Del. Code tit. 9 § 102
New Castle County comprises that part of the State which is bounded as follows: On the north, by the State of Pennsylvania; on the east, by low watermark on the eastern side of the Delaware River within the 12-mile circle described from New Castle, the southerly perimeter of the circle from its intersection with the low watermark westerly to the middle line of Delaware River, the middle lines of Delaware River and Bay southerly to a point in the last mentioned line opposite the mouth of Smyrna River; on the south, by a line drawn by the shortest distance from the last mentioned point in the middle line of the Delaware Bay to the mouth of Smyrna River, thence by a line westerly through the thoroughfare north of Bombay Hook Island, and up the Smyrna River to the mouth of a branch issuing from the main branch of the River, at a point opposite, on the Kent side, land formerly of Enoch Jones, and opposite, on the New Castle side, lands formerly of Richard Nash, and running thence westwardly up the branch the several courses thereof 602 perches to a point, where formerly stood a white oak tree, a corner for lands formerly of Benjamin Hazel and lands formerly of Richard Hollet, at the head of the branch; and from thence continued due west 1708 perches till it intersects the tangent or divisional line between the States of Delaware and Maryland, where the same crosses the Cypress Branch; and on the west by the State of Maryland.
9 Del. C. § 102