The precise timing of when upland terrains first became forested is highly controversial. Pennsylvanian palynoflora and megaflora transported into marine highstand deposits imply that emergent topographic highs may have supported cordaitalean forests. The discovery of a new Pennsylvanian (Bolsovian) plant assemblage in southwest Newfoundland confirms this hypothesis and allows the architecture of these upland trees to be reconstructed in detail. The assemblage includes several hundred calcareously permineralized stumps, trunks, and branches, and represents the remains of shallowly rooted cordaitalean trees that were ≤48.5 m high when mature. The fossils occur in alluvial conglomerates that constitute a 10-km-diameter outlier on the margins of the paleoequatorial Variscan foreland. The paleogeographic setting together with plant taphonomic inferences strongly indicate that these giant trees were transported from nearby upland alluvial plains and deposited in an elevated intermontane basin. This interpretation is supported by analysis of rootstock morphology, which implies tree growth in thin soils consistent with an alluvial gravel substrate. This improved understanding of Pennsylvanian upland forests has important implications for geochemical modeling of the global carbon cycle.