Environmentalists, legal scholars, rural residents and others are anxiously anticipating Interior Secretary Ryan Zinke’s coming national monuments proposal. Following an executive order from President Trump, Zinke is expected to suggest modifying the boundaries of at least some national monuments. While controversial, there is adequate legal and historical precedent to justify such boundary realignments.

Plenty of previous presidents have modified the boundaries of national monuments, but their federal courts have never interpreted how far that power extends. Fully rescinding a monument would be completely unprecedented, but there are arguments over its legality on both sides.

Short of complete revocation, however, reducing the size of a national monument is historically common and legally defensible. If the Department of the Interior finds that any of the national monuments under review fail to comport with the letter of the law, the Trump administration should not hesitate in reducing the size of those monuments.

The Antiquities Act gives presidents the explicit power to create national monuments on federal land. Many presidents have interpreted that same law to give them the implicit power to shrink or change national monuments when previous designations no longer fit within the law’s stipulations.

Numerous presidents have significantly reduced the size of national monuments unilaterally and without legal upset. President William Howard Taft established and subsequently shrunk Navajo National Monument to 89 percent of its original size. Various presidents interfered with the boundaries of Mount Olympus National Monument, eventually leaving it with around 50 percent of its original area. Presidents Harry Truman and Dwight Eisenhower significantly diminished Santa Rosa Island and Great Sand Dunes National Monuments respectively, and other presidents have made even smaller downward adjustments.

These historic reductions were not done in spite of the Antiquities Act, the law that gives presidents the ability to designate national monuments in the first place. Instead, they better aligned monuments with the actual wording and original intent of the act.

The act allows presidents to protect sites only of historic or scientific interest and requires that monuments be “the smallest area compatible with the proper care and management of the objects to be protected.” Its original intent was to defend antiquities from destruction during the West’s rapid development, and the “smallest area compatible” language was added to assuage Western states’ fears that the law would be used to orchestrate massive land grabs.

President Taft’s reduction of Navajo National Monument illustrates how monument reductions can further the Antiquities Act’s goals. His presidential proclamation was centered on a finding that the monument encompassed “a much larger tract of land than is necessary for the protection of such of the ruins as should be reserved.”

Zinke’s opponents are correct that no previous reduction got a formal seal of approval in a federal court, but that is only because no lawsuits ever questioned the president’s authority to shrink monuments. The Supreme Court did adjudicate a case on a separate question, Alaska v. United States, which accepted President Eisenhower’s redrawn boundaries for Glacier Bay National Monument without argument. Even a Congressional Research Service report concludes that “there is precedent for presidents to reduce the size of national monuments by proclamation.”

Zinke’s national monuments review covers monuments from the last 20 years that exceed 100,000 acres. It’s not hard to argue that some of those monuments take up areas larger than the “smallest area compatible” with the protection of historic and scientific sites when many exceed the size of Delaware. If President Trump were to reduce existing monuments to better fit that standard, as many past presidents have done, he would be honoring the Antiquities Act, not ignoring it.