PEDIMENT

in architecture, triangular gable crowning a portice (area, with a roof supported by columns, leading to the entrance of a building); or a similar form used decoratively over a doorway or window. The pediment was the crowning feature of the Greek temple front. The triangular wall surface of the pediment, called the tympanum, rested on an entablature (a composite band of horizontal moldings) carried over the columns. The tympanum was often decorated with sculpture, as in the Parthenon (Athens, 447432 BC), and was always crowned by a raking, or slanted, cornice. The Romans adapted the pediment as a purely decorative form to finish doors, windows, and especially niches. Their pediments frequently appeared in a series consisting of alternating triangular and segmentally curved shapes, a motif revived by High Renaissance Italian designers; particularly fine examples are the window pediments of the piano nobile (floor above the ground floor) of the Palazzo Farnese (Rome, begun in 1517), built by Antonio da Sangallo the Younger. Following a late Roman precedent, in which the line of the raking cornice is broken before it reaches the apex, the designers of the Baroque period developed many varieties of fantastic broken, scrolled, and reverse-curved pediments, an example of which can be seen on the Church of San Andrea al Quirinale (Rome, 165870) by Gian Lorenzo Bernini. In some cases the designers even reversed the direction of the form so that the high points of a broken pediment faced toward the outside of the composition rather than toward the centre; and in the elaborate Churrigueresque, or late Renaissance, architecture of Spain, small sections of pediment were used as decorative motifs. in geology, any relatively flat surface of bedrock (exposed or veneered with alluvial soil or gravel) that occurs at the base of a mountain or as a plain having no associated mountain. Pediments, sometimes mistaken for groups of merged alluvial fans, are most conspicuous in basin-and-range-type desert areas throughout the world. The angle of a pediment's slope is generally from 0.5 to 7. Its form is slightly concave, and it is typically found at the base of hills in arid regions where rainfall is spasmodic and intense for brief periods of time. There is frequently a sharp break of slope between the pediment and the steeper hillside above it. Water passes across the pediment by laminar sheet flow, but if this is disturbed, the flow becomes turbulent and gullies develop. Though features characteristic of pediments attain their fullest development in arid regions, beveled bedrock surfaces also occur in humid areas. In the tropics, for example, the surfaces tend to be mantled with soils and obscured by vegetation. Many tropical towns sited on pediments (which offer easier building sites than the steep hillsides above or the river marshes below) show severe gullying where the water flow has been concentrated between walls and buildings.