A FUNNY THING happened on the way to Western Maryland in 1983, when road crews blasted through a portion of Washington County's Sideling Hill to make way for Interstate 68. After excavating 10 million tons of rock, engineers discovered they had exposed an unusual geologic structure -- a syncline, or U-shaped layers of tightly folded rock strata -- dating back more than 340 million years.

Today, driving west on I-68 about 100 miles from the District, you can see the large gash in the mountain from a dozen miles away -- a deep, out-of-place V-shaped notch in what would otherwise be a smooth mountain ridge. Located about six miles west of Hancock, the Sideling Hill Roadcut, as it is known, boasts a four-story exhibit center, a walkway over the highway (complete with camera ports) and steps leading to the rock formations so visitors can take a closer look.

The road excavation, which was completed in 1984, exposed a ridge of rock from the Mississippian Period, which ranges from 340 million to 365 million years old. Two geologic formations were discovered: the Rockwell formation -- deposited in an alluvial plain, or delta environment, near sea level -- and the Purslane formation, deposited in a fluvial, or river environment, after the Rockwell. The rock exposure is considered one of the best in the region. The various layers and colors of rock make large U-shapes up the side of the cut, and consist of sandstones, silt stones, mud rocks, conglomerates, shale and even coal. Some fossils -- mainly plant remains and some marine invertebrate shell casts -- are found at Sideling Hill, but the site is not considered a prime spot for finding fossils.

"Look, a waterfall!" exclaimed my son Liam, 7, on our last visit to Sideling Hill. Sure enough, a small trickle of water on the south side of the road cut marked the location of one of two aquifers in the hill. On another trip last winter, we saw magnificent ice cascades, or frozen waterfalls, clinging to the rocks on either side of the highway.

After clambering up the steps to examine the formations, a visit to the exhibit center is in order. According to Jo Ann Schetrompf, a supervisor at the state-run facility, more than 120,000 people a year visit the center and have a look at the outdoor geology classroom that the road cut accidentally exposed.

Geology is also the lesson taught inside the exhibit center, where interactive displays, hands-on exhibits, photographs, graphics, maps and descriptions tell the geologic history of Sideling Hill and this part of Western Maryland.

My children especially like the "light up" display, where you pick a time period and choose the mode of transportation that would have been used to cross the mountain. (Even today, some vehicles have to shift to lower gears to make the long climb up Sideling Hill.) There's also a video about the construction of the road cut.

A geologic time spiral starts on the second level of the exhibit center and twists up all the way to the fourth level, giving a visual lesson of the various geologic eras during 500 million years of time. A chunk of the oldest rock found in Maryland -- granite gneiss -- sits stolidly at the bottom of the spiral.

The top floor of the exhibit center shows the kinds of rock found at Sideling Hill and describes how the road cut was made. The elevation of the mountain at the top of road cut is 1,620 feet, and the road cut itself is 340 feet deep; it took five million pounds of explosives to blast the notch for I-68. You can look at quartz crystals through a magnifying lens and touch millions-year-old plant fossils.

Throughout the center, animals of the region are also on display (the stuffed variety, that is). A mother black bear and her cub greet visitors to the third level, and on the second level, a whole host of forest creatures is testament to the variety of wildlife in Western Maryland -- bobcats, coyotes, porcupines, deer, foxes, owls, ring-necked pheasants and more.

When it's time to head back home, consider the many millions of years of Earth's history that take only a few seconds to drive through as you traverse the road cut. Put in that context, the drive home should go by in a flash.

SIDELING HILL ROADCUT -- 3000 Sideling Hill, Hancock, Md. 301-678-5442. www.mdwelcome.org. Exhibit center is open daily from 8:30 to 4:45 except Thanksgiving, Christmas, New Year's day and Easter. Exhibit center can be accessed from the Sideling Hill Visitor Center exit off I-68 westbound, but you will have to travel two more miles before you can get to I-68 east. From eastbound on I-68, you will have to park your car across the highway and take the pedestrian overpass.

Ten million tons of rock were excavated from Sideling Hill in Washington County, Md., to make way for Interstate 68.