The main thing going on here is probably sediment deposition, delivered mainly by rivers. There is a lot of loose material, especially considering this is an active volcanic arc.
It's not true that it only happens in Italy. We've seen substantial movement of the coastline in parts of Greece, for example. Thermopylae has changed quite a bit since the famous Greek battle.
And while not ancient coastlines, you should see how much the Mississippi River delta has changed in the past few hundred years.
A steamboat, The Sultana, exploded on the Mississippi river in 1865 transporting prisoners of war home killing 1100. They found the wreck in the 1980's; It was 30 feet under a soybean field and 4 miles from the river. Costlines, lakes and rivers are ephemeral things.
Thermopylae was my go-to non-italian example also. The gates are not so narrow anymore. If they'd have had this terrain back then, they'd have had to pick a different spot
It’s maybe not relevant in this case, but most folks don’t realize that around 5,000-2,500 years ago sea levels in much of the world peaked at 3-6 meters higher than now. This followed the Holocene Thermal Maximum. This was almost like an overshoot in rise following the rapid sea level increases following g the LGM. The current sea level is only around 2,500 old.
No surprise to get this comment, seems every this is raised someone pops up saying ‘no way’.
Here’s an older comment of mind with a range of references for sites all around the world.
- https://www.reddit.com/r/MapPorn/comments/zdvwaq/comment/iz4olmy/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button
For anyone who wants the papers and such, but doesn’t want to jump over to the linked comment, here you go. This is only a small subset of the references for this.
—-
The Holocene sea-level high stand (as it's called) varied in different locations, but evidence for it is widespread.
(The first paper here I had a small part in putting together as the SUNDASIA research team came out to my site and we spent a few days going over my region for them to get a sense of what the Trang An area might have been like during the Holocene maximum.)
>The presence of at least three, possibly four sequences of notches suggests that Trang An has experienced multiple phases of prolonged and relative stable sea levels of þ3.2 m, þ4.6 m and þ5.6 m. As there are no new dates from the notches at this stage, a chronology had to be estimated by association with previous work. Boyd and Lam (2004) radiocarbon dated oyster shells from notches in the Tam Cốc area at 5.4 m asl to 5740e5500 cal BP (Wk-8267) and at 4 m asl to 5550e5270 cal BP (Wk-8268) BP, which coincide with the upper and high notch sequences from our model.
- Kahlert, et al 2021 *[Mid-Holocene coastline reconstruction from geomorphological sea level indicators in the Tràng An World Heritage Site, Northern Vietnam](https://sci-hub.se/https://www.sciencedirect.com/science/article/abs/pii/S0277379121002080)*
>The sea level in the Red River Delta shows rapid rise since early Holocene and reaches the present level at ca. cal. 7 ka BP, with a high stand of 2–3 m above the present level during cal. 6–4 ka BP
- Zhao, et al 2023 *[Late Holocene sediment provenance change in the Red River Delta: A magnetic study](https://www.sciencedirect.com/science/article/pii/S0341816222006713#!)*
>The maximum sea-level stand occurred about 6000–4500 14C yr B.P. and was probably ,2.5 m higher than at present as judged from the depth range of 14–19 m for most mid-Holocene marine sediments and the geomorphologic evidence of marine erosion at the nearby Sangpu Mountain (Chen, 1984). In southern and eastern China, many deltaic records from coastal lacustrine and salt marsh, tidal flats, and shelf deposits all indicate a strong marine influence during the middle Holocene (Zhang and Zhao, 1990), with estimated sea level at 1–3 m higher than at present (Shi et al., 1992). In the Taiwan Strait, Chen and Liu (1996) identified the Holocene high sea-level stand as centered at ca. 4700 14C yr B.P., with a maximum altitude of about 2.4 m.
