Sketch of the Washington Monument, mid-1800s

Until the 1840s, the Mall had been used primarily for grazing and agriculture. In 1848, construction began on the Washington Monument, designed by Robert Mills. It took nearly four decades to complete (1885). In conformance with the classical geometry of the L’Enfant Plan, the monument should have been placed at a 90 degree intersection with the U.S. Capitol and the White House, but it was moved to higher ground for greater stability. 

Library of Congress