Rock County was fully organized (a third dating, alongside February 13 and February 19 above).

The evidence

• “The county of Rock was set off from Milwaukee county in 1836, but continued with Walworth, a constituent part of Racine Co., until the Spring of 1839, when it was fully organized. At the session of the Legislature at Belmont in winter of 1836-7, the location of the county seat was established upon the same fractional quarter section on which Mr.”

  1856-history-of-rock-county/25-city-of-janesville.md, PDF p. 175 / book p. 159

  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

  

• “In this year the county was detached from Milwaukee county and made a part of Racine county; it was not organized as a separate county until the spring of 1839, though the county seat was established at the legislative session of 1836 and 1837.”

  1908-brown-rock-county/26-history-of-janesville.md, Vol. 2, PDF p. 14 / book p. 529

  Brown, William Fiske, editor. Rock County, Wisconsin: A New History of Its Cities, Villages, Towns, Citizens and Varied Interests, from the Earliest Times, Up to Date. Chicago: C. F. Cooper & Co., 1908. 2 vols. archive.org/details/rockcountywiscon01brow, archive.org/details/rockcountywiscon02brow.

  Grade A: Found on a page of one of our sources · may repeat 1856-history-of-rock-county, so not counted as independent