A 42-year-old Montgomery County woman and five of her six children were killed on the way home from an ocean beach yesterday when their van crashed into a highway guardrail on the Eastern Shore, soared over it and plunged into a creek.

All the victims were believed drowned, according to Maryland state police trooper Calvin Milligan. Milligan said it took about 25 minutes after the 4 p.m. plunge from Rte. 50 in Dorchester County before rescue workers could free the victims from the van, which sank in about 8 to 10 feet of water.

Milligan said "nobody seems to know" the cause of the accident, which took the lives of Carol Holmboe of Olney and five children who ranged in age from 3 to 18.

The only survivor, Kimberly Pipari, 16, of Brookville, a family friend, was rescued by a passer-by after being ejected from the van, apparently as it hit the water. Milligan said the girl told police that just before the accident, the occupants of the van "were talking," and that "the next thing she knew they were in the water."

Witnesses told police that the van veered to the side of a narrow stretch of road just east of the bridge at Vienna, Md., "as if it was going to stop," the trooper said.

But the van "never slowed," he said. Instead, Milligan said, it struck the metal guardrail at the point where it tapers to the ground. The van then rode up the rail, which rises to a height of about three feet, and flew over it into the water, he said.

The accident occurred about 50 miles south and east of the Chesapeake Bay Bridge, at a point at which Rte. 50 narrows to two lanes. Milligan said only a few feet of shoulder separate the road from the guardrail where the highway crosses Bridge Creek, a marshy stream that flows into the Nanticoke River.

Milligan said the van was apparently being driven by Holmboe's 18-year-old son, Mark.

According to neighbors, the Holmboes, who live at 3616 Patrick Henry Drive, were on the way home from the family's cottage in Millville, Del., near Bethany Beach.

In addition to Mark, the other Holmboe children killed were identified as Leigh Megan, 14; Jamie Elizabeth, 12; Scott, 8, and Ryan, 3.

Neighbors said the father of the family, Ernest Holmboe, is an assistant vice president at Pacific Sierra Research Corp. They said that after being notified of the accident he went to inform the couple's surviving child, Keri, 19, a sophomore at Pennsylvania State University at Altoona.

The mother, described last night by a friend as "a very special person" who babysat and provided day-care for many neighborhood children, was said also to have been involved in midwife activities and in birthing groups.

"The whole family was well liked," another friend said. "They were just very very nice people and everybody loved them."