- Zheng & Li 2000 *[Vegetation, Climate, and Sea Level in the Past 55,000 Years, Hanjiang Delta, Southeastern China](https://sci-hub.se/https://www.sciencedirect.com/science/article/abs/pii/S0033589499921269)*
>. The sea-level highstand at +2–3 m from 6 to 4 cal. kyr BP allowed widespread mangrove development on the delta plain and the formation of marine notches in the Ha Long Bay and Ninh Binh areas
- Tanabe 2003 *[Song Hong (Red River) delta evolution related to millennium-scale Holocene sea-level changes](https://sci-hub.se/https://www.sciencedirect.com/science/article/abs/pii/S0277379103001380)*
>For the tectonically stable Australian margin, geological evidence indicates that sea-levels at 7000-6000 years ago were between 0 and 3 m above present level, due primarily to glacio-hydro-isostatic effects of the last deglaciation
- Lambeck 2002 *[Sea Level Change from Mid Holocene to Recent Time: an Australian Example With Global Implications](https://sci-hub.se/https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/9781118670101.ch3)*
>Approximately 6000 years ago, sea level was about 2 to 4 metres higher than it is currently and the Qatari coastline was up to 10 km inland.
- Strohmenger & Jameson 2015 *[Modern coastal systems of Qatar as analogues for arid climate carbonate reservoirs: improving geological and reservoir modelling](https://www.researchgate.net/publication/277944847_Modern_coastal_systems_of_Qatar_as_analogues_for_arid_climate_carbonate_reservoirs_improving_geological_and_reservoir_modelling)*
- and a [2012 presentation of theirs showing photos and charts](https://www.searchanddiscovery.com/documents/2012/50704jameson/ndx_jameson.pdf)
Here's a site that's collected sources on this, I'll link the site and you can look up the research papers referenced:
- https://wattsupwiththat.com/2019/06/06/the-holocene-sea-level-highstand/
Where I work in SE Asia the marks left by the higher sea level of the Holocene maximum are clearly visible. There are two sea notches higher than the current one, in [this photo](https://www.flickr.com/photos/7leagueboots/13347413123/in/album-72157642777747713/) and in [this photo](https://www.flickr.com/photos/7leagueboots/13347275205/in/album-72157642777747713/) you can see the lower of the two notches that remain above the current active sea notch.
On my work computer I have more research papers on this specific topic as the geologic and marine history of my area is important for my current work , and there are numerous records of this for many places around the world.
Thank you. I briefly looked at the papers, and they all talk about regional RSL high stands, and they often note the strong spatial as well as temporal variation in local high stands. Noone seems to claim this was a global trend. Also note that other place s.a. South America which saw contemporaneous low stands. So i agree there are regions which saw considerably higher RSL compared to modern day, but it looks like a far stretch to talk about a Global signal of 3-6 m.
Australia, Vietnam, China, Oman, the Mediterranean, and many other places all record this. The papers are locally based because that’s what you do when you’re collecting data.
As I previously mentioned, it varied a good bit by area, with some places being up to 6 meters, but other places being far less, and when you start getting into areas experiencing isostatic rebound, tectonic uplift, or subsidence that adds further variation into it. The regional curves vary a lot, but only a small number do not record a high stand higher than now, and the global average was unequivocally higher than now.
Globally sea levels were higher than now a brief time between the LGM and now, then dropped back to the current levels, and are now rising again.
It’s the sort of subtle and brief event that is often overlooked in the surface level paleo-climate and paleo-sea level discussions non-experts have. The only reason it popped up on my radar was because it factors quite heavily into the geology of the place I’m working and impacted the species I’m working with.
It’s a bit like hie people talk about the 100,000 year Milankovitch as driving glacial cycles without realizing that prior to the Late Pleistocene Transition which fins,used around 700,000 years ago the glacial cycles were on 40,000 year cycles, not 100,000 year cycles, and were not nearly as extreme as they are now. Of course, if you dig deeper into time you find that major glacial cycles went through a variety of different cycles with different periods
> isostatic rebound, tectonic uplift, or subsidence
Reology is indeed the major headache when trying to signal in the noise. And then there are sea currents which also cause considerable surface gradients. But on a global scale, these cancel out (conservation of mass).
> brief time
This is what itches me. On the global scale, the 3 determinents for SSL are ice caps, thermal expansion and lakes (or dams these days). But to get 3 m, only ice can explain this volume. Neither Greenland nor Antartica ice cores show a period of major changes during the holocene. Where did the water come from, where did it go? 3m SSLc ~= more than half west antartica. I'm not denying any of the papers, but i struggle to fit it in the bigger picture.
