Tag Archives: Sedimentology

Beach microcosms and river analogues

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We are regular visitors to the beach; walks with the kids-grandkids, the dog, swimming, fishing, or just sitting and cogitating. It’s easy to get lost in the timeless rush of waves, their impatient foam. My mind reels at the thought that the sea has been doing this for more than 4 billion years. It’s a bit like getting lost in the night sky. There’s so much to discover.

Beaches are geological domains – part of a continuum that extends to the deep ocean, but a part that is easily accessed.  Geological stuff happens there. My attention is always grabbed by the small streams that drain across beaches at low tide. Whenever we came across one of these my kids would scatter, lest they be regaled yet again about the fascination of miniature worlds. I admit it was a bit over the top, so it goes…

Some beach outflows come and go with the tides, others are more permanent leakage from inland drainage. Some trickle, others rush. They are all fascinating, as microcosms of grander floodplain or massive deltas. Project this microcosm to the real world of geological process, of cause and effect. In doing this, you are engaging in the scientific process of creating your own analogy, an insight into a larger universe.

The streams usually start afresh with each tidal cycle. As tides recede, stream flow begins to erode its channel, deepest at the top of the beach. The channels may be straight and narrow, or broad networks of braided sand. Continue reading

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Atlas of aeolian deposits

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Here are some examples of modern and ancient sand dunes from continental and coastal settings. Click on the images for an expanded view.

The Atlas, as are all blogs, is a publication. If you use the images, please acknowledge their source as indicated below (it is the polite, and professional thing to do).  I retain copyright of all images presented herein

Brian Ricketts –  www.geological-digressions.com

 

This link will take you to an explanation of the Atlas series, the ownership, use and acknowledgment of images.

Click on the image for an expanded view, then ‘back one page‘ arrow to return to the list

 

The images:

Close up of Jurassic Navajo Sandstone dune crossbeds with tangential toe-sets, Zion National Park, Utah. Image height is about 2m. Large sand dune complexes in a continental desert, about 180 million years ago.

 

 

Really big dune foresets (lee-face deposits) in the Jurassic Navajo Sandstone, Zion National Park, of southern Utah

 

 

 

Closer view of the exposure in above image, showing large foresets and individual dune truncation. The person (bottom left) is about 2m tall. Navajo Sandstone, Zion National Park, Utah

 

 

 

Coastal dunes bordering Great Exhibition Bay, northernmost New Zealand. The white sand contains 90-95% quartz.  Much of the sand for the modern dunes comes from reactivation of late Pleistocene dune complexes, and sand stored on the sea floor.

 

Dune sets in Late Pleistocene deposits, Great Exhibition Bay, northernmost New Zealand.

 

 

 

Large, active barchan dunes traverse Kokota sandspit that forms a protextive barrier to Parengarenga Harbour, northernmost New Zealand. This view is of the main lee face. The dune is about 8m high.

 

 

Small lakes and ponds that are sometimes perched between sand dunes, accumulate mud, sand and peaty material from plants. Here are at least 6 interdune pond mud layers, some with fossil roots (also replaced by clays). See also the image below. Late Pleistocene, Great Exhibition Bay.

 

Interdune lake near the inland margin of stabilized, Pleistocene sand dunes, Kariotahi, south Auckland, NZ.

 

 

 

Multiple dune sets in a Pleistocene sand barrier, Kariotahi, south Auckland. Formation of the old sand barrier and dune field eventually created the straightened coastline that typifies the west coast of North Island.  The barrier formed during repeated rises and falls in sea level over about 2 million years.

 

Multiple dune sets in Pleistocene sands (Same cliff face as image above), indicating a fairly consistent orientation of the preserved lee faces, and the direction of dune migration (to the left – east). Kariotahi, Southwest Auckland, New Zealand.

 

 

Barrier and dune sands, Kariotahi, south Auckland. Here an older set of dunes (orange weathering) are capped by an thin soil and bands of iron oxide (along the surface that slopes steeply to the left). The soil would have stabilized the older dune sands. A younger set of dunes formed over the top of the old soil.

