Blake and Lloyd state that the three realms of Ynys
Pridein are bounded by the three principal rivers, given in Enwau Ynys
Brydain as Temys a Hafren a Hwmyr. They are given by Geoffrey’s
geographical introduction (HRB i.2) as the Thames, Severn and Humber.
Once again, Geoffrey’s statement derives from the Historia Brittonum
Chapter 9, where we are told that duo flumina praeclariora ceteris fluminibus
tamensis ac sabrinae (‘two rivers are more notable than the other rivers:
the Tamensis and Sabrina). Geoffrey has added a third river, taken
from the Historia Brittonum Chapter 61, where it names umbri maris
in a context which suggests that it was the boundary of regiones in
sinistrali parte brittanniae (‘the regions in the northern part of Britannia’)
held by Ida, king of Bernicia.
The two principal rivers named in the Historia
Brittonum derive from Gildas Chapter 3, where he describes duorum ostiis
nobilium fluminum tamesis ac sabrinae (‘the mouths of two noble rivers, Tamesis
and Sabrina’), which he says were the main routes through which exotic
goods formerly reached Britain. These two names can be traced back in earlier
writers; the former was first mentioned in the first century BC by Julius Caesar
in de Bello Gallico (v.11, 8 and v.18, 1), while the latter is first
found in the late first century AD in Tacitus’s Agricola (12, 31). Both
of these names occur in Ptolemy’s Geography, compiled around the middle
of the second century AD. He gives latitudes and longitudes for the estuaries
that enable us to fix them as the Thames and the Severn respectively.
In other words, there is a chain of evidence
from the early Roman period that enables us to be certain that when Geoffrey
writes of Tamesis and Sabrina, he means the Thames and the Severn.
Moreover, his source for the names is not a mysterious Welsh document related to
the Brut, but the Historia Brittonum, whether or not at first
hand. Similarly, the Humber is named in the early ninth century as umbri
maris and probably derives from a British *Σumbra via Old Welsh *Humber,
surviving as Modern English Humber.
Notwithstanding these clear links, Blake and Lloyd propose that the rivers have been misidentified since Geoffrey’s time. They link the names Temys a Hafren a Hwmyr in Enwau Ynys Brydain with the three most important cities, which they name as Llundain, Caerlleon and Caer Efrog, although the version in the White Book of Rhydderch refers to thirty-three chief cities (following Chapter 3 of the Vatican recension of the Historia Brittonum, a tenth-century rewriting produced in England!) and the version in the Red Book of Hergest names thirty-two.
However, following their argument, the first of the three names they choose was the most important city of Lloegyr and Kernyw, the second of Cymru and the third of Deifyr, Bryneich and the north (i.e. Alban), although this does not come from Enwau Ynys Brydain. In the conventional explanation, Llundain is London, Caerlleon is Caerleon-on-Usk and Caer Efrog is York. This follows the accepted derivations of Llundain from Old Welsh Lundein (Historia Brittonum Chapter 66a) and Latin Londinium following the usual sound changes from Brittonic to Middle Welsh; similarly, Caerlleon is found in Old Welsh as Cair Legion, where it is used for both Chester and Caerleon-on-Usk, while Caer Efrog is the Old Welsh Cair Ebrauc and Latin Eburacum, York. (all Old Welsh forms found in the Historia Brittonum Chapter 66a).
They suggest that these identifications are all based on Geoffrey of Monmouth’s mistranslation of the Brut and that alternative locations should be sought. These are the first three of the confusions listed below.