is not correct; granite forms by solidification of a melt.

Rock. A coherent, naturally occurring solid, consisting of an aggregate of minerals or a mass of glass.

Sandstone. A sedimentary rock consisting almost entirely of sand grains; the sand is typically composed of quartz.

Sedimentary Rock. Rock that forms either by the cementing together of fragments broken off preexisting rock or by the precipitation of mineral crystals out of water solutions at or near the earth's surface.

Sills. A nearly horizontal (like a windowsill), tabletop-shaped tabular intrusion that injects between layers of preexisting rock.

Subsidence. The vertical sinking of the earth's surface in a region relative to a reference plane. Subsidence creates space for sediment layers to accumulate.

Unconformity. A boundary, between two different rock sequences, that represents an interval of time during which sediments were not deposited and/or were eroded.

Uplift. The vertical elevation of land. It is caused by a variety of pressures, all related to the movement of crustal plates or mantle flow. Uplift can yield mountain ranges.

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