Integrative Natural History of Minnesota's North Shore, Part 3: At the Crest of a Dune | Duluth
(Updated August 15, 2024)
We're still on the great baymouth bar known as Minnesota Point, on the grounds of the Park Point recreational facility. And we're facing northward, toward Lake Superior.
To follow along with what I'm about to say here, it's best to review what I covered in Part 2 of this series. If you haven't looked at it already, I suggest that you do.
In that description, I noted that Minnesota Point has a spine of crescentic dunes. The true shapes and disposition of these aeolian (wind-generated) landforms can only be clearly comprehended using lidar imagery—such as that available on the USGS National Map. And as I also mentioned, crescentic dunes come in two different varieties. Which is the dominant form on this baymouth bar? I'm not completely sure, but here's my thinking so far.
- In the barchan type, the snout faces into the prevailing breeze; the horns point downwind. Barchans mostly form in places where the wind direction is quite unvarying, where there's little or no vegetation, and where the supply of sand is adequate but not abundant. And they often coalesce into long ridges.
- With the contrasting parabolic type, it's the snout that faces downwind and the horns up. Parabolics characteristically form in places with a lot of sand—but sand that's already well fixed by vegetation. Eventually, however, the wind succeeds in excavating blowouts wherever it can. The result is scooped-out central sections and vegetated ridges (the horns) on the sides.
So what it all boils down to on Minnesota Point are these questions:
- What is the prevailing wind direction? In this part of the world, it's predominantly westerly, or from the west and heading eastward. But often along coastlines, local onshore winds (those moving from the water to the land) are also powerful shapers of dunes and beach ridges.
So who's the boss here? The dunes are oriented on a northeast-southwest axis. This seems to be nicely aligned with onshore air flow. On the other hand, the westerly regional prevailing winds would actually have to be southwesterly to fit the dune directions. And when sand does migrate where plant roots are lacking, it moves southwestward, apparently pushed that way by the lake breezes. Such areas of sand migration can be seen at Park Point on Google Earth.
- Did these crescentic dunes form before the vegetation covered them, or after? Those portions of Minnesota Point not altered by urban and residential construction are in modern times mantled with plant growth. But it's possible the dunes did form rather quickly, before substantial plant colonization. And the fact that they do coalesce into a linear beach ridge that extends for several mi / km is very, very barchan-ish.
Accordingly, I'm going with the barchan hypothesis, at least for now. If anyone out there can confirm or obliterate it, please do. Or if you have any other insights that would help at all, let me know.
- - - - -
And now it's time to focus on this particular photo. It shows a small section of one of those crescentic dunes—whatever type it really is. Its crest is about 20 ft (6.1 m) above mean lake level, and 6-8 ft (1.8-2.4 m) above the spot I took this photo. And it is indeed home to quite a diverse array of woody and herbaceous plant species.
Of course, the most conspicuous members of this hardy plant community are the trees: what seem to be a mixture of Red and Eastern White Pines (Pinus resinosa and P. strobus) and white-barked Paper Birches (Betula papyrifera). The last-named are just beginning to leaf out. Spring comes slowly in these northern parts.
Also beginning to emerge are the light-green stems of what I gather is that great hero of shoreline stabilization, Marram Grass (Ammophila breviligulata, also called American Beach Grass).
As it so happens, in terms of its native distribution Minnesota Point is this species' westernmost outpost. It has the amazing ability to grow in pure sand, thanks to its mutualistic relationship with root-dwelling fungi. This mycorrhizal association is found in many other plant species, too, but in this case it allows the grass to obtain nutrients in a resource-starved soil environment.
Because Marram spreads by aggressive rhizomes— stems growing horizontally underground—it creates clonal colonies that -perform the crucial job of anchoring otherwise active dunes in place.
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 3: At the Crest of a Dune | Duluth
(Updated August 15, 2024)
We're still on the great baymouth bar known as Minnesota Point, on the grounds of the Park Point recreational facility. And we're facing northward, toward Lake Superior.
To follow along with what I'm about to say here, it's best to review what I covered in Part 2 of this series. If you haven't looked at it already, I suggest that you do.
In that description, I noted that Minnesota Point has a spine of crescentic dunes. The true shapes and disposition of these aeolian (wind-generated) landforms can only be clearly comprehended using lidar imagery—such as that available on the USGS National Map. And as I also mentioned, crescentic dunes come in two different varieties. Which is the dominant form on this baymouth bar? I'm not completely sure, but here's my thinking so far.
- In the barchan type, the snout faces into the prevailing breeze; the horns point downwind. Barchans mostly form in places where the wind direction is quite unvarying, where there's little or no vegetation, and where the supply of sand is adequate but not abundant. And they often coalesce into long ridges.
- With the contrasting parabolic type, it's the snout that faces downwind and the horns up. Parabolics characteristically form in places with a lot of sand—but sand that's already well fixed by vegetation. Eventually, however, the wind succeeds in excavating blowouts wherever it can. The result is scooped-out central sections and vegetated ridges (the horns) on the sides.
So what it all boils down to on Minnesota Point are these questions:
- What is the prevailing wind direction? In this part of the world, it's predominantly westerly, or from the west and heading eastward. But often along coastlines, local onshore winds (those moving from the water to the land) are also powerful shapers of dunes and beach ridges.
So who's the boss here? The dunes are oriented on a northeast-southwest axis. This seems to be nicely aligned with onshore air flow. On the other hand, the westerly regional prevailing winds would actually have to be southwesterly to fit the dune directions. And when sand does migrate where plant roots are lacking, it moves southwestward, apparently pushed that way by the lake breezes. Such areas of sand migration can be seen at Park Point on Google Earth.
- Did these crescentic dunes form before the vegetation covered them, or after? Those portions of Minnesota Point not altered by urban and residential construction are in modern times mantled with plant growth. But it's possible the dunes did form rather quickly, before substantial plant colonization. And the fact that they do coalesce into a linear beach ridge that extends for several mi / km is very, very barchan-ish.
Accordingly, I'm going with the barchan hypothesis, at least for now. If anyone out there can confirm or obliterate it, please do. Or if you have any other insights that would help at all, let me know.
- - - - -
And now it's time to focus on this particular photo. It shows a small section of one of those crescentic dunes—whatever type it really is. Its crest is about 20 ft (6.1 m) above mean lake level, and 6-8 ft (1.8-2.4 m) above the spot I took this photo. And it is indeed home to quite a diverse array of woody and herbaceous plant species.
Of course, the most conspicuous members of this hardy plant community are the trees: what seem to be a mixture of Red and Eastern White Pines (Pinus resinosa and P. strobus) and white-barked Paper Birches (Betula papyrifera). The last-named are just beginning to leaf out. Spring comes slowly in these northern parts.
Also beginning to emerge are the light-green stems of what I gather is that great hero of shoreline stabilization, Marram Grass (Ammophila breviligulata, also called American Beach Grass).
As it so happens, in terms of its native distribution Minnesota Point is this species' westernmost outpost. It has the amazing ability to grow in pure sand, thanks to its mutualistic relationship with root-dwelling fungi. This mycorrhizal association is found in many other plant species, too, but in this case it allows the grass to obtain nutrients in a resource-starved soil environment.
Because Marram spreads by aggressive rhizomes— stems growing horizontally underground—it creates clonal colonies that -perform the crucial job of anchoring otherwise active dunes in place.
To see the other photos and descriptions of this series, visit
my Integrative Natural History of Minnesota's North Shore album.