U

umbel— A flower cluster in which the pedicels come from a common point, like the rays of an umbrella. (See Fig. 6.1.)

understory— Plants beneath the tree canopy in woodland or forest, commonly composed of shrubs, herbs, and juvenile trees.

ungulate— A hooved grazing animal.

uplift— A structurally high area in the crust, produced by positive movements that raise or thrust the rocks upward.

uranium fission— A geological dating method in which the splitting of the nucleus of a uranium atom results in the emission of energy and physical damage to the mineral structure. The number of fission tracks (damaged paths) is indicative of time.