neither a type locality nor derivation of the name was designated when the term "madison limestone" was first used by peale (1893), but since the original work focused on the area of three forks, montana, it is likely that the name relates to outcrops along the madison river, montana. a reference section has been designated on the north side of gibson reservoir in se/4 sec. 36, t. 22 n., r. 10 w., patricks basin quad, teton co., montana.