Sir William M. Ramsay (1851-1939), archaeologist, scholar, and author, was a champion of the South Galatia position (i.e., the N.T. letter to the Galatians was addressed to Antioch of Pisidia, Iconium, Lystra and Derbe) at a time when the North Galatia theory had been accepted for centuries. In his book, A Historical Commentary on St. Paul’s Epistles to the Galatians, he deals with the geography of north Galatia, giving evidence to eliminate that as the area intended in the book of Galatians. He was seeking to understand the geography of Galatia especially and specifically in the mid 1st century AD.
In that portion of his historical introduction, Ramsay mentions the Halys River and how it divided Galatia into two parts.
The country afterwards called Galatia was in primitive time divided ethnographically and politically into two parts, eastern and western: the division was made by the river Halys, which in this part of its course runs in a northerly direction towards the Black Sea. Galatia east of the Halys seems to have been originally reckoned to Cappadocia, though part of it was probably sometimes described as included in Paphlagonia; but the bounds of those countries were so indeterminate, and the ancient writers themselves were so ignorant of the geography of those lands, that it is quite impossible to say anything positive and certain on the subject. . .
Eastern Galatia lies mostly in the basin of the Halys (Kizil-Irmak, the “Red River”). The Halys itself has very few and quite insignificant tributaries. In Eastern Galatia the Delije-Irmak (whose ancient name is unknown) is the only tributary of any consequence; and most of the country lies in its basin; but the river, though it looks large on the map, carries very little water except in flood, when it becomes a broad and raging torrent, exactly as its name indicates, the “Mad River”. . .
Galatia west of the Halys, which was much larger than the eastern country, was the most important and the most typical part of the country; most of our scanty information relates to it; and in general, when any statement is made about North Galatia, the writer has the western part of it in his mind. This western region was originally part of the vast land called Phrygia; and, clearly, the population of the country in the early part of the fourth century were known to the Greeks as Phrygians (Φρύγες). (Ramsay, W. M. (1900). A Historical Commentary on St. Paul’s Epistle to the Galatians (pp. 15–17).
Sir Ramsay referenced the Halys as the Kizil-Irmak. Note the sign here at the bridge crossing the river.
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Thanks Leon !