An 84-year-old bastion of male supremacy was shattered here today when 26-year-old Joy Slater rode Cancottage to a two-length victory over favored Beech Prince to become the first woman to win America's premier timber steeplechase, the Maryland Hunt Cup.

Art Dom, whose rider, John Bosley, fell off at the 21st fense, then ran alongside and jumped back on, was third, another 30 lengths behind. Speedy Smithwick, whose father D. H. (Mike) Smithwick won the Hunt Cup six times, was fourth on Moon Meeting, another two lengths back.

Kilkea Castle, ridden by Joseph Walker III, fell at the eighth jump while Gutsy, ridden by Lisa Stroud, went down at the 17th. Stroud suffered a concussion and a possible broken collarbone but her horse was not injured.

The time of 8:51 over a "wet but good" course was six seconds off Dosdi's winning time last year and far off the 8:33 3/5 record set by Ben Nevis in 1978. Dosdi, an early favorite for today's race, came up lame and was scratched earlier in the week.

"What a good horse -- what a good horse," Slater, of Unionville, Pa., said after the race. "I hit a couple of fences, not too hard though, except the 11th when he sort of slipped into it."

Jay Griswold, one of the finest riders of Maryland racing, never has won the big one -- the Hunt Cup. Today he took Beech Prince out at the start and it looked to a rainy day crowd of more than 11,000 that this would be his day.

Running easily and jumping, if not flawlessly at least adequately, Beech Prince had command over 18 of the first 19 fences, Cancottage putting his nose in front for a moment at the 10th. At the 16th, Beech Prince had a 15-length lead but Slater was "not too worried."

"I had to take a good hold of him all the way," she said. "He wanted to go."

She let him go on the run downhill between the 19th and 20th and was on top of the landing. Beech Prince came back and both jumped the 21st -- the water hole -- almost together. But again Cancottage was on top at the 22nd and managed to keep the margin down the stretch.

Nine years after Kathy Kusner broke the barrier by becoming the first woman ever to ride in the "Maryland," the combined talents of three women achieved the victory. Cancottage, an English-bred (Northern Ireland to be exact) 10-year-old chestnut gelding, is owned by Mrs. Miles Valentine, trained by Mrs. P. F. N. Fanning and has been ridden by Slater for most of his American career.

Cancottage had run a strong race in the Hunt Cup here last year under then-owner Broderick Munro-Wilson, but had fallen at the 19th jump while pressuring Dosdi. Munro-Wilson remounted and the horse finished fifth.