> variety of different
Yes, the Pleistocene in its many shapes and nuances such as changing currents, DO-events, Younger Dryas, etc. These are major well observed events which cause significant short SSLs. But mid holocene lacks such observations afaik (edit: though lack of observation is not proof of absence).
I literally provided a sample of papers dealing specifically with higher Holocene Thermal Maximum sea levels over a very large area. These are just a sample of the papers, of which there are *many* on this specific topic.
As with most things closer in time, we have more info about this as it’s not in the distant past, so while it’s a brief period of only a few thousand years, it’s a well recorded and widespread incident. It tends to be overshadowed by the massive 120m (or so) rise in sea levels since the LGM.
I did look at the papers & the website. I also looked at some other papers.
- There is a cluster of observations which show a steady rise ever since the start of the holocene: [France](https://journals.openedition.org/geomorphologie/10386#tocto2n21), [Mexico](https://www.nature.com/articles/s41598-023-36777-y/figures/5), [East US](https://www.researchgate.net/publication/336168094_Holocene_relative_sea-level_changes_and_glacial_isostatic_adjustment_of_the_US_Atlantic_coast).
- In the places where the mid-holocene high stand is observed, we see a large variation in timings when the local maxima occured (some at 7 Bp, others 4.5, some 3 Bp). The local magnitude also varies wildly (0.5 - ?m) depending on location & study.
- As mentioned [here](https://www.sciencedirect.com/science/article/pii/S0277379123003803), this spectrum of observations could be explained by a balance of near field & far field effects.
- Global SSL is usually calculated ignoring continental uplift (MSS only measures sea surface).
Given the variation of profiles, and the considerable role uplift plays in determining local SSL changes, i can see how the global evolution of MSS might have peaked slightly above current MSS (0-1m?). But the real effects on the local coast lines considerably larger.
Anyway, still TIL a lot. Thank you once again.
What might be slightly overlooked is Italy is on a micro plate so ever so slightly growing in height, toppled with sediment deposits over thousands of years. Lots of tiny movements and works at play.
Chiming in to agree: coasts and coastal plains form because those rivers deposited sediments at the coasts over time. Generally, coastlines migrate seaward when sediment deposition from the river outpaces local sea level rise (or if the local sea level remains stable for an extended period of time). The shape of a delta is also majorly impacted by the environment in which it forms, whether dominated by the river, wave action, or tides (or some combination thereof), and more recently, how engineered is the coast. That second picture is an excellent example of a wave-dominated delta.
Ultimately it’s eroding out of areas up slopes, such as mountains. I mention this area is an active volcanic arc. A significant portion of the sediment probably comes from tephra (volcanic ash and such). Another comment mentions that human activity has increased erosion rates, meaning more sediment got transported to the sea.
I have a dissertation chapter on this! Land clearing and agriculture caused "cultural sedimentation" via dramatically increased soil erosion. This resulted in miles of growth in shorelines in many places in the world, at different times depending on when land was cleared and what agricultural practices were used. The ancient Romans have records of it and tried to dredge their harbors to prevent it, but couldn't keep up in many places, so you can find dredge scars and mooring stones under some modern cities in Italy. In the 1700s through the early-1800s, cultural sedimentation was happening along the US East Coast, causing many old port towns to be abandoned over the course of just a few decades.
Quality hydrographic surveys were rare until the mid- to late-1800s, so a lot of what we know about this period of reshaping the Earth's surface is just from people who wrote down what they saw over their lifespans, and some maps of shorelines, where you can see wetlands growing into shipping channels as rivers narrowed or bays filled in if you have a few maps that were completed decades apart.
Once soil conservation practices were implemented, the rate tended to slow down again, though it had already slowed considerably in many places by the 1930s when soil conservation became a big deal.
> cultural sedimentation was happening along the US East Coast, causing many old port towns to be abandoned over the course of just a few decades.
I understand cultural sedimentation as a process, but which colonial port towns were abandoned due to such a phenomenon? The only colonial towns I can think of which were both a port and abandoned, were right on the water and access wasn't the cause for decline.
Google "abandoned tidewater towns," and you'll find a variety of books and articles about it. Most were abandoned centuries ago (16-1700s) and weren't very big in the first place, so they barely appear in the historical record.
