South from Tucson, I-19 heads straight for the Mexican border, 65 miles away, passing some tangible reminders of the region's Spanish and Mexican heritage. The first of these, the MISSION SAN XAVIER DEL BAC - the best-preserved mission church in the United States - lies just west of the freeway, nine miles south of downtown Tucson on the fringe of the vast arid plain of San Xavier Indian Reservation. It was built for the Franciscans between 1783 and 1797, and even today its white-plastered walls and towers seem like a dazzling desert mirage; how much more dramatic they must have been two centuries ago, when to Christian missionaries and Apache warriors alike they symbolized the Spanish quest to subdue and convert the native peoples of the Southwest. No one knows the name of the architect responsible for its Spanish Baroque, even Moorish lines - it consists almost entirely of domes and arches, making only minimal use of timber - let alone the O'odham craftsmen who embellished its every feature. Recently restored with the help of Vatican experts previously responsible for work on the Sistine Chapel, it attracts a constant stream of tourists (daily 9am-6pm; donation). The ideal time to come is on Sunday morning, when four separate Masses draw large congregations from the reservation.

Fifty miles further south, barely twenty miles north of the border, stands the evocative ruin of another eighteenth-century mission church, preserved as TUMACÁCORI NATIONAL HISTORICAL PARK (daily 8am-5pm; $4 per vehicle), topped by a restored whitewashed dome, and home only to the birds that fly down from the Patagonia Mountains. Behind its red-tinged, weather-beaten facade, the plaster has crumbled from the interior walls to reveal bare adobe bricks. A few traces of a mural can still be discerned in the raised sanctuary, but little remains of the priests' living quarters alongside. A very good museum is housed in the small visitor center at the entrance, and a self-guiding tour map ($1) tells the mission story.

South to the border: the Mission Trail

• South to the border: the Mission Trail

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