Which law allowed theodore roosevelt to set aside federal land for conservation?

The law that allowed Theodore Roosevelt to set aside federal land for conservation was the Conservation Act.

Conservationists, led by President Theodore Roosevelt and his close ally Gifford Pinchot, pointed out that the majority of natural resources in Western countries should belong to the federal government and the best course of action, they argued, was a long-term plan devised by national experts to maximize the long-term economic benefits of natural resources.

This law, sanctioned by Congress and signed by President Roosevelt on June 8, 1906, conferred on the President of the United States the authority to restrict the private use of public lands owned by the federal government by executive order, without requiring the Congress supervision.

The law was intended to allow the President to reserve certain valuable natural public areas such as parks and protected lands. These areas had the title of "national monuments". It also gave him the power to reserve or accept private lands for that purpose. The objective was to protect all historical and prehistoric sites on federal lands of the United States and to prohibit the destruction or excavation of these antiquities. With this law, this could be done more quickly than by managing the process for the creation of a national park before Congress. The law established that the zones to declare monuments were limited to being the smallest surface compatible with the good care and handling of the objects that had to be protected.

Some areas designated as national monuments were later converted into national parks, or incorporated into existing national parks.