Fore-reef slopes vary from relatively gentle to nearly vertical. The steepnessdepends on the action of wind and waves,the amount of sediment flowing down theslope, the depth and nature of the bottomat the reef base, and other factors. As withother types of reefs, the abundance anddiversity of corals on the fore-reef slopegenerally decreases with depth. Thegrowth form of the corals also changesdown the slope. At the crest, under thepounding of the waves, the corals aremostly stout and compact; many are mas-sive (see Fig. 14.6). Below the crest thereis great variety in form. Whether they form branches, columns, or whorls, coralsin this zone often grow vertically upward. This may be an adaptation for competi-tion. Corals that grow upward like sky-scrapers rather than outward need lessspace to attach. They are also less likely tobe shaded, and if they spread out at thetop, can shade out other corals. Deeper onthe reef slope, corals tend to grow in flatsheets, which probably helps them collectmore light (see Fig. 10.1)