Identification: the river Blyth
This may be a pure river-name, i.e. one made up of Celtic river-letters, but it appears more likely to be a land-name transferred by the Romans to a river. In this case the place called Sidumanis is probably the same place as the Sitomago of the Antonine Itinerary Iter IX and the Sinomagi of the Peutinger Table. The distances given in Iter IX and the Peutinger Table for the journey from Venta Icenorum to Sitomago/Sinomagi are different, but point to a site on the river Blyth at Holton or Halesworth. The various forms of the place-name all include the hill-letters s and m, the corresponding river-letters being b and r. The Celtic river-name including these river-letters, whatever its precise spelling, may have acquired the river-letter t, changed to th, and a name somewhat like Brith may have changed to Blyth at a later date, perhaps even as late as the Norman period. If the t of the Iter IX form is correct then the place was located adjacent a high hill, and if the g of the Iter IX and Peutinger forms is correct then the hill was also considered steep. It is of course possible that Ptolemy’s form is closer to the original Celtic name, the initial Sid pointing to a location on the top of raised ground. There is raised ground at both Holton and Halesworth, so either may have been the location of the Roman fort. Holton is perhaps to be preferred since it is actually on the river Blyth, whereas Halesworth is on a tributary, though not far from the Blyth. The discrepancy in the mileages given in the Peutinger Table and Iter IX is easily explained. The distance between Caistor St. Edmund and Holton/Halesworth along Margary road 36 is about 20 Roman miles, perhaps a little more since the precise route of the road is not known at the northern end. The distance given in the Peutinger Table is 22 Roman miles. The distance if one travels south from Caistor on Margary road 3d, then on the 35 and then on the 34b is about 33 Roman miles, very close to the 32 miles indicated in Iter IX. But while the fort may have been at either Holton or Halesworth, for present purposes the important point to note is that Ptolemy’s Sidumanis river was the river now called the Blyth.
[NB. Detailed information as to the different river-letters and as to how they were combined to form compound river-names, together with information as to the four categories of Celtic river-names, is given in Chapter 19: the rivers of Roman Britain. Detailed information as to the different hill-letters is given in Chapter 1 and information as to how the hill-letters were combined to form compound place-names is given in Chapter 2]
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