SIERRA VISTA, Ariz. — The sweeping view of undefiled wilderness on the border with Mexico long rewarded walkers who finished the Arizona Trail, an 800-mile route winding through deserts, canyons and forests.

Then something else came to focus a couple of weeks back at the forbidding site in the Huachuca Mountains: a lonely sector of the border wall, connected to nothing at all, in a region where migrants rarely even attempt to cross into the United States.

There it was, this bare piece of completely pointless wall, right in this magical place, said Julia Sheehan, 31, a nurse and former Air Force mechanic who trekked into the site with three other military veterans that are hiking the Arizona Trail. “It’s one of the most senseless things I’ve ever seen. ”

The quarter-mile fragment of wall is part of an array of fresh barrier segments along the border, some of them bizarre in appearance and of no apparent utility, that contractors rushed to construct in the waning days of the Trump administration — well after President Biden made it very clear that he would prevent border wall construction.

The imperfect boundary wall, already one of the costliest megaprojects in United States history, with an estimated eventual price tag of more than $15 billion, is igniting tensions again as critics urge Mr. Biden to tear down parts of the wall and Republican leaders call on him to finish it.

The most recent controversy over the wall comes amid a significant increase in migration across the border that is prompting U.S. authorities to search for additional places to hold new arrivals, particularly unaccompanied children and teens. More than 9,400 young migrants arrived along the boundary with no parents in February, a nearly threefold increase over this past year at the same time, creating a significant humanitarian challenge.

The Biden administration suspended structure on the border wall on Jan. 20, the president’s first day in office, announcing a 60-day interval during which officials are determining how to proceed.

Donald J. Trump made the wall as a symbol of his administrations efforts to slash immigration. While many stretches of the 1,954-mile border had some low-level barriers built by previous administrations, the project was mired in controversy from the start.

Only a few miles were built in South Texas, the region most prone to illegal crossings. Instead, much of the building, especially in the Trump administration ’s closing days, has taken place in distant parts of Arizona in which crossings in the past few years have been relatively rare.

U.S. Customs and Border Protection, the agency responsible for choosing border wall building websites, contended in a statement last week which places chosen for new border barriers are “areas of high illegal entry. ”

Border barriers slow and stop illegal activity, said Matthew Dyman, a C.B.P. spokesman.

Alejandro Mayorkas, Mr. Bidens homeland security secretary, has been directed to choose whether to resume modify, or terminate projects when the 60-day suspension ends this month. But the last-minute building efforts, with a lot of the hurried building activity happening in the times between the Jan. 6 attack on the Capitol by Trump loyalists and Mr. Biden’s inauguration on Jan. 20, have left a curious tableau for the new administration to evaluate.

Some stretches of the border, particularly on federal lands which are relatively flat, have long, continuous segments of 30-foot high steel barriers that could endure in the desert for decades to come.

But in other regions, border-crossers can easily tiptoe around far-flung islands of the wall, some of which seem more like conceptual art pieces than imposing barriers to entry.

There are half-dynamited mountaintops where work crews set down their tools in January, leaving a heightened risk of rapid erosion as well as dangerous landslides as the summer monsoon season approaches.

In some regions, colossal piles of unused steel bollards linger at abandoned work websites, next to idled bulldozers and water-hauling trucks. In Arizona, ranchers are complaining that rough roads carved by work crews into hillsides near uncompleted sections of wall now function as easy access points for smugglers and others seeking to enter the once-remote areas across the edge.

Now there are a lot of access roads that its possible for someone to walk up to places where the wall ends, and have somebody just pick them up, said Valer Clark, a conservationist who has purchased and sought to preserve about 150,000 acres of land along the border in both the USA and Mexico.

Ms. Clark said a ranch manager recently quit after a break-in at his family ’s home, the kind of crime which was rare in the area before the new streets appeared.

Altogether, the Trump administration completed about 453 miles of border wall since 2017. Billions of dollars for the wall were diverted from financing originally appropriated to the Defense Department.

The majority of the construction involved upgrading smaller present barriers. In regions where no barriers previously existed, like the rugged terrain in which the Arizona Trail winds to its terminus, the Trump administration built a total of 47 miles of new primary wall.