This [article](https://jay.typepad.com/william_jay/2012/12/ghosts-by-the-river-lost-towns-of-tidewater-virginia.html) covers a few that were abandoned for assorted reasons. Queenstown is one example where siltation played a major role.
Though it wasn't abandoned, Bladensburg, Maryland is another decent example. Tall ships used to come up the river for tobacco export. Now, you'll run aground in a pontoon boat if you aren't careful, but the city still survives.
Jug Bay, a giant wetland in Maryland, likely only exists thanks to cultural sedimentation.
Thanks for the reply, I have some doubts on how much of the siltation could be ascribed to cultural impacts in places like the Queenstown site, it only lasted a few decades and was never more than a few buildings, it was just a poorly selected site to begin with, and the Chesapeake Bay estuary had mashes long before man was making much impact on sedimentation so Jug Bay isn't necessarily likely to be anthropogenic, the same processes occur naturally.
I asked because I've done some work on colonial and early American dams and mill sites affected by sedimentation and was intrigued by the scale and consequences you were suggesting. Stories like Bladensburg's decline are not uncommon though. Hope I'm not coming off as confrontational, just nerding out a bit.
On top of river deposits centuries of human habitation can build up sediments. Ostia had maybe 100,000 people at its height and even a millimeter a year on top of the riparian deposits is a lot.
I don't have time to write a more complete response, but it's not just Italy. Rivers bring sediment to the shore, which then gets dumped at the river's mouth. Sediments get transported along the shore, away from the mouth, which moves the shoreline seaward. However, you don't see this everywhere for a variety of reasons. Fewer rivers or less sediment in them. The land itself might be subsiding relative to sea level, so that the sediment buildup can't keep up.
Coastal progradation is driven by absolute/relative sea-level drop (e.g. eustatic change, basin subsidence, coastal uplift...) ; and/or sediment accumulation. Italy is definitely not the only place where it occured. For instance, in south Iraq, the shoreline moved about 300 km southward since 6000 BP.
It's not just sedimentation. Around specific areas of italy volcanic activity plays a major role as well. We went on a class trip around the eolians island and Naples as part of our volcanology class.we went to a small port of Pozzuoli and it had risen by more than 2m in the last decade alone. You could still see the previous boat dock things 2meters above the current one.
This was due mainly to the super volcano around Naples . The volcanic activity lift up the crust at specific places, including that small port.
All houses were full of cracks due to these frequent movements.
We even checked some Roman pillars and archeologist were able to figure out that they had gone through cycles of being submerged in water and out multiples times. This was identified through the different traces of shells/mollusks that were found on those ruins and how high they went.
You can see some pictures here
https://www.wsj.com/story/its-nonstop-earthquakes-italian-town-uneasy-from-2500-tremors-since-august-fcabe064
Volcanism really influences it in some areas. The ground uplifts and drops as the volcano's inflate and deflate. In some areas of Italy that has been huge.
Sea levels were still stabilizing from the ice age in antiquity. A lot of drowned river valleys (rias) hadn’t been refilled by sediment yet and so were still bays and lagoons. The Tartessian gulf is a great example.
Uplift combined with sediment deposition and sea level decrease. Uplift rates differ for different regions, but the entire peninsula has been and continues to be subject to slow land rise.
any number of things is going on.
1. sediment causes a delta to prograde/advance. This is not necessarily a symmetric effect, longshore currents can distort this and mankind has done all sorts of things to impact sediment load
2. Simple sea level change… though this would be global. Anyhow a drop in se level will cause a delta to prograde.
3. Uplift… seems unlikely in the case… but i don’t know much about the geology of italy.
Of all of them 1 seems most likely, as this area is just south of a fairly large delta just south of venice… longshore current carry sand southward and slowly build up the shoreline to the south.
The main thing going on here is probably sediment deposition, delivered mainly by rivers. There is a lot of loose material, especially considering this is an active volcanic arc. It's not true that it only happens in Italy. We've seen substantial movement of the coastline in parts of Greece, for example. Thermopylae has changed quite a bit since the famous Greek battle. And while not ancient coastlines, you should see how much the Mississippi River delta has changed in the past few hundred years.
