A small hump-backed mountain rises above East Mesa, midway between Las Cruces and the Organ Mountains. It is often called "A" Mountain for the Aggies "A" blazed on its west side like a gargantuan modern pictograph. But I prefer its older name, Tortugas , or "Tortoise" Mountain, for its resemblance - when viewed from the south - to a huge tortoise slowly ascending the bajada.

This odd-shaped, solitary mountain is a chunk of a once-extensive layer of limestone that began as muddy, shell-laden ocean-bottom sediments several hundred million years ago. Some 30 million years ago, the thick layer of limestone was broken by tremendous heaving and wrenching of the earth's crust. East of a now-invisible fault, the rock layers were dropped down to form a deep basin. The basin was later buried under the stream-deposited layers of gravel and sand that now make up the sloping bajada between Tortugas and the Organ Mountains. On the west side of the fault, the layers were thrust up. (The layers were displaced a total of 8,000 feet.) Tortugas, the Doña Ana Mountains, and the Bishop Cap Hills are remnants of the upthrust side of the fault.

Sitting at the east edge of Las Cruces' metropolitan area, Tortugas Mountain remains an island of undeveloped land. But the small mountain is no pristine

wilderness. A few dish antennas sprout from its top. A New Mexico State University-administered "forest" of antennas accompanied by rows of temporary metal buildings sprawls over the bajada west of the mountain's base. Abandoned quarries and fluorite mines pock its sides. Bullet-pitted trash litters its lower slopes. Trails frequented by mountain bikers, four-wheelers, walkers, horseback riders, and others cris-cross the mountain.

Unlike the creosote bush desert of the bajada below, the steep slopes of Tortugas Mountain are dominated by grass: crescent-shaped mats of black grama, clouds of bush muhly, tall stalks of sweet tanglehead and bluestem, fat bunches of sideoats grama . . . Shrubs, cacti, and wildflowers dot the slope too, but the expanse of grasses - a rare remnant of the grassland that once graced this part of the Chihuahuan Desert - is one of Tortugas' charms.

Each December, Tortugas Mountain is lit by remembered magic. During the Village of Tortugas' fiesta in honor of the Virgen de Guadalupe, hundreds of people trek - some on their knees, some barefoot - the three miles from the Pueblo to the top of the mountain for which it is named, for a daylong mass and blessing ceremony. As dusk falls, the orange bonfires blaze from the mountain top. Another answers from the village. Then a huge cross in lights is outlines on the mountain to illuminate the return trek of the blessed. All night long, Tortugas Mountain blazes with light, a beacon of faith in the winter darkness.