The underlying bedrock or limestone of the Everglades basin affects the hydroperiod, or how long an area within the region stays flooded throughout the year.[36] Longer hydroperiods are possible in areas that were submerged beneath seawater for longer periods of time, while the geology of Florida was forming. More water is held within the porous ooids and limestone than older types of rock that spent more time above sea level.[39] A hydroperiod of ten months or more fosters the growth of sawgrass, whereas a shorter hydroperiod of six months or less promotes beds of periphyton, a growth of algae and other microscopic organisms. There are only two types of soil in the Everglades, peat and marl. Where there are longer hydroperiods, peat builds up over hundreds or thousands of years due to many generations of decaying plant matter.[40] Where periphyton grows, the soil develops into marl, which is more calcitic in composition.