A good marker horizon is distinctive, widespread, and represents a relatively short period of geologic time. A spherical or subspherical rock particle which has grown around a nucleus by accretion. Calcareous ooliths which are concentrated in beds to the exclusion of non-calcareous material form oolitic limestones. Some oolitic iron ores may contain primary ooliths of iron minerals. A structure with large spherical or subspherical masses made up of concentric shells of different mineral composition which occurs within certain plutonic rocks such as granites and diorites.
Pegmatite is also the source for many gem-quality precious and semiprecious stones.
The interpretation of geological features as exposed in aerial photographs of Earth or other planets. Aerial photographs commonly reveal details of the geology not detectable on the ground. Detailed study under the stereoscope provides almost enough data to plot a geological map. A process which takes place in a cycle of erosion when a region which has developed a mature drainage system is relatively uplifted with respect to sea level. A process of sediment transport by wind or water in which a particle jumps from one point to another above the ground or stream bed under the influence of a current too weak to keep it permanently suspended.