From the time of its founding, the western border of the Shire had been marked by a line of low rolling hills known as the Far Downs. In the east, the border had traditionally been the River Brandywine, though Gorhendad Oldbuck eventually settled a new eastern 'march' (that is, a borderland) beyond the river in Buckland.
Fastred and Elanor settled at Undertowers on the Tower Hills, and their son Elfstan Fairbairn succeeded his father as Warden. From this beginning, the Fairbairns, hereditary Wardens of Westmarch, would become one of the most influential families in the Shire. They were especially important from a historical point of view, because it was at Undertowers that they preserved the Red Book, the original historical texts holding the stories that would become The Hobbit and The Lord of the Rings.4
Notes
1
We know that the Westmarch lay between the Far Downs and the Tower Hills, but beyond that our knowledge of its geography is limited to a single very small-scale map providing almost no specific detail. The relationships shown here are broadly correct (Undertowers was certainly in the Tower Hills, for example), but the exact locations of the places and borders shown is necessarily speculative.
2
In many earlier editions of the Prologue to The Lord of the Rings, Westmarch's dating is given incorrectly as S.R.1462 (or IV 41). This dating is amended to '1452' in later editions of the book.
3
Strictly, while it's strongly implied that Fastred was the first of the Wardens, the limited evidence on this point means that some other Hobbit may possibly have held the post beforehand.
4
In the context of Tolkien's stories, their origins came from the preserved diaries of Bilbo and Frodo Baggins, alongside additional material prepared by Bilbo in Rivendell. Much of this additional material was related to Elvish history and legend, and was presumably the root of The Silmarillion.