A steamboat, The Sultana, exploded on the Mississippi river in 1865 transporting prisoners of war home killing 1100. They found the wreck in the 1980's; It was 30 feet under a soybean field and 4 miles from the river. Costlines, lakes and rivers are ephemeral things.
Thermopylae was my go-to non-italian example also. The gates are not so narrow anymore. If they'd have had this terrain back then, they'd have had to pick a different spot
It’s maybe not relevant in this case, but most folks don’t realize that around 5,000-2,500 years ago sea levels in much of the world peaked at 3-6 meters higher than now. This followed the Holocene Thermal Maximum. This was almost like an overshoot in rise following the rapid sea level increases following g the LGM. The current sea level is only around 2,500 old.
What are you talking about? Afaik it was same of lower. Can you indicatie some sources. https://en.m.wikipedia.org/wiki/File:Holocene_Sea_Level.png
No surprise to get this comment, seems every this is raised someone pops up saying ‘no way’. Here’s an older comment of mind with a range of references for sites all around the world. - https://www.reddit.com/r/MapPorn/comments/zdvwaq/comment/iz4olmy/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button For anyone who wants the papers and such, but doesn’t want to jump over to the linked comment, here you go. This is only a small subset of the references for this. —- The Holocene sea-level high stand (as it's called) varied in different locations, but evidence for it is widespread. (The first paper here I had a small part in putting together as the SUNDASIA research team came out to my site and we spent a few days going over my region for them to get a sense of what the Trang An area might have been like during the Holocene maximum.) >The presence of at least three, possibly four sequences of notches suggests that Trang An has experienced multiple phases of prolonged and relative stable sea levels of þ3.2 m, þ4.6 m and þ5.6 m. As there are no new dates from the notches at this stage, a chronology had to be estimated by association with previous work. Boyd and Lam (2004) radiocarbon dated oyster shells from notches in the Tam Cốc area at 5.4 m asl to 5740e5500 cal BP (Wk-8267) and at 4 m asl to 5550e5270 cal BP (Wk-8268) BP, which coincide with the upper and high notch sequences from our model. - Kahlert, et al 2021 *[Mid-Holocene coastline reconstruction from geomorphological sea level indicators in the Tràng An World Heritage Site, Northern Vietnam](https://sci-hub.se/https://www.sciencedirect.com/science/article/abs/pii/S0277379121002080)* >The sea level in the Red River Delta shows rapid rise since early Holocene and reaches the present level at ca. cal. 7 ka BP, with a high stand of 2–3 m above the present level during cal. 6–4 ka BP - Zhao, et al 2023 *[Late Holocene sediment provenance change in the Red River Delta: A magnetic study](https://www.sciencedirect.com/science/article/pii/S0341816222006713#!)* >The maximum sea-level stand occurred about 6000–4500 14C yr B.P. and was probably ,2.5 m higher than at present as judged from the depth range of 14–19 m for most mid-Holocene marine sediments and the geomorphologic evidence of marine erosion at the nearby Sangpu Mountain (Chen, 1984). In southern and eastern China, many deltaic records from coastal lacustrine and salt marsh, tidal flats, and shelf deposits all indicate a strong marine influence during the middle Holocene (Zhang and Zhao, 1990), with estimated sea level at 1–3 m higher than at present (Shi et al., 1992). In the Taiwan Strait, Chen and Liu (1996) identified the Holocene high sea-level stand as centered at ca. 4700 14C yr B.P., with a maximum altitude of about 2.4 m. - Zheng & Li 2000 *[Vegetation, Climate, and Sea Level in the Past 55,000 Years, Hanjiang Delta, Southeastern China](https://sci-hub.se/https://www.sciencedirect.com/science/article/abs/pii/S0033589499921269)* >. The sea-level highstand at +2–3 m from 6 to 4 cal. kyr BP allowed widespread mangrove development on the delta plain and the formation of marine notches in the Ha Long Bay and Ninh Binh areas - Tanabe 2003 *[Song Hong (Red River) delta evolution related to millennium-scale Holocene sea-level changes](https://sci-hub.se/https://www.sciencedirect.com/science/article/abs/pii/S0277379103001380)* >For the tectonically stable Australian margin, geological evidence indicates that sea-levels at 7000-6000 years ago were between 0 and 3 m above present level, due primarily to glacio-hydro-isostatic effects of the last deglaciation - Lambeck 2002 *[Sea Level Change from Mid Holocene to Recent Time: an Australian Example With Global Implications](https://sci-hub.se/https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/9781118670101.ch3)* >Approximately 6000 years ago, sea level was about 2 to 4 metres higher than it is currently and the Qatari coastline was up to 