Whereas in books 14-20 the excursuses are all formal, in 21-27 methodological and reflective excursuses appear too.
22 8 The coast of Thrace and Pontus; 14.7-8 The Apis Bull;
Except for the formal excursuses, in books 27-31, Ammianus adds some excursuses in which he expresses his personal comments.
27 3.14-15 Bishops; 4.2-14 Thrace
Concerning his main narrative, Ammianus is still appreciated as an accurate historian. Yet, as mentioned above, modern historians brush his geographical digressions aside as being far more inferior to the military and political ones. That truly is unfortunate, since we can deduce from them a clear picture of ancient geographical knowledge. As such, Ammianus geographical digressions are of particular interest to modern scholars.
The concept of geographical knowledge of Ammianus time was quite different from ours. One of the arguments that can be used to disregard the geographical digressions, is the incorrect topography Ammianus provides his audience with. He does not always list rivers and cities in the correct topographical order, as J.W. Drijvers argues for the Pontus Euxinus (Black Sea). Yet, he agrees with Bekker-Nielsen and Brodersen, who introduced another view toward ancient geographical information. Since todays maps are quite detailed and accurate, it is easy to forget that the ancient Greeks and Romans did not possess this kind of specific information. Their geographical knowledge was captured in a concept called a mental mapperhaps quite equal to the sequences of cities and countries children have to remember and recite nowadays in their geography classes. In this frame of mind, accurate information in the modern sense on geography and topography can hardly be reproduced. Therefore, Ammianus had to use his own observance, along with oral and written information from othersnotably by sailors guides called periploi. Apparently, a general idea of the geographical and topographical structure of the empires provinces and foreign territories sufficed to the Roman readers.