As ever, dear readers, welcome.
I’m always interested in Tolkien’s sources, as much for the fun of looking for them and learning new things as I go as for actually locating definite—or at least possible—ones. The following is from one of my latest forays.
September, 9AD: things were going very badly for Publius Quinctilius Varus (46BC-9AD), governor of the new Roman province of Germania. Trusting an officer of Germanic auxiliary cavalry, Arminius (19/18BC-21AD), who had grown up in Rome as a hostage, and seemed more Roman than the Romans, Varus had marched three of his legions into the depths of the German forests

and into a nightmarish series of ambushes

(by Peter Dennis)
in which he had seen as many as 20,000 soldiers and civilians killed before Varus himself committed suicide, probably convinced that, if taken prisoner, he would have met a much slower and more painful end.
Although the Romans, over the next years, had mounted a number of punitive campaigns into Germania, ultimately, it was decided that the region was best left to itself, but, to try to keep an eye on the tribes and to prevent possible raids, if not outright invasions, the Roman government established what was called the Limes Germanicus, the “German boundary”.

This wasn’t just a striped road barrier and a customs station,

but hundreds of miles of ditch with a palisade and earth wall behind it,

watch towers with beacons ready to be lit,

and Roman military camps behind it.

And this became a pattern for Roman conquest. In Britain, troubled by Pictish tribes to the north, the army set up two walls,

one, the so-called “Antonine Wall”, in Scotland, subsequently abandoned,

which was constructed much like the Limes,
and then the so-called “Hadrian’s Wall”,

more substantially built, to the south, with towers,

“mile castles” at intervals,

and extensive camps, just like the Limes.

Later post-Roman Britain saw other attempts to indicate boundaries, like Offa’s Dyke,

which may have originally resembled a rather rudimentary version of the Limes and which (roughly) marked the separation of England from Wales.
The Carolingian emperor, Charlemagne,

developed this boundary idea farther, establishing a whole series of what were called markas (marcae),which, in English, we called “marches”, and which

colonized, but also militarized, regions along his empire’s borders (when you read “Denmark”, for example, you can think that this was one such zone—the “border region with the Danes”).
As the Normans and their heirs spread westward across Britain and began the conquest of Wales, certain lords—who came to be called “marcher lords”–controlled similar areas throughout the early Middle Ages,

and the troubled border between Scotland and England was, on both sides, divided into East, Middle, and West Marches,

each March supervised by a Warden. This is an area particularly full of romance/adventure for me, not only for its spare landscape, with its fortified houses, called “bastles”,

and scattered castles, like Hermitage,

its Reivers—raiders who lived on the edge, both of the Border and of the law–

(Angus McBride)
and the bold men who enforced the law—or tried to.

(another McBride—this is the “hot trod”, where the Warden summoned all those to help him in pursuit of raiders by raising a burning turf on a spear)
Many of the ballads collected from this region are stories about these people, like that of the rescue of Kinmont Willie from Carlisle castle.

(a third McBride)
(If you’d like to know more, I recommend George Macdonald Fraser’s The Steel Bonnets, as a beginning.)

This brings us to the “Riddermark”, that is, Rohan.
It seems pretty clear,, where Tolkien, first as a classicist, then as a medievalist, got the concept and thus the term from.

After Eorl the Young,

(I’ve always liked this image, which suggests the tapestry identified by Aragorn in Edoras: “ ‘Behold Eorl the Young!…Thus he rode out of the North to the Battle of the Field of Celebrant.’ “ The Two Towers, Book Three, Chapter 6, “The King of the Golden Hall” )
had led his men in helping the Gondorians to deal with “a great host of wild men from the North-east”, Cirion, the Steward of Gondor:
“…in reward for his aid, gave Calendardhon between Anduin and Isen to Eorl and his people; and they sent north for their wives and children and their goods and settled in that land. They named it anew the Mark of the Riders [Riddermark}…but in Gondor their land was called Rohan, and its people the Rohirrim…There the Rohirrim lived afterwards as free men under their own kings and laws, but in perpetual alliance with Gondor.” (The Lord of the Rings, Appendix A, II, “The House of Eorl”)
And in this, we see the ghosts of all those historical border-watchers, from the Romans to the Carolingians to the marcher lords of Wales and the turbulent border of England and Scotland.
Thanks, as always, for reading.
Stay well,
Listen always for the sound of distant horns,
And remember that, as ever, there’s
MTCIDC
O