While a portion of the Union Wharf bulkhead was constructed in the late 18th century, it did not assume its present form until 1845 when merchant John L. Gardner purchased it. The wharf’s construction is typical of Boston’s 19th-century waterfront, which used timber cribs filled with rubble stone to form the perimeter. Earth filled the interior and granite blocks placed alongside the cribs formed a durable, protective seawall. The wharf’s surface is now covered with a variety of paving materials, including granite pavers, asphalt and landscaping. Gardner expanded the wharf to the north to create a larger building site and erected a granite-faced warehouse, which is the major building occupying the wharf today. Completed in 1847 on the northwest section of the property, the warehouse was originally four stories tall. The fifth level is a 19th-century brick addition. Gardner sold the property to the Union Wharf Company in 1847, but apparently retained the principal interest. The United States government subsequently leased Union Wharf as a bonded warehouse. By the late 19th century, the ports of New York and Philadelphia gradually became more important to maritime commerce than Boston. In response to this shift, Boston became a center for steamship lines and railroad terminals.