What you need to know on Wall Street today

Thomson Reuters

Gary Cohn walks through the lobby at Trump Tower in Manhattan, New York City

Federal Reserve Chair Janet Yellen had a clear message for the Trump administration Friday in what could be her final Jackson Hole speech: Undoing the hard work of overhauling the financial system after the financial crisis could have dangerous consequences.

In her keynote address Friday, Yellen was not holding back - in a way suggesting she is not holding her breath for a reappointment from President Donald Trump. Yellen's term as Fed chair expires in February, and Trump is widely expected to nominate Gary Cohn, the former Goldman Sachs president who now leads Trump's National Economic Council, to replace her.

Cohn criticized the White House's response to Charlottesville, Virginia, in his first public remarks about the deadly violence earlier this month. Cohn says he faced "enormous pressure" to resign - and he came extremely close.

In Wall Street news, a portfolio manager at Izzy Englander's $35 billion hedge fund is setting off on her own. And a fresh San Francisco hedge fund is aiming to raise as much as $600 million.

Traders are dumping US stocks to a degree not seen in 13 years. The most important pillar of the bull market is starting to crumble. And history is pointing to a 5% drop in the market over the next month, according to Fundstrat's Tom Lee.

Gary Shilling sat down with Business Insider CEO Henry Blodget on camera this week, and said stocks are expensive, and that a "shock" could send them plunging. You can watch the latest episode of The Bottom Line in full here.

Amazon announced Thursday that Prime members will get special discounts at Whole Foods, once again flashing its ability to destroy the competition. Still, the top Walmart analyst says the company has one big advantage over the tech giant.

In other news, rising avocado prices pose a huge risk to Chipotle's bottom line, according to Credit Suisse.

In tech, Facebook's most successful alum hopes to help ordinary investors make money on startups. WeWork just raised another $4.4 billion from SoftBank. And Meg Whitman is at the center of Uber's boardroom battle.

An entrepreneur who quit private equity to move to a beach in Brazil described the lightbulb moment that made up his mind. A rare Malibu vineyard with ties to liquor royalty has hit the market for $12.5 million.

And 700 Goldman Sachs bankers duked it out in a whiffle ball tournament for charity - see the photos.