Flat spots in the valleys

[Scroggy]

While poring over Boyd's excellent maps, I noticed something I don't quite know how to interpret. Looking, at, say, the Mullica watershed above Batsto, LIDAR shows that the terrain is entirely filled with braided channels. However, on the lower Mechseatauxin below Wescoat Bogs, the braiding marks just sort of disappear and don't reappear until reaching the old bogs just above Batsto. The west side of the Oswego between Martha and Jenkins Neck has a similar appearance.

Is this just an artifact of the LIDAR data, with high surface water hiding the braiding marks? Did the braided channels fill in with sediment in the backwaters behind beaver dams? Curious as to what's going on here.

 

[Boyd]

Links to specific locations on the imagery would be helpful.

 

[bobpbx]

While poring over Boyd's excellent maps, I noticed something I don't quite know how to interpret. Looking, at, say, the Mullica watershed above Batsto, LIDAR shows that the terrain is entirely filled with braided channels. However, on the lower Mechseatauxin below Wescoat Bogs, the braiding marks just sort of disappear and don't reappear until reaching the old bogs just above Batsto. The west side of the Oswego between Martha and Jenkins Neck has a similar appearance.

Is this just an artifact of the LIDAR data, with high surface water hiding the braiding marks? Did the braided channels fill in with sediment in the backwaters behind beaver dams? Curious as to what's going on here.

I know what you are speaking of. I think they originated during the last glacial period due to glacial runoff. I'm about 50% sure of that, but I'll look up some stuff on it. We have a member on here that has ideas on that, maybe he'll chime in.

With that said, it may also be related to the periods the area was underwater. Many times.

 

[bobpbx]

This source is kind of overwhelming, but it has great stuff in it, and it references related maps in the upper right corner.

Here's a comment in the beginning: "Stewart Farrell of Stockton State College contributed to our understanding of the head of the Mullica River basin near Atsion."

https://pubs.usgs.gov/imap/2540d/report.pdf

 

[Spung-Man]

This paleohydrology is from the cold, dry, and windy Pleistocene. We are looking at a Pleistocene alluvial fan where snowmelt flooding occurred over frozen ground. Stewart et al. (1985) first noticed the stream pattern, which Marsh (1985) thought might be flooding across South Jersey caused by ice-damming of the Delaware River. Newell and Wyckoff (1991) found no exotic sediments from ice-dam flooding off the Delaware and suggested the channel form was a signature of past permafrost thaw dynamics. We (e.g., French & Demitroff 2012, Demitroff et al., 2019) agree and have covered the braided channels more in depth should you wish to deep dive.

S-M

1. Farrell SC, Gagnon K, Malinousky T, Columbo R, Mujica K, Mitrocsak J, Cozzi A, Van Woudenberg E, Weisbecker T. 1985. Pleistocene? braided stream deposits in the Atsion quadrangle area, northwestern Atlantic County, New Jersey. In Talkington RW (ed.). Geological Investigations of the Coastal Plain of Southern New Jersey, Part 1: Field Guide. 2nd Annual Meeting of the Geological Association of New Jersey, Geology Program, Pomona, NJ: Stockton State College. A-1 to A-12.
2. Marsh ER. 1985. A Pleistocene lake in central New Jersey. In Talkington RW (ed.). 1985. Geological Investigations of the Coastal Plain of Southern New Jersey, Part 1: Field Guide. 2nd Annual Meeting of the Geological Association of New Jersey, Geology Program. Pomona, NJ: Stockton State College. A-14 to A-28.
3. Newell WL, Wyckoff JS. 1992. Paleohydrology of four watersheds in the New Jersey Coastal Plain. In Gohn GS. (ed.), Proceedings of the 1988 U.S. Geological Survey Workshop on the Geology and Geohydrology of the Atlantic Coastal Plain. United States Geological Survey, Circular 1059, Washington, DC, pp. 23–28.
4. French HM, Demitroff M. 2012. Late-Pleistocene paleohydrology, eolian activity and frozen ground, New Jersey Pine Barrens, eastern USA. Netherlands Journal of Geosciences. 91, 1/2: 25–35.
5. Demitroff M, Wolfe SA, Woronko B, Chemieloska D, Cicali M. 2019. Late Pleistocene Ice-Marginal Dune Fields on the Atlantic Coastal Plain, New Jersey Pine Barrens, USA. T4. Eolian Processes and Landscape Evolution (Posters), Geological Society of America Abstracts with Programs. Vol. 51, No. 5, doi: 10.1130/abs/2019AM-336443 https://www.researchgate.net/public...tic_Coastal_Plain_New_Jersey_Pine_Barrens_USA

 

[bobpbx]

So Mark, this statement from that document is in agreement with your last post?

"The youngest colluvial-alluvial deposits ( Oca 1 b) are mapped only in the headwaters of the Mullica River in southern New Jersey. The surface of these deposits is engraved with a pattern of braided stream channels. Exposures reveal bedforms and sorting that can only be deposited by stream discharges that were orders of magnitude beyond the scale of the largest modern runoff events. The braided-channel surfaces are devoid of frozen ground features as are other postglacial Holocene deposits. Thus, this deposit is interpreted as the product of extreme discharge from large storms combined with meltwater from the final thaw of permafrost (Newell and Wyckoff, 1992). Following permafrost melt and discharge, the surficial aquifer system was reestablished and contributed ground-water discharge directly to streams."

