Ross Grimsley stood on the mound: his hair and eyes wild, and screamed, "What's going on here? I can't believe this."

The roof caved in on the Baltimore Orioles here today and southpaw Grimsley was the innocent standing helplessly in the middle, like the operator of a berserk merry-go-round.

In a bizarre five-run fifth inning that unraveled the O's last sensible pennant hopes, the Birds made four errors and had a passed ball. That's a week's worth of infamy for the meticulous Birds.

When the most embarrassing inning of the Baltimore season was finished, a 4-0 lead over Cleveland had been squandered and the Birds were on the way to a 9-4 defeat.

"We more cruising," said Grimsley who was working on his 14th consecutive shutout inning when it all broke loose. "We had that game in hand. To lose like this . . . it's so frustrating after trying so hard. It's so unlike this team.

"I'll tell you, I still don't believe half the stuff that happened out there."

Even those 6.291 who were here - and dutifully booed every time a score from New York's doubleheader sweep over Toronto was announced - will have a hard time recalling which fly wheel front broke loose and which fly wheel first broke loose and threw a rod through the motor.

Maybe the ball that Mark Belanger, whose percentage proclaims him the best fielding shortstop in history, threw into the box seats to open the fifth, should have been a hint. Certainly left fielder Al Bumbry's two errors - on a grounder through his legs, the other a liner off the heel of his glove - were omens of the first water. Just Saturday Bumbry was jumping above the center field fence to rob Rico Carty of a grand-slam home run.

Dave Skaggs, the catcher with only one passed ball in 75 games, made two today - both on waist-high fast balls, one of them a strike. And even Grimsley contributed to his demise by throwing a pickoff into center field.

Every team fears termites in the bat rack, but who ever heard of pixies in the gloves?

What the Orioles didn't do to themselves in that pivotal fifth, fate did. In that frame the Indians had an infield hit, three looping liners off the end of bats (two on consecutive pitches for doubles by the eighth and ninth hitters), plus the sickest nubber over a drawn-in infield thta ever KOd a pitcher.

"They still haven't hit a ball back any harder than I threw 'em up there," said Grimsley. "And I throw 'em mighty slow."

If the O's had any hopes of a dramatic ninth-inning rally like Saturday's, Carty ended them. After the Indians had scored an insurance run off reliever Dennis Martinez in the seventh on a walk and two singles, Carty had his revenge.

Earl Weaver brought in Nelson Briles to face Carty.

But Carty swung at the aging Briles like a man who knows a gone-to-seed pitcher when he sees one. Briles' third pitch disappeared over the center field wall at the same spot where Bumbry committed his larceny 24 hours before.

As his three-run homer disappeared, Carty, who calls himself "Beeeg Man," might as well have said, "Catch that one, Bumbry."

The Orioles knew this defeat marked the end of their spectacular 54-27 pennant drive since July 1. New York's 15-0 victory in its opener almost seemed like wretched excess.

"Bury us. Go ahead and bury us now," yelled the O's Tony Muser bitterly at a flock of Baltimore reporters that some Birds think underestimated the team all year.

"It ain't over with," snapped back manager Weaver, defusing Muser's grief and getting a point over to his team. "Don't quit now."

But Weaver is no slouch at arithmetic. "Like Yogi Berra said. "It ain't over 'til it's over.' But we're four games behind in the lost column with six to play. That spells it out pretty clear.

"Those four errors . . . I guess we were due. We were close to the alltime league record for fewest errors in a season. Then this. My players are human beings, not machines. But to lose this way after bustin' our butts . . ."

For historians and sadists, the chronology of the inning that pulled the plug on the Birds went like this:

Carty singled to deep short, Belanger's throw into the stands allowing Carty to go to second. After a line out, Fred Kendall's soft single to left skipped past Bumbry, allowing Carty, who had stopped at third, to score.

The next two hitters, Rick Manning and Alfredo Griffin, poked doubles to right, then left, on consecutive pitches. Both landed near O's outfielders. "I thought they were both going to be caught," said Grimsley. "I think we were playing out of position on Griffin."

As a final Hitchcock touch, the next two men who struck crucial blows, did not even start the game.Ron Pruitt, who dumped an RBI single to center, and Dave Oliver, whose liner Bumbry dropped, entered as replacements for Paul Dade and Duane Kuiper, who collided on a Belanger bloop triple and had to leave the game.

Then came a wild pickoff and a passed ball.

Finally, to end all the indignity, the O's came up blank - Larvell Blanks, that is. The Indian thrid baseman's fly to right fell in front of Ken Singleton to score the eventual winning run and make a loser of Grimsley.