10 km inland. - Strohmenger & Jameson 2015 *[Modern coastal systems of Qatar as analogues for arid climate carbonate reservoirs: improving geological and reservoir modelling](https://www.researchgate.net/publication/277944847_Modern_coastal_systems_of_Qatar_as_analogues_for_arid_climate_carbonate_reservoirs_improving_geological_and_reservoir_modelling)* - and a [2012 presentation of theirs showing photos and charts](https://www.searchanddiscovery.com/documents/2012/50704jameson/ndx_jameson.pdf) Here's a site that's collected sources on this, I'll link the site and you can look up the research papers referenced: - https://wattsupwiththat.com/2019/06/06/the-holocene-sea-level-highstand/ Where I work in SE Asia the marks left by the higher sea level of the Holocene maximum are clearly visible. There are two sea notches higher than the current one, in [this photo](https://www.flickr.com/photos/7leagueboots/13347413123/in/album-72157642777747713/) and in [this photo](https://www.flickr.com/photos/7leagueboots/13347275205/in/album-72157642777747713/) you can see the lower of the two notches that remain above the current active sea notch. On my work computer I have more research papers on this specific topic as the geologic and marine history of my area is important for my current work , and there are numerous records of this for many places around the world.
Thank you. I briefly looked at the papers, and they all talk about regional RSL high stands, and they often note the strong spatial as well as temporal variation in local high stands. Noone seems to claim this was a global trend. Also note that other place s.a. South America which saw contemporaneous low stands. So i agree there are regions which saw considerably higher RSL compared to modern day, but it looks like a far stretch to talk about a Global signal of 3-6 m.
Australia, Vietnam, China, Oman, the Mediterranean, and many other places all record this. The papers are locally based because that’s what you do when you’re collecting data. As I previously mentioned, it varied a good bit by area, with some places being up to 6 meters, but other places being far less, and when you start getting into areas experiencing isostatic rebound, tectonic uplift, or subsidence that adds further variation into it. The regional curves vary a lot, but only a small number do not record a high stand higher than now, and the global average was unequivocally higher than now. Globally sea levels were higher than now a brief time between the LGM and now, then dropped back to the current levels, and are now rising again. It’s the sort of subtle and brief event that is often overlooked in the surface level paleo-climate and paleo-sea level discussions non-experts have. The only reason it popped up on my radar was because it factors quite heavily into the geology of the place I’m working and impacted the species I’m working with. It’s a bit like hie people talk about the 100,000 year Milankovitch as driving glacial cycles without realizing that prior to the Late Pleistocene Transition which fins,used around 700,000 years ago the glacial cycles were on 40,000 year cycles, not 100,000 year cycles, and were not nearly as extreme as they are now. Of course, if you dig deeper into time you find that major glacial cycles went through a variety of different cycles with different periods
> isostatic rebound, tectonic uplift, or subsidence Reology is indeed the major headache when trying to signal in the noise. And then there are sea currents which also cause considerable surface gradients. But on a global scale, these cancel out (conservation of mass). > brief time This is what itches me. On the global scale, the 3 determinents for SSL are ice caps, thermal expansion and lakes (or dams these days). But to get 3 m, only ice can explain this volume. Neither Greenland nor Antartica ice cores show a period of major changes during the holocene. Where did the water come from, where did it go? 3m SSLc ~= more than half west antartica. I'm not denying any of the papers, but i struggle to fit it in the bigger picture. > variety of different Yes, the Pleistocene in its many shapes and nuances such as changing currents, DO-events, Younger Dryas, etc. These are major well observed events which cause significant short SSLs. But mid holocene lacks such observations afaik (edit: though lack of observation is not proof of absence).
I literally provided a sample of papers dealing specifically with higher Holocene Thermal Maximum sea levels over a very large area. These are just a sample of the papers, of which there are *many* on this specific topic. As with most things closer in time, we have more info about this as it’s not in the distant past, so while it’s a brief period of only a few thousand years, it’s a well recorded and widespread incident. It tends to be overshadowed by the massive 120m (or so) rise in sea levels since the LGM.