 

[Spung-Man]

Wow, It's nice to see dear Wayne Newell getting due recognition for seeing a strong periglacial (cold, non-glacial) signature here. He spent summers on his grandfather's farm along Stop-the-Jade east of the PPA's Bishop House, working the family cranberry bogs toward New Lisbon. I argue that our ice age inheritance is what makes the Pine Barrens unique. The actual land-surface processes at work are even more complex than what we knew back in 2000 when Newell and all wrote that. Long retired from the USGS, Newell remains a close friend and mentor. The first to recognize frozen ground here was my old professor from the old ag school, Cook College., Peter Wolfe. Wolfe grew up in Nesco and we talked for hours about spungs between classes.

1. Wolfe PE. 1943. Soil and Subsequent Topography. Journal of Geology. 51: 204–211.
2. Wolfe PE. 1953. Periglacial frost-thaw basins in New Jersey. The Journal of Geology. 61, 2: 133–141.

 

[pinelandpaddler]

Fun stuff.

"Exposures reveal bedforms and sorting that can only be deposited by stream discharges that were orders of magnitude beyond the scale of the largest modern runoff events."

This has been my understanding as well. Mark, do you see it this way?

 

[bobpbx]

By the way Mark, I like that term in the one paper: "underfit" stream channels. It really does fit for our streams.

 

[Scroggy]

The pointer is centered in one of the flat zones here: https://boydsmaps.com/#15.00/39.685276/-74.687625/mbx3dmidatl/0.00/0.00

Notice how on LIDAR all the braiding marks just disappear in this area.

Back upstream of Wescoat a bit and near the active bogs, there are also some sort of interesting raised features cutting perpendicular to the general direction of streamflow, diverting water behind them towards the Sleepy: https://boydsmaps.com/#16.00/39.702820/-74.708589/mbx3dmidatl/0.00/0.00

The stream diverted north by the large one centered there has flooded the old road connecting Dave's Road with the Atsion-Batsto road. The features show up on the 1930 aerials, so I initially took them to be the beaver dam complexes mentioned in an old account Mark quoted in a thread a few years ago. On close examination of the old aerials, a sand road branched off Dave's Road to run to the north end of the feature under the pointer, so perhaps the remains of a beaver dam were rebuilt into a causeway to carry it. It's not very clear what happened to the road at the south end.

 

[Spung-Man]

Scroggy,

You make wonderful observations. The remarkable preservation of undisturbed terrain and its degradation is easily seen on lidar imagery. Dating and characterizing these sediments might make for an interesting story in light of new information. How do you account for the flat spots?

'Paddler,

The paleohydrology story is complex.

S-M

 

[Scroggy]

I was hoping you knew!

Three hypotheses, in (IMO) increasing order of probability:

1. These areas were never channeled. Seems very unlikely: the flat area isn't raised above the area carved by meltwater and there's no trace of other features that would have sheltered it from runoff.
2. They were channeled, but the channels were subsequently infilled by sediment deposition. You can see there seems to be less relief to the channels as you approach the border of the "flat spot" and a few raised mounds that were presumably cut around still show deeper into the flat spot. The area behind the beaver/causeway features near the active bogs also looks pretty flat, so maybe beaver dams trapped organics and a bit of mineral sediment and the area filled in. (On reflection, I think I'm subconsciously channeling Walter & Merritts here.) The problem with this is that there's no trace of those elevated features cutting perpendicularly across the channels in the LIDAR view of the big flat spot. If this was historically covered with enormous beaver impoundments, I'd expect some trace of that.
3. This is all a LIDAR artifact. I don't know how it works, but I know the LIDAR point cloud has to be algorithmically processed and sorted into returns from vegetation, land, and water before the sorted points are joined into the surface we view. My suspicion is that in these areas, the water table is very close to the surface or just above it at low-lying points, and the algorithm is breaking down trying to properly assign points bounced off of shallow surface water pools, sand, and white-cedar root hummocks. The braiding is probably there in the flat spot, just mostly submerged.

 

[Spung-Man]

Oh my, channeling Robert & Dorothy's legacy is quite a feat. Assuredly, both remain milling about. Again, it would help if we knew the chronostratigraphy and morphoscopy of the sediments. Send me an email as the conversation is going beyond what is published.

 

[pinelandpaddler]

I was hoping you knew!

Three hypotheses, in (IMO) increasing order of probability:

1. These areas were never channeled. Seems very unlikely: the flat area isn't raised above the area carved by meltwater and there's no trace of other features that would have sheltered it from runoff.
2. They were channeled, but the channels were subsequently infilled by sediment deposition. You can see there seems to be less relief to the channels as you approach the border of the "flat spot" and a few raised mounds that were presumably cut around still show deeper into the flat spot. The area behind the beaver/causeway features near the active bogs also looks pretty flat, so maybe beaver dams trapped organics and a bit of mineral sediment and the area filled in. (On reflection, I think I'm subconsciously channeling Walter & Merritts here.) The problem with this is that there's no trace of those elevated features cutting perpendicularly across the channels in the LIDAR view of the big flat spot. If this was historically covered with enormous beaver impoundments, I'd expect some trace of that.
3. This is all a LIDAR artifact. I don't know how it works, but I know the LIDAR point cloud has to be algorithmically processed and sorted into returns from vegetation, land, and water before the sorted points are joined into the surface we view. My suspicion is that in these areas, the water table is very close to the surface or just above it at low-lying points, and the algorithm is breaking down trying to properly assign points bounced off of shallow surface water pools, sand, and white-cedar root hummocks. The braiding is probably there in the flat spot, just mostly submerged.

I think we can confidently rule out #3. The area in question is covered in cedar swamp that is demarcated by the braided area.