 

 

A more panoramic view of the old soil and dunes in the image above.

 

 

 

The landward margin of Pleistocene sand dunes, now stabilized. The steep slopes are the old lee slopes (the steep surface of the dune that faces down-wind). When active, the dunes would have migrated towards the viewer. Kariotahi, south Auckland.

 

 

Reddened sandstone with large dune crossbedding, in the New Red Sandstone on Arran, west Scotland. The red colours are formed by iron oxides, after the sand was deposited during the Permian through Triassic – 280 to 200 million years ago.

 

 

Multiple dune crossbed sets in New Red Sandstone, Arran, west Scotland, 280 to 200 million years old.

 

 

 

Salt-tolerant grasses and shrubs extend their roots deep into the dunes to capture as much moisture as possible, and in so doing help to stabilize the dune.

 

 

Mesquite Flat dunes, Death Valley, near Stovepipe Wells.  An inland ‘sea’ of sand, usually organized as dunes, is called an erg.

 

 

 

Dune sand at Mesquite Flat, migrating over desiccated pond silts. Death Valley

 

 

 

At least two generations of wind ripples in Mesquite Flat dunes. All those trails attest to numerous insects and the larger critters that hunt them.

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Submarine mud flows and landslides modified Kaikoura canyon during the 2016 M7.8 earthquake

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A slashing blow by some mythical behemoth, knifing effortlessly through earth’s rocky foundations; a (seemingly) bottomless chasm, a canyon, with nothing but the wind between you and whatever lies below. A bit over the top perhaps, but canyons often spark the imagination – standing on the lip can feel like being perched on the edge of the world, vertiginous for some.  Most canyons have been carved by the relentless churning of stream and river, incising the layers of rock and removing the sediment to distant shores.

Terrestrial canyons have their submarine counterparts, that transect the submerged, outer margins of continents and volcanic islands. Submarine canyons commonly mark the transition from shallow continental shelves and platforms to ocean basins, acting as conduits for sediment delivery from rivers, deltas and shallow seas, to the deep oceans.  And like their land-based cousins, they are deep (1 to 2 km, or in the case of Grand Bahama Canyon, 5km), steep-sided incisions in the ocean floor.  More than 600 submarine canyons have been identified world-wide from bathymetry maps.

The 3-dimensional bathymetric reconstruction of Monterey Canyon (top image), about 100km south of San Francisco, illustrates common attributes of these structures. The canyon cuts deeply into the break between the shelf and steeper marine slope – in this image the break is a definite line separating light blue from darker shades.  The Monterey Canyon head encroaches onto the shallow shelf to within a few 100m of the shoreline; this is actually atypical of most other canyons where incision of the sea floor usually begins closer to the shelf edge. The canyon channel snakes down slope, eventually flattening out on the deep ocean floor; the main channel is joined by several smaller tributaries. Several smaller gullies are also incised into the shelf edge and slope.

The general opinion among earth scientists is that submarine canyons are formed by two main processes: Erosion by sea-floor hugging flows of mud and sand (given the general name sediment gravity flows), and by collapse of the steep margins, producing submarine landslides (and potentially, tsunamis).  Common triggers are thought to include storm surges and earthquakes. The primary basis for this interpretation is abundant geological evidence of past events, combined with some experimental work, but it remains a largely theoretical interpretation because there have been very few direct observations of either process in action.  The reasons for this disparity are that submarine flows of mud and sand are relatively rare events (at least on a human time scale), and because of the difficulties inherent in witnessing such processes in deep water. For this reason, recent events in Kaikoura Canyon (southeast New Zealand) have sparked significant international interest.

