It was a memorable Fourth of July, full of the bizarre, the predictable and the unexpected. Margaret Dickins won't soon forget it. She left Arlington to visit relatives in New York, and while she was gone an errant fireworks rocket landed in the gutter of her house, set leaves afire and caused $65,000 damage to the house.

One mounted U.S. Park Police officer won't forget it either. He was patrolling on the Mall when his 14-year-old, 1,500-pound thoroughbred horse dropped dead of a heart attack. The officer was not injured, but officials declined to identify the horse until his "next of kin," the person who gave him to the Park Police, could be notified.

About a quarter-million Metro subway riders won't forget Independence Day 1985 either. The biggest one-day group of riders in Metro's nine-year history, they crammed and jammed, pushed and punched their way aboard the system's 354 cars after the fireworks show at the Washington Monument, but it was hours later before many got home, with frayed nerves and a sour taste for the experience.

Twenty to 30 Metro riders will remember ending up in a heap at the bottom of a runaway L'Enfant Plaza station escalator that abruptly speeded up and sent them hurtling downward.

A total of 119 revelers will remember it as the day they got arrested on one charge or another. Thousands more of the celebrants in the crowd, estimated by D.C. police at 650,000, were said to be too bombed on booze to remember much of anything. A couple hundred people on at least three commercial jets flying along the Potomac River may recall it as the day their flights were bombed by private aerial fireworks as they headed toward National Airport.

Yesterday, 90 National Park Service workers and 25 Youth Conservation Corps members were left with the residue from the Thursday night celebrants on the Mall, at the Capitol and at various vantage points across the Potomac in Virginia: a pungent collection of 300 tons of rotting trash that looked like a wintertime blizzard covering the usually pristine landscape.

Also, the Park Service was left to wonder about the magnitude of the crowd, its choice of the Beach Boys for entertainment and the complaints it was getting about its decision to delay the start of the fireworks from 9:15 to 9:50 p.m. because the Beach Boys showed up late for their concert.

Even before the mass of humanity descended on the Mall for the annual celebration of the nation's freedom from British tyranny, of summer, and of good times, Manus J. (Jack) Fish, the Park Service's regional director here, ordered a study of how future Fourths of July should be commemorated on the Mall.

Park Service spokeswoman Sandra Alley said, "We'll be considering what type of program should be put on for the Fourth of July, whether it ought to be traditional, with a military band, or continue with a rock band or something else. The problems are inherent when you put on anything this big with the huge crowds."

This year, she said, the Park Service for the first time put out a request for anyone who wanted to perform on the Mall to make a proposal. The Park Service then selected the Beach Boys, who had performed in recent years and whose Love Foundation agreed to underwrite the estimated $100,000 cost of the extravaganza, including the expected $30,000 cleanup.

By 1 a.m. yesterday, work crews equipped with four back-end loaders, 12 garbage trucks, three giant vacuum cleaners and seven dump trucks had already started to sweep the debris from the Mall and other nearby areas. Alley said the work will be 85 percent completed by this afternoon but will not be finished until Monday.

John Peden, 25, said he, along with his girlfriend, Liv Reinecke, and her two young daughters, were the last persons to get on the runaway L'Enfant Plaza Metro escalator when suddenly "it was going four or five times faster than normal. It felt like it was accelerating," he said. "This thing was ripping. It was like a roller coaster.

"People couldn't get out of the way at the bottom. There were people screaming. They couldn't jump off fast enough, and 20 or 30 people fell. People behind us realized what was happening" and stayed off.

Metro spokeswoman Beverly Silverberg said two or three persons received minor injuries, mostly bruises, in the incident, but the cause of the mishap has not been determined. On the whole, Silverberg said, Metro, with 59 six-car trains, performed well and delivered most riders to their destinations by 2 a.m. But many riders, perhaps unaccustomed to the annual crush that results after the fireworks display, seemed to resent the turmoil.

"I was totally bathed in tears and sweat by the time I got onto the correct train to Shady Grove," said Mary Nix, who along with her family got caught in the throngs trying to work their way into subway cars at Metro's Smithsonian station, the only one on the Mall. "Through the sheer grace of God we feel we got home safely. Every square inch of space in the Smithsonian station was mobbed with people so that you couldn't move except to fall into the pit," she said. "People were shoving from all directions and being shoved from all sides."

Silverberg said a few Metro passengers, perhaps a half-dozen, fainted in the subway crush. "Flow control is the single most serious problem for us, how we allow people onto the platforms and how we allow people to transfer" to other Metro lines, she said. "We'll be studying that and the frequency of trains."

Meanwhile, in the air, Federal Aviation Administration air traffic controllers were forced to alter the north-to-south landing patterns of jets heading to National Airport Thursday night after three pilots reported either being hit by aerial fireworks fired from the Virginia side of the Potomac or seeing them close by. All three planes landed safely. Later, south-to-north landings and takeoffs were ordered.

On the ground, D.C. fire officials reported they treated 357 persons for a variety of cuts, bruises, injuries from their own fireworks or heat exhaustion. Another 71 people had to be taken to hospitals, but apparently no one was seriously injured during the day of concerts, picnics and merrymaking on the Mall.

But D.C., park and Metro police said they did charge six persons with assault, four with robbery, three with theft, 24 with illegal drug use, 32 with disorderly conduct, 35 with illegally selling food and souvenirs without licenses and 15 with other offenses.