A six-foot-wide pothole, which apparently developed overnight on the Capitol Beltway near Colesville Road in Silver Spring, caused an eight-mile traffic tieup during the morning rush hour yesterday, one of the longest Beltway jam-ups in recent months.

In order to cope with the massive pothole, which was several inches deep, Maryland State Police has to close two lanes of 1-495 during the peak of the morning rush hour, forcing four lances traffic to squeeze into two.

The pavement cave-in, which construction workers eventually covered with metal plates, was six feet by 1 1/2 feet, police reported.

The pothole was discovered about 5 a.m. by a westbound motorist who drove through it and bent one of his car wheels, police said. By 8:16 a.m., when the full four lanes were open again, traffic was backed up as far east as the Baltimore-Washington Parkway.

"We never saw a policeman or anything. We didn't even know what was wrong," said Fredreick Townshend, a publications officer for the U.S. Food and Drug Administration who works at the FDA's Rockville offices. "All of a sudden we just came to a standstill. We never did see what was wrong."

Townshend, who commutes 50 mile each way from southern Prince George's County to Rockville, was especially irriated because he said he had heard an erroneous radio report that the highway department was simply "repairing the road."

"I was cussing the state highway people from one end of the traffic jam to the other, thinking they had just chosen that time to make their routine repairs," Townshend said. The delay added 45 minutes to his normal 65-minute drive.

The Colesville road intersection with the Beltway is just east of the heaviest traveled section of I-495, according to state highway department. Each day more than 109,000 cars pass along the Beltway where it meets Georgia Avenue, according to department statistics.

Morning commuters can regularly expect to crawl along between Georgia and connecticut avenues during a particularly snake-like Scurve stretch of the highway.

"But when I saw this tieup come up even earlier, I said, 'late to work again,'" recalled Bruce Butts, administrator of the Noyes Children's Center near Gaithersburg.

"Every little fender-bender backs up the road," said Butts, who commutes from Glen Burnie outside Baltimore along I-95, I-495 and I-270 to his office. Usually the trip takes an hour and 15 minutes; yesterday, it lasted close to two hours.