Sea ice forms and remains fast along the coast, where it is attached to the shore, to an ice wall, to an ice front, between shoals or grounded icebergs. Vertical fluctuations may be observed during changes in sea level. For example, vertical movement may be evident due to tidal oscillations. Fast ice may be formed in situ by freezing of seawater, or by freezing of pack ice of any age to the shore. Fast ice may extend a few yards (meters) or several hundred miles (kilometers) from the coast. Fast ice may be more than one year old and may then be prefixed with an appropriate age category (old, second-year, or multiyear). If it is thicker than about 7 ft (2 m) above sea level, it is called an ice shelf.
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