Rock County was set off from Milwaukee County by act of the territorial legislature and named for the "big rock" on Rock River; the county was attached to Racine County until fully organized in 1839.

The evidence

• “The city of Janesville is the county seat. Rock was set off from Milwaukee county, December 7, 1836, and fully organized February 19, 1839, It is composed of twenty organized towns, of six miles square, each— 720 square miles—460,800 acres.”

  1856-history-of-rock-county/03-rock-county.md, PDF p. 41 / book p. 23

  Guernsey, Orrin, and Josiah F. Willard, editors. History of Rock County, and Transactions of the Rock County Agricultural Society and Mechanics' Institute. Janesville, Wis.: William M. Doty & Brother, 1856. Cornell University Library via the Internet Archive, archive.org/details/cu31924028871593.

  Grade A: Found on a page of one of our sources