Cefn hamlet is in the south-east of the parish, and includes Trelewis, Llancaiach fawr, Gelligaer church and village, Penybryn, Penallta and that part of Ystrad Mynach to the west of Nant Cylla (the stream that runs under the middle of Ystrad Mynach).
Its boundaries in 1750 were given as “It begins where Cayach Brook goes to the River Rumney by Ystrad Manerch then along Cayach Brook upwards till it goes to Bargod Taff then along the River Bargod to nant y Garth then along y nant by Craig Vargod Meeting house directly to Funnon Cluide Trauska then along the High way Eastward Towards Pen Ca Mawr till it comes to the way that leadeth from Pen y Brin Ore to Pen yr Hewl Ddu then along that way downwards till it comes to an old ditch that goeth Eastward from the said way near Ca yr Capel then from that ditch to a stone standing upright on the common opposite Mardy Bach House where nant Goch springs out, then along nant goch till it comes to Pen y Garreg Farm then along the hedge between Bryn coch and Pen y garreg farms directly westward till it comes to the common then along the hedge of Pen y Garreg Farm to a white thorn in the said hedge and a stone on the common by spring head of Killa Brook Between the high way and the hedge then along Killa Brook to Ystrad Manerch afore said whear it goes to Rumney River then along Rumney River to the Entrance of Cayach Brook to it as aforesaid”.
Although its population doubled between 1801 and 1871 it was still only 654 in 1871. The population in each sucessive census is shown below. By 1881 population was no longer recorded by hamlet as their function had all but disappeared.
The farms as recorded in the 1841 tithe schedule are shown below. Many of the farms are still shown on the modern OS map, although with urbanisation some have disappeared.
The tithe value has been converted to modern pounds/pence; it gives an indication of the relative values of the farms in 1841
** Dan y Pentre and Rhos y Bedw, in Hengoed Hamlet, were recorded as one 78 acre farm. Although the two farms are close they do not share a boundary.