Fortin, probablement de la fin de l'époque romaine, réutilisé au Moyen Âge. Il est situé à côté de la voie romaine, devenue un grand axe routier. Le Quesnel (Somme).

When it comes to more ancient periods of time, it is a delicate operation to identify and accurately date structures located from the air, without recorse to test pits or archaeological excavation, the ultima ratio of archaeologists.

Nevertheless, lines with very characteristic geometric forms do exist, as well as ancient villas with their typical floor plans. For example, rural dwellings in northern Gaul, with their proportions and their lengthwise layout (generally flanked by a gallery façade) are easily identifiable from the air. In the same way, the layout of Roman retrenchments is also characteristic.
As always, caution is called for — some structures, such as the titulum, which are thought to be typically Roman, were brought back into use under Henri IV. Naturally, one has to be careful to avoid illusory keys to interpretation, even when taking into account fairly characteristics dimensions.
Not every small square enclosure is necessarily from the Gallic era, even if this is generally the case. In the same way, moated circles are not always from the Bronze Age, because they were still in existence in the Merovingian era and afterwards. They also had difference uses: surrounding certain isolated trees, as roadside crosses or windmill mounts, and even for protecting batteries of anti-aircraft guns during the Second World War. Small differences can help tell them apart.

Fortin en étoile du siège d'Amiens par Henri IV en 1597 et tranchées en zigzag de 1918. Nord d'Amiens (Somme).

Be careful of formal similarities, which are often false. As Montaigne remarked, with his usual lucidity, "Resemblance does not so much make one, as difference makes another." It is the little "differences" to which we need to pay particular (but not exclusive) attention.

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Butte de moulin entourée d'un fossé de drainage irrégulier et un cercle protohistorique bien régulier. La ligne droite blanchâtre est un ancien chemin d'exploitation rurale. Nord d'Abbeville (Somme).

Butte de moulin entourée d'un fossé de drainage irrégulier et un cercle protohistorique bien régulier. La ligne droite blanchâtre est un ancien chemin d'exploitation rurale. Nord d'Abbeville (Somme).