From ArborWiki

A much-painted boulder at the corner of Washtenaw Avenue and Hill Street in tiny Washington Park (a sample picture: [1]).

History, from the UM Campus Information Center:

"Ann Arbor's most famous Glacial Erratic" is the painted rock at the corner of Washtenaw Avenue and Hill Street. This huge chunk of Canadian limestone, deposited by glaciers in a gravel pit on Pontiac Trail, was moved to Ann Arbor by Eli Gallup in 1932 to become a memorial honoring George Washington on his 200th birthday. Buried in the foundation of the rock is a box containing its history and origin. Originally the rock was painted grey and had a copper plaque and tribute to President Washington.