Fisgas do Ermelo waterfall is located on the Olo river, a tributary of the Tâmega river, showing a river break point between a western sector dissected by the Atlantic network and a sector to the east corresponding to the tectonic block raised from the mountains of Alvão and Marão, to be captured by the same network.

Fisgas do Ermelo lie on the edge of an intermediate erosive plateau at about 800 m high, immediately to the east of the waterfall, and extending northward, marking the transition between the western sector and the Alvão plateau.

The Fisgas do Ermelo owe its existence to the resistance that the Ordovician quartzites offer locally.

This knick point is thus one of the places where the tectonic and lithologic factors, that control the incision of the fluvial network in the Hesperian Massif, are more evident.