If as the tall tales have it, the Rio Grande was dug with a tree trunk by Pecos Bill, then Mather Gorge was created when an ancient god slammed his fist down on the Piedmont Plateau, shattering and splitting the bedrock and letting the Potomac rush in.

Fanciful, yes. But it fits the special qualities of this stunning geologic treasure below Great Falls that compresses the Potomac into its narrow rock-girded channel for more than a mile and a half.

This is the best time of the year to visit the gorge -- while the air is clean and crisp, after the leaves have gone and before the snows come to stay.

Those who do will be treated to a rich pallette of colors and textures, painted by light and moisture and sculptured by wind, water and time.

Trails lead to and along the gorge from the visitors center in both the Maryland and the Virginia Great Falls parks.

Or, if you have the skill and the water in the river is low, you can enter the gorge on the river as we did.

The ifs are important. At normal water levels, for instance, three- to eight-foot waves and boat-keeping whirlpools keep paddlers too busy to sightsee.

We made our way to the river on the Maryland side paddling upstream along a secondary channel between the Rocky Islands and Maryland.

In the morning sun, the rugged pyramid shapes of the islands glow silver and blue. After a rain, they appear green instead, and splotched by a brown lichen that's almost invisible when dry.

We emerged from behind Rocky Island just below the last of four fishladders built around Great Falls in the 1880s. According to Corps of Engineers historian Frank N. Schubert, they were begun in conjunction with the aqueduct dam above Great Falls, but probably were never completed; official mention of them in Corps annual reports stops in 1898.

Across the river from us, a steep rock wall rises some 40 feet straight out of the water. Its face is cut in angular blocks as though it had been quarried and a bare tree trunk wedged between two outcrops near the top reminds us of the power of the Potomac in flood.

Great Falls is a quarter of a mile upstream and out of sight around a sharp bend. All the water in the Potomac's main channell is marshaled into a course only about 30 feet wide and, with full force, it slams into the battered shank of Rocky Island. Unable to penetrate the hard bedrock, it reels backward, right and then left, and staggers into a narrow cut created from an ancient fault line.

In our canoes, we carefully negotiate these same turns and, with the river, enter Mather Gorge.

In here, between Rocky Island and Virginia, the river can be 60 feet deep, but is only 40 feet wide in places, and the sheer rock walls on each side rise as high as 70 feet above us.

It's awesome, this gorge named for the first director of the National Park Service, but comprehensible.

Our eyes can take the measure of the stunted trees and scraggly shrubs that cling to the sides. Our fingers can touch the rock and feel the smooth surface below the water line, or trace thin veins of quartz protruding from the weathered face above.

As always, the sights in the gorge set off an explosion of joy. We smile and shout to one another, pointing to things we've seen before as if we had never seen them.

Like the ridge of rock jutting into the channel where a pothole has been scoured by churning sand and pebbles. We joke about coming back in a thousand years to see if it has been cut through.

Until two million years ago, more or less, the river here ran through a broad valley, at about the same level as McAthur Boulevard today. Then, in the Ice Age, it began cutting down, seeking paths of least resistance through the hard rock, such as the fault line here in the gorge.

As the river cut, it exposed the crumbled, folded layers of ancient rock that underly the Piedmont, leaving us a roadmap into geologic time.

We can identify the bedrocks, large chunks of blue, gray and silver schist and metagray-wacke, which began as mud, shale and sand on the floor of the ocean some 600 million years ago.

They were fused and hardened by metamorphosis, then folded and stressed and thrust above the surface by convulsions deep in the earth. Geologists think that the Piedmont may once have been as high as the Rockies.

As the earth's crust rose, the bedrock cooled and fractured. Over some 150 million years, periodic injections of molton granite, quartz and a lavalike rock called lamprophyre, among others, filled some of the cracks.

The sheets and veins of younger rocks add to the colors and textures in the gorge wall. Overall, the rocks appear to be blue-gray, with a silver cast that probably comes from the tiny flecks of mirror-surfaced mica imbedded in the schist. Looking closer, one of the blues has a sugary texture. From another angle, the wall takes on a pink cast, and there's a deep green in the shadow areas.

From each angle, each perspective, the appearance of the walls changes. And, as the light changes during the day, the colors range from an iridescence in early morning to a stark white at midday. After a rain the mica is muted and the colors shift and deepen.

Fascinated by the interplay of light, water and rock, James Reber spent 10 years photographing the falls and Mather Gorge. The result is a visual hymnal, 60 photos published in "Potomac Portrait" showing the river and rock in a multitude of moods and seasons -- in winter snow and summer fog, ice-coated and flood-swept.

Yet, today, Reber says, "I am impressed when I go there by all the pictures I did not take."

As we reach the end of the Rocky Islands and the fault line that let the river cut such a deep, narrow channel for the upper gorge, the river widens to nearly 60 feet. Then it squeezes down again temporarily to plunge over a small ledge at Wet Bottom Chute, a favorite playground for whitewater boaters.

Along here, also, we begin to spot rock-climbers, groping for hand and footholds in the cracks and crevices of the rock face.

Hikers on the Virginia shore wave to us and help us locate the narrow, straight-sided cut that marks the exit of the 18th-century Potowmack Canal. The trail that brought them has passed by ruins of locks from that canal, detoured to the abandoned site of the town of Matildaville, and passed river-scoured potholes and outcrops of rock from the age before the river cut down into the gorge.

One of the sharp-eyed among us spies an iron eyebolt still embedded in the gorge wall from the days when ropes were attached to boats coming up the gorge. g

The gorge remains wider below Wet Bottom, and sand and rock beaches become more frequent. On the Maryland side, the Rocky Islands have been replaced by Bear Island, which has not really been an island for thousands of years.

Some of the river's most spectacular pothols and rock sculptures can be found along the trail of Bear Island. They were created by a combination of river scouring and glacial erosion.

Bear Island also offers a unique collection of plant life, according to Tom Janosky, a botanist with the U.S. Geological Survey.

One of only three places where post oak grows in the area, it's also the only place on the river where you can find prickly-pear cactus.

Janosky points out that from a plant's viewpoint, the island is a dry environment in spite of being on the river. Most of it is barren rock; its soil is mostly thin and sandy and open to the sun.

Flood-deposited soils have collected in hollows and low spots, though, and runoff from the rock remains in potholes and abandoned channels, so that Bear Island also has some ponds and even some swampy areas where black tupelo grows.

The rock shore of Bear Island gets progressively lower to the water as the gorge nears its end and the breaks between rock masses are longer.

Mather Gorge ends as it began, where the river meets impenetrable rock. At the lower end, though, the river is much wider and split into three rapids separated by barren islands.

Reluctant to leave the gorge, we tarry at the Maryland chute, watching a young man put on a display of acrobatics in his kayak. When he hits it right, the force of the current stands his boat on end and walks him backward, and he grins like an Olympic gold medalist.

It seems a fine way to end a day of spendid sights, so we wish him well and depart, leaving the gorge for another day, soon, while the leaves are gone and before the snows come to stay.