Collecting the water of pristine mountain tributaries like the Cacapon and growing as it goes, cleansing itself of the North Branch's load of trouble, the river finds its way at last out of the washboard and meanders among silver maples and great sycamores across the productive populated expanse of the Great Valley that runs athwart the whole Basin from north to south. The Potomac is in thickly historic country now as it flows under the contemplative eyes of fishermen and past old villages and the relics of generations of human activity going back before written records, for here and there the funnel shapes of stone Indian fishing weirs can still be seen at shallow places and the durable fragments of their way of life can be scratched up along high shores. Of many Civil War clashes in the valley, Antietam was the most crucial; the Potomac shaped Lee's strategy there, and still ripples across fords by which his troops came to that violent place and afterward escaped it. At Harpers Ferry on the Valley's eastern edge, the river is reinforced by the waters of its greatest tributary, the Shenandoah, rolling north out of the limestone country that fed the gray armies till Sheridan put a stop to that. Then it rams through the high wall of the Blue Ridge and out of the Valley into the Piedmont, and still gathering strength from tributaries like the Monocacy, dotted with big islands and frequented by waterfowl and good fish, moves powerfully downcountry past further mists and layers of history to Great Falls and the rushing, crashing descent through the gorge to tidewater at the capital.