Last week President Donald Trump issued an executive order directing Interior Secretary Ryan Zinke to conduct a review of at least 24 large national monuments on federal public lands and to make recommendations for their reduction or revocation. While size appears to be a central concern—the president specified that all monuments spanning at least 100,000 acres must be reviewed—the executive order leaves the door open for Zinke to review any monument designated since January 1, 1996.

So when is a monument too big? The Antiquities Act instructs that a monument “be confined to the smallest area compatible with proper care and management of the objects to be protected.” But sometimes historic and scientific “objects” are pretty, well, monumental. Take the Grand Canyon.

In 1908, less than two years after passage of the Antiquities Act, President Theodore Roosevelt designated the Grand Canyon National Monument. He declared 800,120 acres of the canyon an “object of unusual scientific interest.” It was the largest of the 18 monuments Roosevelt created, and it paved the way for the canyon’s subsequent designation as a national park by Congress and later as a World Heritage Site by the United Nations Educational, Scientific and Cultural Organization.

The text of the presidential proclamation that designates each monument guides how public lands included in it are managed. Proclamations generally accommodate a variety of uses and inevitably reflect compromises. At the Grand Canyon, for example, President Roosevelt included a “savings clause” to protect existing mining operations within monument boundaries while preventing new claims. Regardless, the designation drew fierce criticism from the mining industry in the Arizona Territory.

Businessman and politician Ralph H. Cameron was one of those critics. He improved 20 acres at the trailhead of the Bright Angel Trail, building a stable and other structures, under the premise that he held a lode mining claim that predated the monument designation. Perhaps not coincidentally, the claim included a primary entry point for tourists to access the canyon.

Following the monument designation, Cameron defied the president and refused to leave. The federal government sought and successfully obtained a court order to reclaim the parcel. Cameron lost in lower courts and appealed to the U.S. Supreme Court, which heard his case in 1920.

In addition to arguing that he had a valid mining claim, Cameron urged the court to find the monument designation invalid for its size, arguing that it did not meet the “confined to the smallest area” standard. Justice Willis Van Devanter flatly rejected this argument. Writing for a unanimous court, he explained:

[T]he act under which the President proceeded empowered him to establish reserves embracing "objects of historic or scientific interest." The Grand Canyon, as stated in his proclamation, "is an object of unusual scientific interest." It is the greatest eroded canyon in the United States, if not in the world, is over a mile in depth, has attracted wide attention among explorers and scientists, affords an unexampled field for geologic study, is regarded as one of the great natural wonders, and annually draws to its borders thousands of visitors.

The court did not need to reach this conclusion in order to dispose of the case; it had already rejected Cameron’s mining claim, barring further access. But by upholding the designation of a large monument, the court affirmed for future presidents great discretion in the use of their congressionally delegated authority under the Antiquities Act. The result has been unparalleled and lasting protections for some of the country’s most important cultural, historic, and scientifically unique landscapes.

Indeed, every subsequent federal court to consider a legal challenge to a national monument has upheld the designation. Most recently, the U.S. Court of Appeals for the D.C. Circuit rejected a challenge to six monument designations by President Bill Clinton, citing Cameron v. United States to support the proposition that monuments need not include solely man-made objects. District courts have ruled similarly. In 1945 a court upheld President Franklin D. Roosevelt’s designation of the 221,610-acre Jackson Hole National Monument, finding that “objects” can include fur trapping and hunting trails, structures of glacial formation, peculiar mineral deposits, indigenous plant life, and wildlife habitat. And in 1980, in Anaconda Copper Co. v. Andrus, a court denied a copper company’s challenge to President Jimmy Carter’s designation of three large national monuments in Alaska, which together constitute nearly 20 million acres.

The future of the Antiquities Act remains to be seen. Several bills have been introduced in Congress that would weaken the president’s discretionary authority to protect historic and scientific objects on public lands. On Tuesday, May 2, the House Natural Resource Committee’s Subcommittee on Federal Lands held an oversight hearing on the Antiquities Act related to the president’s recent order. The hearing memo uses strong language to condemn the act itself, calling it “outdated,” boldly suggesting that designations “lock up” land.

The National Trust weighed in, expressing our strong support for the law and its legacy. As the administration and Congress consider changes to the Antiquities Act, or existing designations that protect irreplaceable historic and cultural resources, we will continue to make our voices heard and encourage our supporters to do the same.