Not far out of the gates of the “Central Iowa Rock Road Endurance Metric” (or CIRREM as it’s known in gravel circles), riders started going down on the dirt road in the middle of Iowa. A big guy on my left spilled hard and almost took me out. Another one up front went over and slammed his helmet into the ground. I slipped on the ice a few times, but managed to stay upright. In the lead pack, a rider broke away and the others started to chase him. Nearly all of them went over, too.

We were just a few miles into the late-February, 63.5-mile bike race that brings out the hardest of the hardcore groadies (gravel roadies). Gravel riding, or “gravel grinding” as it’s known, is a different sort of race than the ones that came before. These are epic rides on forgotten, unpaved roads covered in crushed rock. They’re more relaxed, more low-brow, and more hardcore than you average road crit.

Not only that, but they open up a vast new territory for cycling. At last count, there were 1.3 million miles of unpaved roads throughout the United States. Cyclists are just beginning to discover these as a new frontier where there are no rules, no governing bodies, and where you can just announce a race and people will show up to ride 60, 100, 200 miles or more.

“EVER SINCE 2005, IT’S been on the upswing,” says Mark “Guitar Ted” Stevenson, who runs the 320-plus-mile Trans Iowa gravel race, as well as the Gravel Grinder News website. “It’s been astounding to see the growth of it.”

Stevenson says the number of gravel races was up by 50 percent last season, and the number of racers has exploded. Last year, for example, CIRREM made no opening announcement but filled up in 24 hours. The Almanzo, a 100/160-miler in Minnesota had 350 people in 2011 (only 170 finished), they registered 800 in 2012, and this year they’ve added the 400-mile “Nellie.” Even the brutal, 200-mile Dirty Kanza in Kansas filled its 450 slots in two-and-a-half hours. Getting into the Trans Iowa these days has almost become as hard as the ride itself.

In Europe, too, new gravel races have popped up and taken off, like the Strade Bianche in Italy, the Dengie Marshes Tour in England, and the Gran Premio Canal de Castilla in Spain. “They’ve been catching on everywhere,” says Ed Pickering, who covers European cycling for the U.K.-based Cycle Sport Magazine. “I think it’s because Paris-Roubaix and the Tour of Flanders are held in such high esteem.”

The Paris-Roubaix and the Tour of Flanders are two of the Spring Classics held in Europe early in the year. They’re grueling and glorious, going through mud and over old cobblestone roads; they seem closer to the sport’s pre-paved roots than other major road races, retaining all the romance of old-world cycling. “The appeal is retro,” said Pickering. Now, with the fall of Lance Armstrong, and the pall he cast on the Tour de France, that appeal seems likely to grow.

“Everyone wants to be in Paris-Roubaix,” says Drew Wilson, who went on to finish third at CIRREM. “That’s what you dream about. But the USA Cycling-sanctioned road races have too many set-up costs and rules to stage a race on this same scale. So these gravel rides are as close as we can get to having classics-style courses.”