Gerrhos

Gerrhos (Greek "reed-swamp") is a place in Scythia essential to interpreting Herodotus' world-map, for it formed one of the corners of the great square that defined Scythia. A more familiar "Gerrhos" or reed-swamp — from the Alexandrian point of view — lay to the east of the mouth of the Nile. Herodotus drew a meridian between the two "Gerrhoi": that in Scythia was considered the source of the "Boristhenes" Dnieper. In the words of Herodotus (IV.53):
Livio Catullo Stecchini, the unrivalled historian of earth-measuring, assigned as "Gerrhos", the area of swamps to the northeast of Smolensk, today considerably reduced by post-glacial warming and drying of the climate and by the conscious drainage and intrusion of agriculture. Later classical historians and geographers, such as Pomponius Mela mislocated this "Gerrhos".