From The Long Roll (a novel) by Mary Johnston, with illustrations by N. C. Wyeth (Boston: Houghton Mifflin Company, 1911):

Being largely Valley of Virginia Virginians, Louisianians, Georgians, Texans, and North Carolinians, the army had acquaintance slight or none with the country through which it was passing. Gordonsville left behind, unfamiliarity began. "What's this county? What's that place over there? What's that river? Can't be the Potomac, can it? Naw, 'taint wide enough!"—"Gentlemen, I think it is the Rappahannock."—"Go away! it is the headwaters of the York."—"Rapidan maybe, or Rivanna."—"Probably Pamunkey, or the Piankatank,

Where the bullfrogs jump from bank to bank."

From Recollections, 1844-1909 (autobiography) by Henry Clay McDougal (Kansas City, Mo.: Franklin Hudson Publishing Co., 1911)

She was born and reared down in the tide-water country of Virginia, and in that part of the footstool, away back in Colonial days, when the planter did not wish to disclose his exact location, or the human interrogation point propounded the inquisitive question, he had the answer: "From Pianketank (sic), where the bullfrogs jump from bank to bank."

From Wikipedia:

The Piankatank River is a river in the United States state of Virginia. Located on the Middle Peninsula, between the Rappahannock and York Rivers, it was the site of numerous actions during the American Civil War.