The boundary between Delaware and Maryland was originally
set to be the line dividing rainfall that flowed into the Chesapeake Bay
from rainfall that flowed into the Delaware River and Bay.
This MD-DE Divide is shown as a broad serpentine line on the map below.
Travel in the 1700s was primarily by water, so the people who settled west of the MD-DE Divide came primarily from either direct immigration through an Eastern shore Maryland port or moved from what is now Maryland while those who settled east of the MD-DE Divide, especially in New Castle and Kent counties, came primarily from direect immigration through a Delaware port or moved from Pennsylvania and New Jersey.
Before 1764, when a team of British surveyors led by Mason and Dixon set the western and southern borders for Delaware, deeds for land in the shaded area of the map below [* Yes we need a new map here, since the shading is not present in the image below *] were often granted by Maryland, which claimed all of present Delaware as "Durham Co."
The borders of the Delaware hundreds (see the page on Delaware's Hundreds) as they had evolved by 1775 remained unchanged until 1830, but they were sometimes designated by more than one name; for example. Dover Hundred was also called St. Jones Hundred and Broadkill Hundred was also called Broadkiln Hundred
CREDITS: The "Delaware Divide" map appears on page 51 of the Delaware Genealogical Research Guide - 2nd edition (1997) and was taken from the Delaware Geological Survey (Newark DE). The shaded area for Maryland deeds was designated by George L. Caley, and the dashed-line boundaries for hundreds and counties were added by Thomas P. Doherty.