Once upon a time, before global positioning systems, maps, compasses and bread crumbs, trekkers still had to find their way from point A to point B.

And back.

That's no great problem on a worn footpath replete with wooden bridges over brooks, yellow brick roads and signage.

But what happens when the destination is at an altitude at or near tree line? When obvious footpaths disappear into scattered collections of gravel and scree? When the navigational hints of an occasional juniper tree or a patch of manzanita give way to a long stretch of smooth granite and there's no clue about the most direct or least risky potential switchback?

If you've hiked much in the Sierra, you've been there, done that. For those uneasy with a map and compass, there are times when you're awfully glad somebody came to that spot before you and left a cone-shaped pile of rocks called a cairn.

Cairns are found throughout the world – and throughout history. The higher, rockier and more featureless the terrain, the more likely you are to find them on trails.

Some, like those at Dartmoor in England, have spurred debate about their origin and significance in the region near Plymouth, located just above the English Channel. Archaeologists posit that beyond their navigational purpose dating back to the Bronze Age, they could have been war memorials as well (www.legendary dartmoor.co.uk/ cairns_ moor.htm).

If you hop on the Internet and Google "cairn imagery," you'll see some very stout and artistic examples around the world. Otherwise known as "ducks," these mini-pyramid rock piles – like dots you connect along the way – can signal that you're on the right path.

Or not.

Overzealous cairn-builders have been known to create so many routes along a stretch that, well, they are the opposite of helpful.

"That's confusing," says Leon Nelson, 74, an avid hiker who specializes in the Trinity Alps to the north in California. A retired Redding dentist, he's been backpacking for 60 years.

"I'd say I spend at least 30 days a year out there backpacking … I've hiked every one of the lakes in the Trinity Alps." (He says there are 256.)

His point is that cairns or ducks can serve a valuable purpose.

"On occasion, I find they're extremely helpful. It permits off-trail travel," he says.

If one wants to explore off the beaten path, a system of cairns will come in handy at the end of the exploration and allow the hiker to connect the dots back to the designated trail.

However, Nelson says, hikers who mark such a route should disassemble it on the way back.

With a few exceptions.

When a hike takes you above the tree line along a smooth, rocky surface – at a place where you really can't tell where others have hiked before you – the cairn can serve you well.

"Anything at tree line and above – and on hard surfaces" would be an acceptable place for semipermanent cairns, Nelson says.

Unless you're in a federally designated wilderness area. That's where many duck the issue of cairns.

Nearly 45 years ago (Sept. 3, 1964), Congress gave birth to the Wilderness Act. Therein, precisely at Section 2, subsection (c), a wilderness is defined as "in contrast with those areas where man and his own works dominate the landscape, is hereby recognized as an area where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain," an undeveloped area "without permanent improvements."

You could argue that cairns violate the act when constructed within a federally designated wilderness area such as the Desolation Wilderness in the nearby Eldorado National Forest.

If there is a specific reference in U.S. Forest Service policy forbidding the construction of cairns, Frank Mosbacher is hard-pressed to find it.

He's the public information officer for the Eldorado National Forest, which has 600,000 acres – including about 100,000 acres combined in the Desolation Wilderness at the north end and the Mokelumne Wilderness at the south end.