The name « Larch River Formation » was proposed by Bergeron (1954) for a sequence of sandstone, siltstone and mudstone outcrops along the Mélèzes River (formerly the Larch River) in the northern part of the Labrador Trough. The rocks of the formation were previously mentioned by geologists working for Fenimore Iron Mines (e.g. Moore, 1948). When mapping the area straddling the Mélèzes River, Bérard (1965) referred to the unit as the « Rivière Larch Formation ». Sauvé and Bergeron (1965) used this name west of Gerido and Léopard lakes. The unit was also described by Fahrig (1965).

The unit known as the Larch River was mapped between Dusay Lake (58°21′N) to the north (Bérard, 1965) and Minguarutittalik Lake (formerly Laurin Lake) (57°13′N) to the south (Fahrig, 1965; Dressler, 1979), over a distance of ~132 km on a NW-SE axis. At Mélèzes River, the width of the unit is 19 km. From this river and northwards, the unit’s width and thickness become smaller at Dusay Lake. It may extend north of Dusay Lake, but its schistose nature prevents it from being followed further (Bérard, 1965). Near its southern end, the unit ends in the axial part of a syncline at Minguarutittalik Lake (Dressler, 1979). However, small areas of the unit were mapped even further south (Fahrig, 1965). Its thickness was estimated at >45 m at Jourdan Lake (formerly Merchère Lake) and 8 m at Gourdon Lake (Bérard, 1965). Fahrig (1965) characterized the unit as « thick ». Because of numerous folds and faults affecting the unit, the actual thickness cannot be determined precisely (Bérard, 1965; Clark, 1979).

North of latitude 58°N, slaty schists of the Larch River Formation overlie the Abner dolomite (now the Denault Formation) in paraconformity (Bérard, 1965). North of the Mélèzes River (57°35′N), the Larch River Formation apparently conformably covers a thin unit of cherty ironstone, but the contact is not visible on outcrops (Fahrig, 1965; Clark, 1979). The formation is structurally overlying the Wishart and Ruth formations near latitude 58°N (Bérard, 1965). Thus, a fault appears to mark its structural base. The eastern edge of the Larch River Formation is marked by a thrust fault (Fahrig, 1965; Clark, 1979). Depending on the location, this fault would have transported dolomite of the Denault Formation (former Abner Formation) or slate and siltstone corresponding to the « lower phyllites » of Sauvé and Bergeron (1965) (equivalent to the lower Baby member) above sedimentary rocks of the Larch River Formation (Clark, 1979).

The stratigraphic position of the Larch River Formation is uncertain (Clark, 1979; Clark and Wares, 2004), as the unit may be equivalent to the Menihek Formation, the lower Baby member or the upper Baby member. To explain the fact that the Larch River Formation is structurally overlying, depending on location, the Denault, Wishart, Ruth or Sokoman formations, its western boundary has been interpreted as a thrust fault. Clark and Wares (2004) recommended that the name Larch River Formation be abandoned as they consider it to be equivalent to part of the Baby Formation. Finally, the Larch River Formation was interpreted as equivalent to the Menihek Formation, which has similar sedimentological characteristics (Clark, 1977). Given the assumption that the Larch River Formation is equivalent to another geological unit, the name Larch River was abandoned.

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