John Lyman in 1919 and modeled after the University of Virginia. The campus is roughly divided into quadrants. The north and south sides of campus are delineated by a grassy expanse called the Mall, which stretches from Old Main eastward to the campus' eastern border at Campbell Avenue (a major north–south arterial street). The west and east sides of campus are separated roughly by Highland Avenue and the Student Union Memorial Center (see below). The science and mathematics buildings tend to be clustered in the southwest quadrant; the intercollegiate athletics facilities to the southeast; the arts and humanities buildings to the northwest (with the dance department being a major exception as its main facilities are far to the east end of campus), with the engineering buildings in the north central area. The optical and space sciences buildings are clustered on the east side of campus near the sports stadiums and the (1976) main library. Speedway Boulevard, one of Tucson's primary east–west arterial streets, traditionally defined the northern boundary of campus but since the 1980s, several university buildings have been constructed directly on, and north of, this street, expanding into a neighborhood traditionally filled with apartment complexes and single-family homes. The university has purchased a handful of these apartment complexes for student housing in recent years. Sixth Street typically defines the southern boundary, with single-family homes (many of which are rented out to students) south of this street. Park Avenue has traditionally defined the western boundary of campus, and there is a stone wall which runs along a large portion of the east side of the street, leading to the old Main Gate, and into the driveway leading to Old Main. Along or adjacent to all of these major streets are a wide variety of retail facilities serving the student, faculty and staff population (as is the case in other similar university neighborhoods throughout the United States): shops, bookstores, bars, banks, credit unions, coffeehouses and major chain fast-food restaurants such as Chipotle, Panera Bread and Pei Wei. The area near University Boulevard and Park Avenue, near the Main Gate, has been a major center of such retail activity going back to the university's early decades; many shops dating from the 1920s have been renovated since the late 1990s, other new retail shops have been built in recent years, and a nine-story Marriott hotel was built in this immediate district in 1996.