SOEMIL AS THE 'SEPARATOR' OF DEIRA FROM BERNICIA: CONFIRMATION OF THE DERVENTIO THEORY

I've researched the possible origin of the Dere (genitive Dera) kingdom name rather extensively.  And have made some false steps along the way.  My errors have consistently been due to an attempt to reconcile the Welsh forms of Dewr or Deifr with the Anglo-Saxon name for the place.  As it turns out, in this particular case we are much better off going with the English name.  

Dewr, as I once pointed out, is homophonous with W. dewr, 'brave, bold' (see Koch, CELTIC CULTURE: A HISTORICAL ENCYCLOPEDIA, under the entry for Deifr).  Deifr, on the other hand, has been linked with the W. word for 'waters' (also Koch).  

But Dere/Dera has very plausibly been linked to W. dar.  From the GPC:

dâr

[H. Grn. dar, Gwydd. Diw. dair: < IE. *der- neu *dər- o’r gwr. *deru- ‘pren bychan’]

eb. ll. deri, deiri.

Derwen, coeden o rywogaeth y Quercus a nodweddir gan braffter ei chyff, gan ei rhisgl trwchus rhychiog, ei dail llyfn danheddog o wyrdd tywyll, ac a adwaenir yn enw. wrth ei ffrwyth, sef y mes; yn ffig. blaenor mewn brwydr, &c., arweinydd galluog, arglwydd cadarn:

oak-tree; fig. foremost warrior, leader, mighty lord. 

And given the location of the Derventio [1] Roman fort at Malton, and the course of the River Derwent through what was Deira (a region spanning the area from the Humber to the Tees), it would seem very reasonable to conclude that Dere/Dera is for "oak/oaks" - or, more specifically, the River Derwent, which is defined by Rivet and Smith as 'oak-river' or 'river in an oakwood.'

Site of Derventio/Malton Roman Fort

We might be tempted to see in the Dewr/Deifr of the Welsh the term 'waters', here referring to the very same River Derwent.  How the Welsh term came about is hard to determine.  I suppose a metathesis of *deru- or *daru- to dewr is not impossible.  That the area in question was dominated by a river named for the oaks on its banks may have been a contributing factor.

But how do we know for certain that Derventio was the origin of the English kingdom name?

Well, I think the "proof", as it were, exists in the cryptic statement recorded in the HISTORIA BRITTONUM that the 5th century Deiran king Soemil "separated" Deira from Bernicia.  

The Roman fort of Derventio was garrisoned (NOTITIA DIGNITATUM) by a unit from Petuaria, i.e. Brough on Humber.  According to Rivet and Smith and others, Petuaria means "fourth" and tells us that the Parisi tribe had perhaps been divided into four pagi.  As separo in Latin has among its meanings "divide", I would make a case for the statement regarding Soemil as a garbled memory of the Petuarian unit's presence at Derventio/Malton. 

Alan James rather likes the idea.  He said:

"Divisions of land into notional 'quarters' seems to have been a strongly-rooted custom among the Celtic speaking peoples (even when there were more or less than 4 divisions, they still tended to call them by words meaning 'quarter'!) And the source of HB seems to have hadd a notion that Northumbria began as one big unit and Deira was carved out of it, which is pretty certainly confused."

[1] Derventio, according to Rivet and Smith, derives from *daru- *deru- 'oak', with *ent- and *-io-(n) suffixes.