The boundary stone, the landmark of the roads, is Hermes
Often the traveler, especially Walker, sees along the road or at the entrance of the cities stone markers, landmarks and even piles of stones.
Often we also see these milestones, blocks of stone or cairns with the function of marking a territory, designate a special place, as the grave of a deceased, or define a space. In the cult language we call these milestones "herm" or "Herma", in ancient Greek ἕρμα. word that means "rock, mound". They also had a protective and apotropaic function to "turn away" all harm or evil influences (from Greek ἀποτρέπω, apotrépō, "away, apart", of ἀπό, apo, "away" and τρέπω, trepo, “climbed, spin"). These cairns were usually crowned with the bust or figure of Hermes or Mercury.
But Hermes is a god in Greek mythology, son of Zeus and Maya. He is the messenger of the gods, the protector of the travelers, the god of merchants and thieves (the ancients had gods for everything). The Greek Hermes corresponds to the Roman god Mercury (note the connection with the word "market").
So the milestone, the stone block represents the god Hermes and so in the ancient world are very common these stone markers square shaped finished with bust of the god Hermes, sometimes adorned with male genitalia, especially with the phallus or penis as a symbol of fertility.
This god in this function is similar to the Roman "Terminus" (meaning more evident and clear). These "Hermes", as I said, were placed along and at the crossroads of the roads (among other things Hermes is the god of travelers), at the gates of the cities, at the limits and boundaries of populations, etc. . They may have their origin in the early Greek and Roman custom of forming piles of stone in certain points along the way; the traveler passing threw its little stone by making a wish. The fact that they are of stone gives us back to ancient beliefs and cults in which the stone has a spirit or special religious value.
In Spain remains from old time that custom in some parts: in Aragon are called "peirones", in the Molina de Aragón (Guadalajara) (where they are called "pairones") are numerous vertical prismatic stones, whose top is usually a religious image. They are certainly related to the old "Hermes".
No less striking is how endure to this day customs that have lost all meaning and how persist old names, although reduced already to specially educated circles.
Regarding "terminus", is convenient to make some observations. Personified and deified is a Roman god who protects the limits of both private and public property. Tradition attributes to Numa the custom of placing markers or boundary stones to delimit the property, which is protected by Iuppiter Terminus, in whose honor festivals Terminalia are celebrated in Februar, 23.. (Fast Ov.. Ii. 639 ff).
By extension the word "terminus", which went to Spanish (“término”) also means limit, end of something and territory bounded by terms (for example: municipality), and even the word that defines a concept (linguistic term).
Along the noun is the verb "terminar" "end" to mean stop or end of something and the compounds 'determine' (set limits or terms between which something is in real or figurative sense) and "exterminate" (completely finish , destroy, take you beyond the limits or terms).
I then offer translation of Ovid's text in which he describes the festival of Terminalia to deepen the idea of the importance of religious rites and acts to set the rules and laws of society, in this case, the private ownership of land : Ovid: Fasti, 2, 639 ff. (Translated by JAMES G. FRAZER )
When the night had passed, see to it that the god who marks the boundaries of the tilled lands receives his wonted honour. O Terminus, whether thou art a stone or stump buried in the field, thou too hast been deified from days of yore. Thou art crowned by two owners on opposite sides; they bring thee two garlands and two cakes. An altar is built. Hither the husbandman’s rustic wife brings with her own hands on a potsherd the fire which she has taken from the warm hearth. The old man chops wood, and deftly piles up the billets, and strives to fix the branches in the solid earth: then he nurses the kindling flames with dry bark, the boy stands by and holds the broad basket in his hands. When from the basket he had thrice thrown corn into the midst of the fire, the little daughter presents the cut honeycombs. Others hold vessels of wine. A portion of each is cast into the flames. The company dressed in white look on and hold their peace. Terminus himself, at the meeting of the bounds, is sprinkled with the blood of a slaughtered lamb, and grumbles not when a suckling pig is given him. The simple neighbours meet and hold a feast, and sing thy praises, holy Terminus: “Thou dost set bounds to peoples and cities and vast kingdoms; without thee every field would be a root of wrangling. Thou courtest no favour thou art bribed by no gold: the lands entrusted to thee thou dost guard in loyal good faith. If thou of old hadst marked the bounds of the Thyrean land, three hundred men had not been done to death, nor had the name of Othryades been read on the piled arms. O how he made his fatherland to bleed! What happened when the new Capitol was being built? Why, the whole company of gods withdrew before Jupiter and made room for him; but Terminus, as the ancients relate, remained where he was found in the shrine, and shares the temple with great Jupiter. Even to this day there is a small hole in the roof of the temple, that he may see naught above him but the stars. From that abide in that station in which thou hast been placed. Yield not an inch to a neighbour, though he ask thee, lest thou shouldst seem to value man above Jupiter. And whether they beat thee with ploughshares or with rakes, cry out, ‘This is thy land, and that is his.’” There is a way that leads folk to the Laurentine fields, the kingdom once sought by the Dardanian chief: on that way the sixth milestone from the City witnesses the sacrifice of the woolly sheep’s guts to thee, Terminus. The land of other nations has a fixed boundary: the circuit of Rome is the circuit of the world.
Nox ubi transierit, solito celebretur honore
separat indicio qui deus arva suo.
Termine, sive lapis sive es defossus in agro
stipes, ab antiquis tu quoque numen habes.
te duo diversa domini de parte coronant,
binaque serta tibi binaque liba ferunt.
ara fit: huc ignem curto fert rustica testo 645
sumptum de tepidis ipsa colona focis.
ligna senex minuit concisaque construit arte,
et solida ramos figere pugnat humo;
tum sicco primas inritat cortice flammas;
stat puer et manibus lata canistra tenet. 650
inde ubi ter fruges medios immisit in ignes,
porrigit incisos filia parva favos.
vina tenent alii: libantur singula flammis;
spectant, et linguis candida turba favet.
spargitur et caeso communis Terminus agno, 655
nec queritur lactans cum sibi porca datur.
conveniunt celebrantque dapes vicinia simplex
et cantant laudes, Termine sancte, tuas:
'tu populos urbesque et regna ingentia finis:
omnis erit sine te litigiosus ager. 660
nulla tibi ambitio est, nullo corrumperis auro,
legitima servas credita rura fide.
si tu signasses olim Thyreatida terram,
corpora non leto missa trecenta forent,
nec foret Othryades congestis lectus in armis. 665
o quantum patriae sanguinis ille dedit!
quid, nova cum fierent Capitolia? nempe deorum
cuncta Iovi cessit turba locumque dedit;
Terminus, ut veteres memorant, inventus in aede
restitit et magno cum Iove templa tenet. 670
nunc quoque, se supra ne quid nisi sidera cernat,
exiguum templi tecta foramen habent.
Termine, post illud levitas tibi libera non est:
qua positus fueris in statione, mane;
nec tu vicino quicquam concede roganti, 675
ne videare hominem praeposuisse Iovi:
et seu vomeribus seu tu pulsabere rastris,
clamato "tuus est hic ager, ille tuus".'
est via quae populum Laurentes ducit in agros,
quondam Dardanio regna petita duci: 680
illa lanigeri pecoris tibi, Termine, fibris
sacra videt fieri sextus ab Urbe lapis.
gentibus est aliis tellus data limite certo:
Romanae spatium est Urbis et orbis idem.