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You stand at the base of massive Santa Elena Canyon in Big Bend National Park, and dip your toes in the placid Rio Grande. Mexico is but a stone’s throw away.

Then you cast your eyes up and up and up. The rock wall — so sheer only birds can negotiate it — rises 1,500 feet, the equivalent of 150 stories. The canyon stretches 50 miles in Mexico and 10 miles in the United States.

“Looks like somebody already built a wall,” a 20-something visitor declared the other day. “Nobody could build a wall as good as God’s wall.”

To see the park and Santa Elena Canyon is to understand how a wall could ruin some of the most majestic scenery in the United States.

Yet President Donald Trump plans to wall off about 2,000 miles of the U.S.-Mexico border, 1,250 miles of it in Southwest Texas along the Rio Grande. His budget proposes $1.5 billion to start on a project that could cost more than $20 billion.

“Splendid isolation” is how the national park describes its 800,000 rugged acres, as remote a place as any in the lower 48. It has mountains, desert, canyons, wildlife, millions of stars in an obsidian sky — and 118 miles of Rio Grande border.