NPS Photo / Mike Plummer-Steen
Middle Precambrian Rove Formation 1,850-1,750 Million YBP (years before present)
When you arrive to Grand Portage National Monument and get out of your vehicle, your eyes will likely be drawn up to piles of broken rocks with a gray-green cast. Above the piles are cliffs in which you may be able to see horizontal layering; at your feet you may notice the layers slightly tilt towards the lake. Nearly two billion years old, these rocks are known as the Rove Formation. Beautiful and durable, masons have used the rock to trim the Heritage Center and build the tallest fireplace in the county inside. Reconstructed buildings in the historic depot also have fireplaces and foundations built from this stone as did the North West Company who completed the originals in 1789. On each side of the parking lot are dark near vertical rocks which stick out as walls because they are much harder than the Rove rocks between and are much harder for water and ice to break apart.
NPS Photo / Mike Plummer-Steen
Late Precambrian Midcontinent Rift 1,109-1,000 Million YBP
About 1,200 million years ago, fragments of continental crust pushed together by tectonic motion began to assemble the giant continent of Rodinia. Only 109 million years after forming, Rodinia began to break apart along a giant rupture that cut through northeastern Minnesota all the way to Kansas, the legendary Midcontinent Rift. A mantle plume or "hot spot" caused molten magma to rise into vertical cracks in the older graywacke and shale of the Rove Formation. The magma cooled in the cracks into a hard, tough rock known as diabase, the fillings are known as dikes. Diabase is Greek for "crossing over" as a chemically similar middle rock representing a change between coarser, deeper cooled gabbro rock and instantaneously cooled basalt lava at the surface. Diabase, thus, represents direct feeders for surface lava flows. The linear hills in Grand Portage, suggest that basalt lava flowed onto the surface through long fissures. Modern fissure eruptions are observed in Iceland a "hot spot" on the Mid-Atlantic ridge where North America has been rifting from Europe for about 280 million years from the break-up of the most recent supercontinent Pangaea.
Diabase dikes record two episodes of magma influx along the rupture in Rodinia. An older set of 50 small, low dikes 10-20 feet wide known as the Grand Portage dike swarm and a more massive younger set of at least 14 dikes referred to as the Pigeon River dike swarm. Pigeon River dikes are the youngest rocks in Grand Portage. Smaller Grand Portage dikes can be seen east and west of the Heritage Center. High hills in Grand Portage contain cores of large Pigeon River dikes which support Mts. Josephine, Rose, Maude and Sophie. The highest waterfalls in Minnesota, at Grand Portage State Park, flow over the Wauswaugoning Bay-Pigeon River dike. Age dating matches Pigeon River diabases to the Lutsen basalts suggesting the diabase feeders may have produced lavas 50 miles west of Grand Portage.