Shelves are not of uniform width. They vary considerably in size off the coasts of the United States. They are almost negligible along Southern California and Florida's southeast coast. Off Florida's west coast however, the shelf extends 200 miles into the Gulf of Mexico. The average width worldwide is about 40 miles.
Shelves look like the adjacent land.
Coastal areas that are mountainous will have a continental shelf with similar features, which is the case off Southern California. Coastal areas dominated by low hills or plains are likely to have a shelf of nearly flat plains or low hills. This is observed along most of the U.S. Atlantic and Gulf Coasts. Shelves off the mouths of larger rivers such as the Mississippi, are usually broad with a large mud base of continental sediments. Continental shelves along the coasts of the United States cover an area of about 891,000 square miles.