The great barrier of Comb Ridge forms the east margin of the Bears Ears National Monument. The Abajo Mountains are the background. Comb Wash is the prominent drainage. (Robert Fillmore/Robert Fillmore)

MORE THAN a century ago, Congress gave the president extraordinary powers to protect precious public lands, in part to enable the White House to quickly preserve the Southwest’s Anasazi ruins from looters. Since then, presidents have used the 1906 Antiquities Act to create scores of national monuments on hundreds of millions of acres. But a president has never tried to undo a national monument, and it is unclear whether he has the authority to do so.

For that matter, it is unclear whether President Trump will try to do so. But there are ominous signs that he is considering at least shrinking protected areas. In an executive order last week, Mr. Trump asked for a review of monuments that are more than 100,000 acres and were designated in 1996 or later. “Historically, the Act calls for the President to designate the ‘smallest area compatible with proper care and management of the objects to be protected,’ ” Interior Secretary Ryan Zinke said in a statement. “Despite this clear directive ‘smallest area’ has become the exception and not the rule.”

In fact, recent presidents have used the Antiquities Act ambitiously, but, for the most part, not in unprecedented ways. From the start, presidents used the act to preserve huge tracts of land. Teddy Roosevelt designated the Grand Canyon, at more than 800,000 acres, as a national monument. Franklin D. Roosevelt vastly expanded protected zones in the Grand Tetons. Jimmy Carter created the largest land-based national monument, 10.95 million acres in Alaska’s Wrangell-St. Elias National Monument.

The Antiquities Act has protected California’s redwoods and Washington State’s Olympic Peninsula, Wyoming’s dramatic Devil’s Tower and Arizona’s Petrified Forest. The act also preserves historical sites, such as the Obama-designated monuments dedicated to Cesar Chavez and to the Reconstruction Era.

Mr. Obama created more large monuments than any of his predecessors. He and President George W. Bush each set aside hundreds of millions acres, but their largest by far preserved big portions of Pacific Ocean habitat, which do not seem to be the object of Trump administration scrutiny. Instead, the prime targets appear to be two national monuments in southern Utah: President Bill Clinton’s 1.7 million-acre Grand Staircase-Escalante National Monument and Mr. Obama’s 1.35 million-acre Bears Ears National Monument. The former is a spectacular red-rock landscape that tells a geologic story over hundreds of unspoiled miles. The latter also has glorious landscape, and contains priceless Native American ruins.

There is much good in the Antiquities Act track record, because presidents, in their broad view of the national interest, have generally used their powers judiciously. Moreover, Congress has not been absent from this process. Very often it has ratified presidential actions, and at times it has checked the president’s use of the act, an interaction that restrains presidential abuse. Mr. Trump should keep this broad national interest, and long view, in mind.