In the column regarding Death Valley National Monument , it is stated that National Parks are created by an act of Congress, and national monuments by order of the President via the Antiquities Act of 1906. So, with the stroke of a pen the President can create a National Monument – is it that easy to make a National Monument (or National Park for that matter) not one anymore? What hoops need to be jumped through to reverse National Monument or Park status?
It depends. If it’s been a national park for more than a trimester, it takes a declaration by the Supreme Court, but then a partial park reversal can be negotiated… if it’s covered by your HMO.
Cecil is able to see at least 12 years into the future. In the Death Valley column, written in “27-Aug-1982” he says:
I bow to you, omniscient one. 
The Antiquities Act is included in the United Stated Code as Title 16, Section 431. This section appears to be a one-way mechanism for creating National Monuments:
It is also apparent that the Congress can modify or possibly even overturn these declarations. For instance, Section 431a forbids creation or expansion of National Monuments in Wyoming without Congressional authorization.