Integrative Natural History of Minnesota's North Shore, Part GPSP-A: Through the Mist and Spray | Grand Portage State Park
(Updated on July 26,2025)
Looking north-northwest toward the High Falls of the Pigeon River.
In this region, this stream marks the boundary of the United States and Canada. Here it spills over a northeast-southwest-oriented dike of Mesoproterozoic diabase, a dark wall of highly resistant mafic igneous rock. Then it rushes down a canyon deeply incised into much softer shale, siltstone, and graywacke of the Paleoproterozoic Rove Formation. These sedimentary strata can be dimly discerned through the mist at lower left and right.
The tremendous visceral impact of the surging water and splashing spray is matched by the spectacle of geologic time presented at the High Falls. The Rove beds were deposited between 1.836 and 1.78 Ga ago in the Animikie Basin, a downwarped section of the Earth's crust linked to the Penokean mountain-building episode.
Then, at about 1.1 Ga and long after the lofty Penokean Mountains had been beveled flat by erosion, the magma that would become the diabase was injected into a large vertical fissure in the Rove. This occurred during the cataclysmic episode that created the Midcontinent Rift (MCR) and almost tore North America apart. While specialists still debate the details of its origin, the MCR formed in a time of crustal extension and thinning. Its massive outpourings of lava may also be linked to mantle-plume activity.
To see the other photos and descriptions of this series, visit my
Integrative Natural History of Minnesota's North Shore, Part GPSP-A: Through the Mist and Spray | Grand Portage State Park
(Updated on July 26,2025)
Looking north-northwest toward the High Falls of the Pigeon River.
In this region, this stream marks the boundary of the United States and Canada. Here it spills over a northeast-southwest-oriented dike of Mesoproterozoic diabase, a dark wall of highly resistant mafic igneous rock. Then it rushes down a canyon deeply incised into much softer shale, siltstone, and graywacke of the Paleoproterozoic Rove Formation. These sedimentary strata can be dimly discerned through the mist at lower left and right.
The tremendous visceral impact of the surging water and splashing spray is matched by the spectacle of geologic time presented at the High Falls. The Rove beds were deposited between 1.836 and 1.78 Ga ago in the Animikie Basin, a downwarped section of the Earth's crust linked to the Penokean mountain-building episode.
Then, at about 1.1 Ga and long after the lofty Penokean Mountains had been beveled flat by erosion, the magma that would become the diabase was injected into a large vertical fissure in the Rove. This occurred during the cataclysmic episode that created the Midcontinent Rift (MCR) and almost tore North America apart. While specialists still debate the details of its origin, the MCR formed in a time of crustal extension and thinning. Its massive outpourings of lava may also be linked to mantle-plume activity.
To see the other photos and descriptions of this series, visit my