Much of the Camino in this stretch is on an old Roman road and the Roman tradition of leaving stones at high passes in gratitude to Mercury for helping in safe passage also stems from the tradition you find at the summit of Monte Irago.
If Mercury was honored here there is a very good chance that Lugus was too among the Celtiberians before them. Lugus was one of their most popular gods and he possesses Mercurial qualities. Similarly, the Celts piled rocks in high points, placing cairns as markers, offerings, and memorials.
More recently, the wooden post with the iron cross mounted on top is a later Christianization of the hill. That dates to the 12th century when a hermit named Gaucelmo placed it in the center of the hill of stones.
He did it to take over the pre-Christian rite, a known sacred spot, but he also used wood from an oak tree, one of the holiest trees to the Celts (and one that is planted, or selected, around so many churches and shrines in Spain, including all along the Camino). In effect, he succeeded in doing both, Christianizing the hill and reminding everyone about who was here before.
Known often simply as the iron cross—cruz de ferro in Gallego, cruz de fierro in Leónés, and cruz de hierro in Castilian—it is a challenging hike from Rabanal del Camino to get to Monte Irago. Once here, placing a stone of gratitude can feel liberating just as it may have to a Celtic or Roman traveler in these same hills.
This is a culmination point in the ritual process of the Camino. Many pilgrims arrive here having carrying a special rock, perhaps from home, or from Saint-Jean-Pied-de-Port. Some pick one up on a whim along the way.
They anticipate the moment of release when they deposit the rock with so many others before them on the impressive heap that holds the iron cross in the center like a beacon (or a stupa, borrowing from the Buddhist idea of the pointed steeple delivering one’s prayers to heaven).
Some see it as a chance to unburden heavy psychological matters. Others see it as a simple act of gratitude, much like the Romans and Celts. A possible Christian interpretation is that the rock carries ones sins and transfers them from the sinner. It may be a reference to Jesus’ admonition, “let he who casts the first stone be free of sin.” It offers a chance for catharsis and reconciliation just by doing something conscious and intentional about it.
An extension of the ritual here at Monte Irago occurs all across the Camino. Notice all those stacked stones on way markers and on hills by the side of the path. Pilgrims have intuitively already been casting their stones/sins from themselves as they walk, a process that takes the entire Camino.
It is also a beautiful form of communication. Each pilgrim who passes such a pyramid of stones feels redeemed knowing others are shedding, who have issues, feel guilty, had hard times, and are all striving for clarity and lightness. The feeling of adding your stone to the top of those is powerful and liberating.