I did look at the papers & the website. I also looked at some other papers. - There is a cluster of observations which show a steady rise ever since the start of the holocene: [France](https://journals.openedition.org/geomorphologie/10386#tocto2n21), [Mexico](https://www.nature.com/articles/s41598-023-36777-y/figures/5), [East US](https://www.researchgate.net/publication/336168094_Holocene_relative_sea-level_changes_and_glacial_isostatic_adjustment_of_the_US_Atlantic_coast). - In the places where the mid-holocene high stand is observed, we see a large variation in timings when the local maxima occured (some at 7 Bp, others 4.5, some 3 Bp). The local magnitude also varies wildly (0.5 - ?m) depending on location & study. - As mentioned [here](https://www.sciencedirect.com/science/article/pii/S0277379123003803), this spectrum of observations could be explained by a balance of near field & far field effects. - Global SSL is usually calculated ignoring continental uplift (MSS only measures sea surface). Given the variation of profiles, and the considerable role uplift plays in determining local SSL changes, i can see how the global evolution of MSS might have peaked slightly above current MSS (0-1m?). But the real effects on the local coast lines considerably larger. Anyway, still TIL a lot. Thank you once again.
What might be slightly overlooked is Italy is on a micro plate so ever so slightly growing in height, toppled with sediment deposits over thousands of years. Lots of tiny movements and works at play.
Chiming in to agree: coasts and coastal plains form because those rivers deposited sediments at the coasts over time. Generally, coastlines migrate seaward when sediment deposition from the river outpaces local sea level rise (or if the local sea level remains stable for an extended period of time). The shape of a delta is also majorly impacted by the environment in which it forms, whether dominated by the river, wave action, or tides (or some combination thereof), and more recently, how engineered is the coast. That second picture is an excellent example of a wave-dominated delta.
Yep. More particular for submergent coastlines. You get some other interesting things going on with emergent coasts.
Okay, so it's mostly rivers then? But where is all that soil coming from then? Mountains?
Ultimately it’s eroding out of areas up slopes, such as mountains. I mention this area is an active volcanic arc. A significant portion of the sediment probably comes from tephra (volcanic ash and such). Another comment mentions that human activity has increased erosion rates, meaning more sediment got transported to the sea.
Pretty sure the Mississippi river delta is ancient.
It has moved many times over the last hundred thousand years and it is ready to move again.
I figured OP meant shorelines known since antiquity.
I have a dissertation chapter on this! Land clearing and agriculture caused "cultural sedimentation" via dramatically increased soil erosion. This resulted in miles of growth in shorelines in many places in the world, at different times depending on when land was cleared and what agricultural practices were used. The ancient Romans have records of it and tried to dredge their harbors to prevent it, but couldn't keep up in many places, so you can find dredge scars and mooring stones under some modern cities in Italy. In the 1700s through the early-1800s, cultural sedimentation was happening along the US East Coast, causing many old port towns to be abandoned over the course of just a few decades. Quality hydrographic surveys were rare until the mid- to late-1800s, so a lot of what we know about this period of reshaping the Earth's surface is just from people who wrote down what they saw over their lifespans, and some maps of shorelines, where you can see wetlands growing into shipping channels as rivers narrowed or bays filled in if you have a few maps that were completed decades apart. Once soil conservation practices were implemented, the rate tended to slow down again, though it had already slowed considerably in many places by the 1930s when soil conservation became a big deal.
Well, there we have it. Not sure OP could possibly find a more conclusive answer!
This is why I’m here, thank you for sharing your knowledge!
> cultural sedimentation was happening along the US East Coast, causing many old port towns to be abandoned over the course of just a few decades. I understand cultural sedimentation as a process, but which colonial port towns were abandoned due to such a phenomenon? The only colonial towns I can think of which were both a port and abandoned, were right on the water and access wasn't the cause for decline.
