The Magnificent Morton Gneiss, Part 10: Two Glances at the Archean Eon | Morton, Minnesota, USA

The best way to appreciate this photo couplet is to first refer to Part 9 of this set, which provides topographic and lidar maps of this locale as well as its description.

The upper shot was taken facing eastward, along Route 71 just a few yd/m north of the entrance to the access drive leading into the gas-station-and-motel complex at the corner of 71 and Minnesota 19.

The lower image faces northeastward. It was taken from the upper portion of the access drive cited just above.

Now that I've presented a succession of photos of the Morton Gneiss in architectural applications like this one, it's a nice change of pace to show the amazingly ancient migmatite in its native habitat.

The easily overlooked hillock shown here is a combination of rocky bald spots and patches of vegetation. It came into being when it was exposed, scoured, and sculpted by the surging waters of River Warren, the drainageway for the immense proglacial Lake Agassiz that formed late in the Pleistocene epoch.

If you wander around the top of this erosional knob—it is now the Morton Outcrops Scientific and Natural Area—you'll find remnants of old quarrying operations as well as "potholes" ground into the hard gneiss by loose stones caught in eddies of the River Warren megaflood.

Not surprisingly, in many places in the preserve the Morton is either mantled in lithophytic plant communities or is heavily weathered, and the details of its complicated composition are difficult to impossible to decipher. Luckily, there is one roadcut on the side of the hillock, situated very close to the lower image's location, where we can see some less altered faces. I'll be showing that in the next few pics of this set.

In the meantime, keep in mind that the unprepossessing rock outcrops you see here originated as early as 3.524 Ga ago, and not in one of the cratons of Laurentia (ancestral North America), but rather as part of the enigmatic Minnesota River Valley Terrane. That peripatetic section of the Earth's crust dates back at least as far as the Paleoarchean era. Where exactly it came from, we're not yet sure, but it merged with the Superior Craton at about 2.68 Ga ago.

To see the other photos and descriptions in this set, visit my Magnificent Morton Gneiss album.

Taken in: United States / Minnesota / Morton / Morton (show map)

Taken on: December 27, 2024

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