By the 1930s, multiple factors led to the neighborhood's decline, including the Great Depression, the dawn of the automobile, the demand for more heat-efficient houses, and the younger generations of the families that had built these homes largely moved to Westmount. During the Quiet Revolution, some of the businesses created in Montreal, on whose fortunes the Square Mile had been built, moved to Toronto. In this period, the Square Mile evolved to gradually become the central business district, and many of grand houses were demolished. The face of the Square Mile was altered, leading to the formation of Heritage Montreal to preserve architecture in the city.