The Hudson is sometimes called, in geological terms, a drowned river. The rising sea levels after the retreat of the Wisconsin glaciation, the most recent ice age, have resulted in a marine incursion that drowned the coastal plain and brought salt water well above the mouth of the river. The deeply eroded old riverbed beyond the current shoreline, Hudson Canyon, is a rich fishing area. The former riverbed is clearly delineated beneath the waters of the Atlantic Ocean, extending to the edge of the continental shelf. As a result of the glaciation and the rising sea levels, the lower half of the river is now a tidal estuary that occupies the Hudson Fjord. The fjord is estimated to have formed between 26,000 and 13,300 years ago.