Decades of geological field mapping at Gros Morne showed that the Tablelands is a slab of ancient ocean floor and upper mantle rock, thrust onto the continent as an ancient ocean closed around 470 million years ago. It's one of the clearest visible pieces of evidence anywhere for how plate tectonics works.
Citation
Field mapping and interpretation by geologists Harold "Hank" Williams and Bob Stevens, primarily 1960s–1980s, with geoheritage significance documented by the International Union of Geological Sciences and Parks Canada.
Study overview
Newfoundland's west coast preserves an unusually complete cross-section of ancient oceanic crust and mantle rock, known as the Bay of Islands and Gros Morne ophiolite complex. Geologists mapped this rock in detail over several decades, working out how a slice of ocean floor ended up sitting on top of the continent rather than buried beneath it, as would normally be expected.
Methodology summary
The work relied on field mapping and rock and mineral analysis across the region, identifying the ultramafic peridotite that makes up the Tablelands as mantle-derived rock rather than ordinary crust, and tracing how it related to surrounding rock formations.
Key findings
- The Tablelands preserves a near-complete cross-section from ancient oceanic crust down toward the mantle, including a section of the Mohorovičić discontinuity — the actual boundary between crust and mantle.
- This rock was thrust onto the continental margin roughly 470 million years ago as an ancient ocean, the Iapetus Ocean, closed and the Appalachian mountain belt began forming.
- The mapping contributed directly to establishing plate tectonics as accepted scientific theory in the 20th century, and geologist John Tuzo Wilson used the Tablelands as supporting evidence in his own work on the theory.
Limitations
This isn't the result of one single study but of decades of accumulated fieldwork and geological consensus. The precise sequence and timing of how the rock was thrust onto the continent is still refined by ongoing research, and public explanations — including this one — necessarily simplify a genuinely complex geological history.
Practical meaning
When you walk the Tablelands interpretive trail, the barren orange-brown rock underfoot is functionally mantle material, not ordinary ground — which is also why almost nothing grows there.
Last updated: August 2, 2026 · Editorial team