Delmar Boulevard cuts across St. Louis, but where it meets Euclid Avenue, the nation’s differences in race and class are stark. (Lee Powell/The Washington Post)

To get a sense of the fracture that cuts this city in two, drive along Delmar Boulevard, a major four-lane road that runs east to west. Hit the brakes when you see an Aldi grocery store and put your finger on the blinker. Decide which world to enter.

In the blocks to the immediate south: Tudor homes, wine bars, a racquet club, a furniture store selling sofas for $6,000. The neighborhood, according to U.S. Census data, is 70 percent white.

In the blocks to the immediate north: knocked-over street signs, collapsing houses, fluttering trash, tree-bare streets with weeds blooming from the sidewalk. The neighborhood is 99 percent black.

The geography of almost every U.S. city reveals at least some degree of segregation, but in St. Louis, the break between races — and privilege — is particularly drastic, so defined that those on both sides speak often about a precise boundary. The Delmar Divide, they call it, and it stands as a symbol of the disconnect that for years has bred grievances and frustrations, emotions that exploded into public view on the streets of the majority-black suburb of Ferguson after a white police officer fatally shot an unarmed black teenager. Ferguson is north of Delmar; the suburb of Crestwood, where the officer lives, is south.

Even the way people perceive the Aug. 9 shooting and the street protests that have followed is influenced by geography.

Map of St. Louis, Missouri (The Washington Post)

“I’m one of those people that feels sorry for the officer,” said Paul Ruppel, 41, a white business owner who lives just to the south of the divide. “For the most part, I believe the police of St. Louis are doing a great job.”

Said Alvonia Crayton, an African American woman who lives just to the north of Delmar: “My reaction is, what took them so long? Michael Brown was basically the straw that broke the camel’s back.”

St. Louis’s geographic divide stems from a legacy of segregation — legal and illegal — and more recent economic stratification that has had the effect of reinforcing racial separation. Even now, some tony suburbs maintain large-lot single-family zoning, essentially closing the door to lower-earners who might want to subdivide a property.

St. Louis, its urban center hollowed out, has had far less of the gentrification that has transformed other Rust Belt cities, including Chicago and Pittsburgh. Look at a map of St. Louis, color-coded by race, and majority-African American communities sit almost exclusively to the north — that is, above Delmar.

“You have a division between the haves and have-nots,” commented Carol Camp Yeakey, founding director of the Center on Urban Research & Public Policy and Interdisciplinary Program in Urban Studies at Washington University in St. Louis. “People on one side are prospering, and the people on the other side are not.”

The divide is hardly absolute. Middle-class and well-off African American families are scattered throughout the northern part of the city and St. Louis County. Some areas, like University City and Florissant, have long been considered appealing places to live.

Researchers from Washington University and Saint Louis University highlighted the “Delmar Divide” in a lengthy report on the city’s disparities published this year. They analyzed the data of abutting, several-square-block areas north and south of Delmar, right near the Aldi. To the south, home values were $310,000 on average, and 67 percent of adults had bachelor’s degrees. To the north, home values were $78,000. Only one in 20 had college degrees.

Although the divide spans most of Delmar’s 10 miles, it’s seen most sharply near the Aldi, where two neighborhoods share a Zip code but have almost nothing else to do with one another.

The wealthier and majority white neighborhood that starts south of Delmar, known as the Central West End, publishes a community map showing 125 businesses, including a whiskey bar and an independent bookstore. St. Louis Blues hockey star T.J. Oshie lives in the area. So do university professors and vice chancellors. Residents have also noticed a black Lincoln Navigator, with a driver, that’s often parked on a gated, private street, ready to transport one wealthy homeowner at a moment’s notice.

The neighborhood, residents say, is relatively diverse. It’s home to some students, blacks, Asians, Hispanics. But there are also residents who say they’ve been made uncomfortable by police officers’ targeting of minorities.

When Chris Hand, a white law student from the West Coast was moving into this neighborhood a year ago, he saw two black men who were “dressed a little raggedy” walking down the street, Hand said. Then, a police officer stopped them, patted them down and told them to sit on the curb.

“He started interrogating them and said, ‘Are you to be panhandling?’ ” Hand recalled. “He booted them out of the neighborhood,” telling them to head north, toward Delmar Boulevard.

“It was just a little shocking,” Hand said.

Some in the Central West End say there is a reason to be vigilant in an area packed with commerce that is seeking new development. Residents of some sections of the neighborhood have elected to pay an extra tax, most of which is used to pay for more officers to patrol the neighborhood by bike. The police are off-duty from their regular jobs but come to the area to moonlight, said Jim Whyte, executive director of the Central West End Neighborhood Security Initiative, a group formed in 2007 that works in cooperation with the St. Louis Metropolitan Police Department.

When a reporter walked through the neighborhood with a camera, one police officer on a bike came by to check on matters. Whyte soon followed, introducing himself and offering a tour of his office, where a tackboard displayed six photos of “Known Panhandlers” — all African Americans. The reporter told Whyte that he was about to head north of Delmar.

“Just be careful,” said Whyte, a retired St. Louis city police officer. “I’m serious.”

Urban planners worry that the racial divide is self-reinforcing, with home values linked to property taxes and quality of schools. Even if development pushes north of Delmar, lower-earners might be flushed out, chased away by home prices they can no longer afford.

That dynamic leaves St. Louis locked in what Jim Dwyer, a longtime Central West End resident, called a “two-world existence.” Some working-class residents from north of Delmar venture south for a meal or some shopping. But very few from the south go north.

The emotions over Ferguson events remain raw.

“I don’t think anybody expected this — even after the shooting,” Ruppel said, referring to the unrest.

Like Ruppel, Jill Boudreau, who was shopping in the Central West End on Wednesday, is willing to give the officer, Darren Wilson, the benefit of the doubt.

“That kid [Michael Brown], he probably did something” to merit a response from the officer, she said. “We don’t know all the facts.”

Just to the north of Delmar, in the almost entirely black area of Fountain Park, frustrations have long festered, but residents say their neighborhood is improving.

Homes were foreclosed en masse after the 2008 economic crisis, and that’s left a quieter, somewhat emptier area populated by aging homeowners. Many are working-class. There are barbecue picnics on weekends, and a sunflower and vegetable garden has sprouted in an area of razed lots. There are also bargains to be had: Turn-of-the-century mansions, with servants’ quarters, run for under $100,000 on the market.

Still, it has the markings of a tough neighborhood. Restaurant options run a limited gamut from fast-food burgers to takeout Chinese. Residents can tick off violent crimes that happened on this corner and that. Toughest of all, many north of Delmar say they’ve become inured to the divide — so accustomed to it that they sometimes have to remind themselves that it’s a problem.

“It’s life in St. Louis,” said Lawrence McKnight, a custodian at Centennial Christian Church in Fountain Park. “Some factions have it harder than others.”

“It’s always been the same,” said Jeanette Jones, a mail carrier who has worked both to the north and south.

“Once you cross Delmar — I don’t know, it’s a different world.”