To search for terms starting with a certain letter, click a letter in the bar below.



A soil order ; these are soils with subsurface clay accumulations that possess low native fertility and are often red hued (due to the presence of iron oxides). They are found in humid tropical and subtropical climates.

ultramafic rocks

Igneous rocks with very low silica content (< 45%), which are composed of usually greater than 90% mafic minerals. The Earth’s mantle is composed of ultramafic rocks, which are dark green to black in color due to their high magnesium and iron content.


The relation between adjacent rock strata for which the time of deposition was separated by a period of nondeposition or erosion ; a break in a stratigraphic sequence.

underlying structure

A tectonic structure of rock located beneath younger beds that were not affected or were only slightly deformed by tectonic movements.


Upward movement of the crust due to compression, subduction, or mountain building. Uplift can also occur as a rebounding effect after the removal of an ice sheet reduces the amount of weight pressing on the crust.

Utica Formation

An organic-rich black shale from the Middle Ordovician. It is found throughout New York, Pennsylvania, Ohio, West Virginia, and other portions of eastern North America. The Utica Formation is an extremely rich source rock for oil and natural gas.
See also: natural gas, petroleum, shale