<P> Beachrock units form under a thin cover of sediment and generally overlie unconsolidated sand . They typically consist of multiple units, representing multiple episodes of cementation and exposure . The mineralogy of beachrocks is mainly high - magnesium calcite or aragonite . The main processes involved in the cementation are: supersaturation with CaCO through direct evaporation of seawater, groundwater CO degassing in the vadose zone, mixing of marine and meteoric water fluxes and precipitation of micritic calcium carbonate as a byproduct of microbiological activity . </P> <P> On retreating coasts, outcrops of beachrock may be evident offshore where they may act as a barrier against coastal erosion . Beachrock presence can also induce sediment deficiency in a beach and out - synch its wave regime . Because beachrock is lithified within the intertidal zone and because it commonly forms in a few years, its potential as an indicator of past sea level is important . </P> <P> Beachrocks are located along the coastline in a parallel term and they are usually a few meters offshore . They are generally separated in several levels which may correspond to different generations of beachrock cementation . Thus, the older zones are located in the outer part of the formation when the younger ones are on the side of the beach, possibly under the unconsolidated sand . They also seem to have a general inclination to the sea (50--150). There are several appearances of beachrock formations which are characterized by multiple cracks and gaps . The result from this fact is an interruptible formation of separated blocks of beachrock, which may be of the same formation . </P> <P> The length of beachrocks varies from meters to kilometers, its width can reach up to 300 meters and its height starts from 30 cm and reaches 3 meters . </P>

Where do rocks on the beach come from