The Upper Ordovician (449-454 Ma) Galets Formation was introduced by Sinclair (1953) in the Lake Saint-Jean area. This formation belongs to the St. Lawrence Platform, which forms a slice in the Grenville Province. All Ordovician sedimentary rocks in this region were assigned to the Trenton Group by several authors from a chronostratigraphic perspective (e.g., Dresser and Denis, 1946; Sinclair, 1953). Other authors have done so on lithostratigraphic bases (Benoit and Valiquette, 1971; Harland et al., 1985, 1987). Instead, Desbiens and Lespérence (1989) proposed the creation of a new group for these rocks, but did not. Although spatial correlation between these Ordovician rocks in the Lake Saint-Jean area and those of the Trenton Group of the St. Lawrence Platform is not formally established, the Galets Formation is maintained in this group.

The Galets Formation consists of a single echinoderm calcarenite (grainstone) facies. This calcareous facies is bedded (<1 m, 50 cm on average), well sorted and bioclastic. It was deposited in a high-energy and shallow sedimentation environment (turbulent regime), locally affected by tidal currents (subtidal zone; Jones and Desrochers, 1992). Shale beds are scarce. Calcarenite is massive and contains calcisiltite (packstone) beds approximately 5-10 cm thick and that can make up 30% of the sequence. Sedimentary figures such as cross-lamination, cross-bedding and hummocky cross-stratification are observed in the Galets Formation. The latter is also characterized by high P₂O₅ content (0.07-18.91%; Jones and Desroches, 1992).

In the Lake Saint-Jean area, the Galets Formation and the three other related formations (Shipshaw, Simard and Tremblay) cannot be mapped at the 1:50 000 scale because they are generally not more than 50 m thick and subhorizontal (<4° dip). On the geological map of the Saguenay–Lac-Saint-Jean region (sheets 32A08 and 22D), the Trenton Group is hence shown instead of these four related formations.