before union square (san francisco) was union square what was it called and why?

Answer 1

Before John Geary bought the land and deeded it to the city in 1850, it was a sandbank called O'Farrell's Mountain.

Who was O'Farrell?

Jasper O’Farrell
Made First Comprehensive Survey of San Francisco

Employed early in 1847 to make a survey of San Francisco and extend the mapped limits of the city. His map covered the area bounded by Post, Leavenworth and Francisco streets, and the Bay.
It was probably O’Farrell who had the most to say about naming the streets within that area and who chose so many pioneers.
This 1847 map also included the water lots granted to the city by General Stephen W. Kearny, which were subsequently sold at auction.
The location and present width of Market Street, parallel to the Mission road, were established by O’Farrell.
He was a graduate civil engineer, acquired wealth in land holdings, served as a state senator and was a popular resident of early San Francisco.

Answer 2

San Francisco's Union Square is the third-largest shopping area in the United States.

The 155-year-old square, named during the Civil War for Union rallies.