Orange County's 50,000-acre Irvine Ranch Land Reserve -- essentially a big olive branch extended to the County of Orange by the Irvine Company in exchange for development rights in that county's remaining open spaces -- is bit by bit being transformed into public parkland. About 5000 of those acres, east of the city of Irvine and west of Santiago Canyon Road, lie in what is known as Limestone Canyon Preserve. The preserve is not yet in full public ownership, but will someday become a fully accessible wilderness park. Meanwhile, the Nature Conservancy is charged with offering guided hiking, equestrian, and mountain biking tours of the property, ranging in length from 2 miles to 15 miles.

Limestone Canyon Preserve's most popular tour features an easy, two-mile round-trip hike to a viewpoint overlooking the Sinks, a colorful, dramatically sheer piece of landscape that looks out of place amid the gently rolling foothills of the Santa Ana Mountains. No one is allowed unaccompanied by a guide on the Limestone Canyon property, and roving guards ensure enforcement of that rule.

Contact the Nature Conservancy at 714-832-7478 or e-mail [email protected] for a tour schedule and reservation instructions. For the popular, short hike to the Sinks, participants will either rendezvous at the trailhead (parking space permitting) on Santiago Canyon Road near the tiny community of Modjeska, or gather at a car-pool site not far away. The short hike is offered about twice monthly during morning hours, and typically once a month as an evening hike by the light of the full moon.

You'll begin with a crossing of the wide, bouldery bed of Santiago Creek; then you follow an old dirt road heading up a small canyon on the far side. Graced with shade-giving live oaks and lush riparian vegetation, the canyon is recovering from a century or more of cattle-grazing use. Expect progress to be slow; your Nature Conservancy guide will likely point out every significant type of plant, and spin yarns about various birds and animals that may come into view.

At 0.8 mile you reach the head of the small canyon, where you meet a ridge-running dirt road. You turn north and continue about 0.2 mile to the best viewpoints overlooking the steep-walled sandstone gorge named the Sinks. Erosion of the soft sedimentary rock here has led to the formation of a receding cliff -- most dramatically sheer on the north side, where it exhibits a relief of about 150 feet. Iron oxide is responsible for the ruddy color of the rock. The scene is impressive under mid-morning sunlight, and eerily spectacular in bright moonlight.