fiddlestix1224 added this to her Ancestry.com Tree on 25 Mar 2008
Two histories explain the boundaries of the counties of Somerset, MD and Sussex, DE:

[The following is from rootsweb page for Sussex County, DE at: http://www.rootsweb.ancestry.com/~desussex]
History of Sussex /Somerset Counties--Boundary Disputes with MD/DE, Maryland, Delaware
" Sussex County, the southernmost county in Delaware, is bordered on the north by Kent County, on the east by the Atlantic Ocean, and on the west and south by Maryland's Caroline, Dorchester, Wicomico, and Worcester Counties. The boundary between Delaware and Maryland was long disputed. Until the 1760s and the Mason-Dixon survey, Maryland's claim extended well into what is now Sussex County. The border was undefined, but there were Somerset land claims roughly up to the Indian River, and some beyond that. If you've got lines in Sussex County, especially in the southern part, you would be wise to check Maryland records, too."

[The following is from Somerset County, MD genweb page. It explains the old Somerset County, MD/Sussex County, DE boundaries. Page includes many excellent maps. See page at: http://www.rootsweb.ancestry.com/~mdsomers/lyonmaps/index.html ]
Somerset County MDGenWeb     John C. Lyon's   Old Somerset Hundreds and Land Grant Maps
    "The map series here is based on an ongoing, now largely complete mapping of all original private Provincial and State land grants (patents) made from 1662 to the present in the original ("Old") Somerset County, Maryland. This region now includes the present areas of Somerset, Wicomico and Worcester Counties, and almost half of Sussex County, Delaware. This includes both original surveys of "vacant" lands and all Provincial or State level revisions to those through various authorized resurveys. The mapping also includes all surviving unpatented certificates of survey, which - while they may not have been finally established as grants for a variety of reasons - offer insights into the development of the County, the time of arrival of early settlers or presence of individuals not elsewhere recorded.
    "Records of these grants and surveys are maintained in the Maryland State Archives in Annapolis, which has assumed the responsibilities of the old Land Office which oversaw the granting of lands from the early Colonial period. Most of the original certificates of survey survive, but for the earliest lands, transcripts were made in the colonial period of the original surveyor submissions (few of which survive before 1700) and documented in the Patent records. In all, over 10,000 survey descriptions have been fully extracted from the source records, expressed in digital mapping software and platted on the landscape.
    "The status of this tract mapping at this time (May, 2004) is essentially complete at a neighborhood level for the three modern Maryland counties, with fine placements and corrections of errors still ongoing. Complete and accurate placement of tracts in present-day Delaware is work yet to be done. Some such placements have been made where it is possible to identify landmarks with some reliability.
    "Using this underlying map in tandem with other records, an accurate depiction of the geographic development of the area emerges. Various other understandings of the economics and demographics are possible because of the rich detail in the source records, almost all of which has been transcribed and in the mapping database. Presented here are a few products. More may be added periodically, and slight revisions to the ones presented might occasionally be expected.
    "The first series here exhibits several views of the county from the beginning through the end of the Colonial period. Three of these are completely new views of the Hundreds of the region, before and after the partitioning of Worcester. The 1734 Hundreds map is quite different from several renditions seen before..."