In the United States, a National Monument is a protected area that can be created from any land owned or controlled by the federal government. There are however a handful of “monuments” that are thought of as official US parks, but are not. Four Corners and Monument Valley are two of the more prominently known of those.
In the past, many National Monuments have eventually changed designation and eventually became a National Park. This can only be done by proclamation of the President of the United States or an act of Congress.