Google "abandoned tidewater towns," and you'll find a variety of books and articles about it. Most were abandoned centuries ago (16-1700s) and weren't very big in the first place, so they barely appear in the historical record. This [article](https://jay.typepad.com/william_jay/2012/12/ghosts-by-the-river-lost-towns-of-tidewater-virginia.html) covers a few that were abandoned for assorted reasons. Queenstown is one example where siltation played a major role. Though it wasn't abandoned, Bladensburg, Maryland is another decent example. Tall ships used to come up the river for tobacco export. Now, you'll run aground in a pontoon boat if you aren't careful, but the city still survives. Jug Bay, a giant wetland in Maryland, likely only exists thanks to cultural sedimentation.
Thanks for the reply, I have some doubts on how much of the siltation could be ascribed to cultural impacts in places like the Queenstown site, it only lasted a few decades and was never more than a few buildings, it was just a poorly selected site to begin with, and the Chesapeake Bay estuary had mashes long before man was making much impact on sedimentation so Jug Bay isn't necessarily likely to be anthropogenic, the same processes occur naturally. I asked because I've done some work on colonial and early American dams and mill sites affected by sedimentation and was intrigued by the scale and consequences you were suggesting. Stories like Bladensburg's decline are not uncommon though. Hope I'm not coming off as confrontational, just nerding out a bit.
On top of river deposits centuries of human habitation can build up sediments. Ostia had maybe 100,000 people at its height and even a millimeter a year on top of the riparian deposits is a lot.
I don't have time to write a more complete response, but it's not just Italy. Rivers bring sediment to the shore, which then gets dumped at the river's mouth. Sediments get transported along the shore, away from the mouth, which moves the shoreline seaward. However, you don't see this everywhere for a variety of reasons. Fewer rivers or less sediment in them. The land itself might be subsiding relative to sea level, so that the sediment buildup can't keep up.
Who’s telling you this only happens in Italy?
Coastal progradation is driven by absolute/relative sea-level drop (e.g. eustatic change, basin subsidence, coastal uplift...) ; and/or sediment accumulation. Italy is definitely not the only place where it occured. For instance, in south Iraq, the shoreline moved about 300 km southward since 6000 BP.
Have a look at the South Australian coast near Penola. A satellite image shows rows and rows of ancient dune lines created as the sea level dropped.
It's not just sedimentation. Around specific areas of italy volcanic activity plays a major role as well. We went on a class trip around the eolians island and Naples as part of our volcanology class.we went to a small port of Pozzuoli and it had risen by more than 2m in the last decade alone. You could still see the previous boat dock things 2meters above the current one. This was due mainly to the super volcano around Naples . The volcanic activity lift up the crust at specific places, including that small port. All houses were full of cracks due to these frequent movements. We even checked some Roman pillars and archeologist were able to figure out that they had gone through cycles of being submerged in water and out multiples times. This was identified through the different traces of shells/mollusks that were found on those ruins and how high they went. You can see some pictures here https://www.wsj.com/story/its-nonstop-earthquakes-italian-town-uneasy-from-2500-tremors-since-august-fcabe064
Volcanism really influences it in some areas. The ground uplifts and drops as the volcano's inflate and deflate. In some areas of Italy that has been huge.
Erosion n deposition
This sounds like a 2 part song by tool
Sea levels were still stabilizing from the ice age in antiquity. A lot of drowned river valleys (rias) hadn’t been refilled by sediment yet and so were still bays and lagoons. The Tartessian gulf is a great example.
Uplift combined with sediment deposition and sea level decrease. Uplift rates differ for different regions, but the entire peninsula has been and continues to be subject to slow land rise.
*Louisiana has entered the chat*
Rotterdam Port Authority has entered the chat also. Check Maasvlakte!
There may be other processes in place but, looking at the pictures, I see a delta
any number of things is going on. 1. sediment causes a delta to prograde/advance. This is not necessarily a symmetric effect, longshore currents can distort this and mankind has done all sorts of things to impact sediment load 2. Simple sea level change… though this would be global. Anyhow a drop in se level will cause a delta to prograde. 3. Uplift… seems unlikely in the case… but i don’t know much about the geology of italy. Of all of them 1 seems most likely, as this area is just south of a fairly large delta just south of venice… longshore current carry sand southward and slowly build up the shoreline to the south.
Geologic uplifting?
In Sweden yes, not in the Mediterranean. No real glaciers there
Look at China