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Glossary
Term:
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| Ultimate phalange | The last digit. |
| Ultramafic rock | An igneous rock composed entirely of ferromagnesian minerals. |
| Ultraviolet radiation | That part of the electromagnetic spectrum ranging in wavelength from 0.2 to 0.4m m. Just past the visible violet spectrum. |
| Unconfined aquifer | An aquifer with no confining geologic structure on the aquifer (aquifer pressure about the same as atmospheric pressure). |
| Unconformity | A discontinuity in the succession of rocks, containing a gap in the geologic record. A buried erosion surface. See also nonconformity. |
| Unconsolidated | Loose earth material such as gravel, sand, or lava that hasn't solidified yet. |
| Ungulate | a term that refers to hoofed animals. This includes Family Cervidae (deer) and Bovidae (sheep and goats) as well as the pronghorn. They typically have antlers or horns and are herbivores. |
| Uniformitarianism | The theory that geologic events in the past were caused by natural processes which are operating at the present time. |
| Unsaturated zone | The zone below the surface and above the water table, in which pore spaces are usually filled with air. Synonymous with phreatic zone. |
| Uplift | A structurally high area in the crust, produced by movements that raise the rocks, as in a broad dome or arch. |
| Upwarp | An arched or uplifted segment of the crust. Synonymous with uplift. |
| Urostyle | A rod-like bone composed of fused tail vertebrae, present in frogs and toads (Halliday 1987). |
| U-shaped valley | A valley with a U-shaped cross-section caused by glacial erosion. Contrast with the V-shape of a typical stream valley. |
| Uterus | the hollow muscular organ in female mammals in which the fertilized ovum usually becomes embedded, and in which the developing embryo and fetus are nourished (Morris 1992). |