Not named for its rocky soil (Gannett, Place Names, p. 222), nor for Rock Prairie therein (Wis. Hist. Colls., i, p. 113), but for its principal river. This stream was denominated by the early French explorers, "des Kickapoo," for a village of that tribe found upon it. In the eighteenth century it was called "Riviere de la Roche," which was variously translated into Stony, Rocky, and finally Rock River. This was no doubt a translation of the Indian word, given because of the chain of rocks at the mouth of the stream, causing the rapids beside which is now the city of Rock Island, Ill.

Description from John W. Hunt's 1853 Wisconsin Gazetteer: "ROCK, County, is bounded on the north by Dane and Jefferson, on the east by Walworth, on the south by the State of Illinois, and on the west by Green. The county seat is at Janesville, on Rock river. It was set off from Milwaukee, Dec. 7, 1836, and fully organized Feb. 19, 1839. The county is about equally divided between prairie and oak openings, with no large bodies of heavy timnber. It is situated on both sides of Rock river, the valley of which is as rich soil as can be found in any part of the country. The prairies are some of them quite large, but beautifully undulating, and productive in the highest degree, and are being settled and cultivated to the very centre. The different varieties of soil -- upland, bottom land, prairie and openings, afford facilities for cultivating all the productions of the climate to the greatest advantage -- wheat upon the rolling prairies and openings -- the coarser grains upon the bottom lands -- and tame and wild grasses upon the low prairies and marshes, flourish best, though each class of soil is adapted more or less to all these products. It is watered by Rock river and its branches. The principal villages are Janesville, Beloit, Fulton, and Milton. ¿ Its population in 1840 was 1,701; 1842, 2,867; 1846,12,405; 1847, 14,720; 1850, 30,717. Square miles, 720. It has 3,631 dwellings, 1,975 farms, and 126 manufactories¿"

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[Source: Kellogg, Louise Phelps. "Derivation of County Names" in Proceedings of the State Historical Society of Wisconsin for 1909, pages 219-231.]