Integrative Natural History of Minnesota's North Shore, Part 10: Examining Lester's Mouth | Duluth
This shot complements the Part 9 photo. It's taken in just about the same place, at the southern end of Kitchi Gammi Park. But now we've rotated 180 degrees and are facing west-southwestward.
From our low perch on this shoreline outcrop of late-Mesoproterozoic North Shore Volcanic Group basalt, we can see the Duluth skyline, as well as the heights beyond. They are composed of Duluth Complex gabbros and anorthosites, and they, too, are Midcontinent Rift rocks dating to the Lower Mesoproterozoic.
Much closer at hand, at center right, is the mouth of the Lester River. This Lake Superior tributary has its headwaters a little less than 20 mi (32 km) to the north. On this particular day, there was no sandbar spanning the mouth.
In the foreground, we see more of the glacial-abrasion effects discussed in Part 9. To my eye at least, there are some ice-sculpted forms here that are or at least suggest roche moutonnée structures.
Also known as sheepbacks and whalebacks, these glacial landforms are good indicators of the direction of ice-sheet flow. Their tapered, shallowly sloped sides point upstream; their higher, more steeply sloped portions point downstream.
In my experience, roche moutonnée features, including these, are rarely perfectly shaped, though I have one pretty good candidate for that distinction. My rule of thumb is that if you're not sure if a roche is really moutonnée, see if its orientation matches other directional clues nearby. If it does, one might as well relax and just call it that.
And these lumps of streamlined basalt do conform to the striations (glacial grooves) that will be shown at closer range in photos to follow.
To see the other photos and descriptions of this series, visit
my Integrative Natural History of Minnesota's North Shore album.
Integrative Natural History of Minnesota's North Shore, Part 10: Examining Lester's Mouth | Duluth
This shot complements the Part 9 photo. It's taken in just about the same place, at the southern end of Kitchi Gammi Park. But now we've rotated 180 degrees and are facing west-southwestward.
From our low perch on this shoreline outcrop of late-Mesoproterozoic North Shore Volcanic Group basalt, we can see the Duluth skyline, as well as the heights beyond. They are composed of Duluth Complex gabbros and anorthosites, and they, too, are Midcontinent Rift rocks dating to the Lower Mesoproterozoic.
Much closer at hand, at center right, is the mouth of the Lester River. This Lake Superior tributary has its headwaters a little less than 20 mi (32 km) to the north. On this particular day, there was no sandbar spanning the mouth.
In the foreground, we see more of the glacial-abrasion effects discussed in Part 9. To my eye at least, there are some ice-sculpted forms here that are or at least suggest roche moutonnée structures.
Also known as sheepbacks and whalebacks, these glacial landforms are good indicators of the direction of ice-sheet flow. Their tapered, shallowly sloped sides point upstream; their higher, more steeply sloped portions point downstream.
In my experience, roche moutonnée features, including these, are rarely perfectly shaped, though I have one pretty good candidate for that distinction. My rule of thumb is that if you're not sure if a roche is really moutonnée, see if its orientation matches other directional clues nearby. If it does, one might as well relax and just call it that.
And these lumps of streamlined basalt do conform to the striations (glacial grooves) that will be shown at closer range in photos to follow.
To see the other photos and descriptions of this series, visit
my Integrative Natural History of Minnesota's North Shore album.