Task: Question Answering
Input (English): Passage: The Chinnerys were wealthy and influential. William Chinnery's father, also named William, owned trading ships and named one Gilwell in 1800. William and Margaret Chinnery initially resided in London, and after three years of marriage and inheriting Gilwell in 1792, they moved to Gilwell in 1793. They soon shocked the populace by renaming Osborne Hall to "Gilwell Hall". William Chinnery expanded Gilwell's land holdings through significant purchases over 15 years and, with his wife, transformed it into a country estate with gardens, paths, and statues. Parts of the garden, paths, and dwelling modifications exist into the 21st century. William Chinnery was exposed as the embezzler of a small fortune from the British Treasury where he worked and was dismissed from all his posts on 12 March 1812. Margaret Chinnery was forced to sign over Gilwell Estate to the Exchequer on 2 July 1812.The Chinnery family was prominent enough that members of the English nobility visited often during the 1790s and early 19th century. King George III visited on occasion, and the Prince Regent, who later became George IV, was a regular visitor. George III's seventh son, Prince Adolphus, became a family friend, lived at Gilwell for a while, and tutored their eldest son George.Gilpin Gorst bought the estate in 1815 at public auction, and his son sold it to Thomas Usborne in 1824. When London Bridge was replaced in 1826, Usborne bought pieces of the stone balustrades, which date to 1209, and erected them behind the White House around the Buffalo Lawn. The estate changed ownership more times, but these families did not maintain the property and it fell into disrepair by 1900. Reverend Cranshaw, a local resident, bought the estate in 1911 and was the last owner prior to the Boy Scout Association, as it was then known. 
Question: What did Thomas Usborne erect behind the White House around the Buffalo Lawn?
Output (English): stone balustrades.