basement rock -- n. The oldest rocks in a given area; a complex of metamorphic and igneous rocks that underlies the sedimentary deposits. Usually Precambrian or Paleozoic in age.
Basement Rock Very old granite and metamorphic rocks found in continental crust. These rocks make up the continental shield. Basic Substance having a pH greater than 7.
In some areas such as shields the basement rocks may be exposed at the surface.
Basic Rock:
An igneous rock that has a relatively low silica content. Examples are gabbro and basalt.
 See also: Basin, Erosion, Origin, Surface, Sedimentary rock
  
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