WALKING on the beach this afternoon I took some photographs of beach mounds. I’m referring to piles of shells and pebbles regularly positioned between, and parallel to, the high and low tide marks.
Are these beach mounds a consequence of the ebb tide dragging the sand away from the shells and pebbles or a consequence of swash action dropping shells and pebbles?
During periods of global sea level rise there is typically an overall increase in the amount of sand deposited along a beach.
But a small change in the relative strength of the ebb tide can presumably significantly change the patterns we see on beaches. How different would our beaches look if global sea levels were falling rapidly?
Large mounds dominated by a single shell species near Weipa on Cape York Peninsula (North Queensland) were once considered aboriginal middens but may in fact have been beach mounds. According to Tim Stone from the Australian National University they are not middens by rather a natural consequence of local chenier plain development.
 Shell mound formation in coastal northern Australia by Tim Stone
Marine Geology, Volume 129: 77-100. 1995
Shell mounds are late Holocene deposits typically dominated by a single shell species. In northern Australia these mounds are associated with prograding coastal plains. The largest and most numerous are at Weipa on Cape York Peninsula. Archaeologists claim that these mounds were formed by generations of shellfishing Aborigines. This hypothesis is false because most of the shells from the type-site are of a similar radiocarbon age. Mapping and augering of two contrasting shell mound environments along the Mission River at Weipa demonstrates that mound formation is a natural consequence of local chenier plain development. This is supported by shell ages from across the Weipa landscape. The shell mounds at Prumanung originated as a coarse shell berm. The large mounds on the Uningan plain originated as small shell cheniers. The only reasonable explanation for the transformation of these natural shell deposits into tall, steep-sided mounds is the mound-building behaviour of the Orange-footed Scrubfowl Megapodius reinwardt. Similar mounds composed predominantly of sand and gravel are also present at these localities. The strong likelihood that the shell mounds are natural shell deposits raises serious questions about basic principles of shell midden archaeology. New methods for distinguishing between cultural and natural shell deposits are needed.
Doug Procftor says
One of the more unusual things that happens with wave action on shells is that the top and bottom shell halves, even if mirror images, become separated and concentrated separately. This is because upside down the slopes of the shells are opposite laterally when aligned the same way, and different longitudinally when they are oriented the same way.
An odd thing I noted years ago on the west coast of Vancouver Island was that a shipment of new running shoes, Adidas, that had gone overboard in the north Pacific, showed up as primarily left shoes on the beach I was on. Shoes have the same mirror symmetry that shells have, so the concept that sorting occurs is somewhat reasonable, though unexpected by the experts in Seattle (amused, though).
So: the middens/shell mounds need to have top and bottom halves (pedicle and brachial valves) counted. The midden will have equal amounts of each. Mounds will be consistently under or over represented in one valve.
“New methods for distinguishing between cultural and natural shell deposits are needed”.
Jen; those shell swirls in your photo can’t be mistaken for middens in any stage of SL movement. Our average human deposits build over hundreds if not thousands of years and most are associated with fire places.
The most important feature is large quantities of carbon get mixed up with the lime so they are quite outstanding.
I anticipation of such a post I found this link last night from Google “Bass Strait land bridge”
Ian Thomson says
Just a note on beach sand behaviour.
Some time last century I grew up beside Chrystall’s Beach , about 75 kms south of Dunedin in NZ.
It is a very dangerous beach , with a sharp drop off and bad undertow so nobody attempted to swim there.
However, periodically, a flat beach of finer sand would form for some distance out and it was cautiously usable.
Historically this happens when the sand is ‘out’ at Dunedin’s St Claire Beach , giving it a rough year.
Doug, Thanks for that advice: Mounds will be consistently under or over represented in one valve. That would seem like a good and easy test to apply to the Weipa mounds/middens.
Gavin, Of course my picture is of temporary mounds not old middens, because they weren’t there the day before! And there are more pebbles than shells etcetera, etcetera but it simply amuses me to ponder the pattern on the beach and how it was formed by the particular wave and tide action. Thanks for the link.
spangled drongo says
The existence of middens except in caves and shelters needed for survival always seems a little hard to believe.
Having been involved with aboriginals in open mustering camps in unfenced country in warmer regions, a group of people doesn’t camp in the same place as they did a few months earlier [unless they absolutely have to] for obvious reasons.
Campsites are used on a rotational basis which would mean that both shells and other detritus would be well spread.
John Sayers says
Ian – you wouldn’t want to swim at Chrystall’s Beach because the water is freezing!! even in Summer. Same at St Claire and St Kilda.
I used to drive north over the hill to Warrington Beach where the current from the north had warmer water 😉
“The existence of middens except in caves and shelters needed for survival always seems a little hard to believe”
Ever watched Time Team SD? For cooler and wetter climates, we slept by the fire and despite the constant consumption of wood, each site grows vertical soil layers. It’s the same for caves, huts and middens. With the latter, sleeping right on the mound even under makeshift shelter is not hard to imagine if all the embers are properly covered. Keeping it going through the rain would be a major chore.
For a 2nd hand story; trappers working in the frosty Tasmanian ranges between the wars lost their old trench coats to scorch holes while huddled under smoldering logs overnight. They slept too well after a hard day’s tramping with their traps and game over the shoulder. Managing fire in a damp rotting log at the edge of old rain forest or along some new railway line is another skill we have probably lost.
spangled drongo says
No group of people continuously sleeps in the same place in a warm climate and not even in a cold/wet one unless it is the only place of shelter.
Schiller Thurkettle says
I don’t want to make trouble, but it appears that the beach mounds are perpendicular to, rather than parallel with, the shore. Maybe I’m looking wrong at the picture.
What you are looking at is a wash or series of washes, big stuff to the right, little stuff to the left. Think of gold mining with a pan or tin mining and shaking tables for “sizing” via slightly sloping fluidised beds.
Perpendicular? Sure, and we can assume the slope is towards the waves in general despite some debate about Jen’s camera angle at the horizon. However there is often an apparent depression running parallel with the dunes with various accumulations between tides, Also; all the light debris thus accumulated can be pushed much higher with changing weather and tides leaving a shell line as the rest dries and blows away.
Svensmark’s paper has at last been published and should expand our knowledge of our planet’s history over the last 500 million years.
He shows that the big gaps in the fossil records and sometimes near extinctions of life in the oceans may have a cosmic origin.
Could the climate also be changed by the rate of cosmic ray bombardment over the planet recently as well?
Schiller and Gavin
It may be the angle of the photograph that is confusing you, but the mounds were arranged parallel to the beach and parallel to the high and low water mark i.e. formed by the same set of waves at the same time.
Tony Price says
spangled drongo said:
“No group of people continuously sleeps in the same place in a warm climate and not even in a cold/wet one unless it is the only place of shelter.”
I do, but more seriously, groups revisited sites seasonally or intermittently, and middens built up over time, as sediment or soil layer analysis shows, with carbon dating evidence