Matthew Nelson, the executive director of the Arizona Trail Association, wondered why the Kiewit Corporation, the Nebraska construction giant with the lucrative wall contract for the region, rushed to construct a small piece of wall in January — within an area conservation activists were fighting to preserve — when it was likely that construction could be stopped anyway after Mr. Biden took office. He questioned if it was an effort to pressure the new administration to proceed with additional construction at the site.

Mr. Nelson said the location of this road from the Coronado National Memorial, a secure expanse managed by the National Park Service, was chosen because of its natural beauty and its location along a relatively safe section of the border where few migrants cross.

“Why rush to put a quarter-mile wall in the middle of nowhere in a place which has never been identified as a high-profile boundary crossing? ” Mr. Nelson asked.

Kiewit officials didn’t respond to a request for comment regarding the part of wall the company built in January at the end of the Arizona Trail.

C.B.P. declined to provide specific details regarding border crossings at the location.

Rodney S. Scott, the chief of the Border Patrol, conceded in November that constructing in South Texas, rather than Arizona, was a “ greater priority for the U.S. Border Patrol. ” But he said “we elected to go ahead and shift down to a lower priority because I could make a difference there and then. ”

The area near the Arizona Trail was not the only place where there was a flurry of construction activity in the final days of the previous administration. Between Jan. 4 and Jan. 8 alone, Customs and Border Protection began construction on 12 additional miles of the border wall, according to the agencys disclosures.

In some places along the border, such as Guadalupe Canyon in southeast Arizona, dynamiting crews were blasting hillsides on Inauguration Day.

Even before the latest construction, there’d been anomalies, such as an island of wall in the Texas border city of Los Indios, and some of them attracted the attention of federal overseers.

A 34-month audit of boundary wall structure by the Department of Homeland Security’s Office of Inspector General last year identified significant issues, such as in decisions on where wall segments would be built.

Customs and Border Protection did not use a sound, well-documented methodology to identify and prioritize investments in areas along the border that would best benefit from physical obstacles, the auditors determined.

The Biden administration has not made clear exactly what plans it has for the wall. But in February, after temporarily suspending building actions, President Biden rescinded the national emergency that his predecessor used to justify advancing building.

Democratic members of Congress from border states, including Veronica Escobar of Texas and Teresa Leger Fernández of New Mexico, wrote to Mr. Biden this month urging him to cancel all remaining construction contracts and divert residual funds to removing portions of the wall in regions with “ especially destructive environmental damage and destruction of sacred sites. ”

At exactly the same time, Republicans are positioning themselves around the gaps in the boundary wall, sometimes literally, in an effort to portray Mr. Biden as soft on immigration.

Senator Lindsey Graham, Republican of South Carolina, complained after a trip in February to the border in Arizona a gap in the wall that there was allowing migrants to enter the country across an unprotected wash.

Nothing around here makes sense unless you plug this hole, Mr. Graham said.

In spite of the quitting of new construction, parts of the federal bureaucracy are gradually continuing forward with the property acquisition process, alarming some landowners who had hoped Mr. Biden would heed their anxieties about the possibility of living in the barrier’s darkness.

Ricky Garza, a staff lawyer for the Texas Civil Rights Project, said that the federal government still has nearly 150 open lawsuits against landowners from South Texas to survey and seize land and potentially begin construction on the boundary wall or other measures that could be used to discover migrants.

Mr. Garza, who represents some landowners in the region, said Justice Department lawyers have responded to Mr. Biden’s suspension of wall construction by asking the courts to delay legal cases for 60 days.

However, some landowners continue to face pressure from the federal government in court, he said. One of them is Melissa Cigarroa, who said the government is still seeking a “right of entry ” to her ranch in Zapata County, Texas, where she has raised Barbary sheep.

“Why would they go with these cases if he already indicated he wasn’t going to create another foot and said he would withdraw? ” Ms. Cigarroa said. “It felt like the government had been working in bad faith. ”

Simon Romero reported from Sierra Vista, and Zolan Kanno-Youngs from Washington.