Kaikoura Canyon (New Zealand), 60km long and up to 1200m deep, is located along the tectonically active Hikurangi margin, close to the Alpine Fault system that transects northern South Island and the adjacent submarine shelf.  At its deepest extent (about 2100m) the main canyon channel merges with Hikurangi Channel, which at more than 1500km, is one of the longest deep-sea channels in the world; Hikurangi channel wends its way across the more subdued ocean floor towards the abyssal Pacific Ocean.  The submarine canyon head is an uncomfortable 1000m from the coast, a spitting distance that elevates the risk of destructive tsunamis that can evolve from submarine landslides along the canyon walls. November 2016, and the magnitude 7.8 Kaikoura earthquake, provided a rude reminder of the potential for disaster. The seismic jolt activated slope collapse and sediment movement down the canyon slopes and main channel; fortunately, the ensuing tsunami was small, but the bonus for science was huge. Mapping  of the canyon head and main canyon channel, fortuitously three years before the earthquake and three months after the event, has enabled scientists to track the changes to channel morphology and sediment distribution that can be attributed solely to the earthquake (The project was coordinated by NIWA – New Zealand’s National Institute of Water and Atmospheric Research).

The first two images show before (2013) and after conditions at a location near the canyon head (closest to shore). Large swaths of muddy sediment were dislodged from the ridges and slopes, cascading into the main channel; most of the canyon head is now devoid of its sediment mantle.  Parts of the canyon floor are 50m deeper than before the earthquake, because of erosion by the moving sediment.

The second set of images show before-and-after scenes of the canyon floor at 1800 to 2100m water depth. The striped pattern is formed by large, ripple-like gravel waves, or dunes, that under normal conditions would migrate slowly downslope. However, most of the gravel dunes were moved at least 500m downslope by the rapidly transiting muddy flow.

Much of the dislodged sediment continued as a turbulent muddy flow down the main canyon channel and thence to the deeper Hikurangi Channel; the flow had sufficient momentum to carry it more than 680km from its source. Evidence for this comes from deep-sea cores taken 4 days, 10 weeks, and 8 months after the earthquake.  Cores were taken from the floors of both canyon channel and the more distant Hikurangi Channel, plus the flatter area, or overbank, beyond the channel banks (analogous to a river floodplain).  The reasoning here is that, if sediment gravity flow deposits can be identified in the overbank region, it means that the flow itself was deeper than the channel and, given that we know how deep the channel is, an estimate can be made of the minimum depth of the actual flow.  Overbank deposits were detected in cores, indicating that the moving flow was at least 180m thick, 680km from Kaikoura Canyon. As Joshu Mountjoy (one of the project leaders for NIWA) has pointed out, this has proved to be one of the few occasions in which actual flow dimensions in a deep-sea channel could be measured.

From the Kaikoura event we have confirmed that seismicity can trigger physical modifications to submarine canyons and submarine slopes, and that sediment is flushed from canyons to the deep ocean by far-travelled, turbulent muddy flows (i.e. sediment gravity flows). We have learned something of the stability of the canyon itself and the sediment that gradually mantles the sea floor. The legacy of the Kaikoura earthquake (or any major earthquake for that matter) is often voiced in terms of broken lives, disrupted highways, and the costs of rebuilding. There should be no attempt to minimise these outcomes, but we should also remind ourselves of the advances in scientific understanding of earthquakes, and the geological consequences that accrue from an event like this. We should applaud these gains in knowledge because ultimately such knowledge will help save lives and property.

Most of the information for this post is gleaned from NIWA news articles and publications, linked in the text above.

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In the field: Mountain storms and surprise encounters, northern British Columbia

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The interior of northern British Columbia is rugged, mountainous country. Roads, that tend to be quite rough were frequently opened to provide access to mines and small settlements. It is an isolated part of the world, beautiful, even majestic, but also unforgiving.  East of the Coast Mountains and about 200km south of Yukon, is a huge swath of sedimentary rocks, referred to collectively as Bowser Basin.  The rocks are Jurassic to Cretaceous, recording a history of about 70 million years duration. Humungous volumes of sediment were eroded from older rocks to the north, that were uplifted and deformed as tectonic plates, or terranes, collided with the ancient margin of North America. Gravel, sand and mud were carried by braided rivers, supplying coarse sand and gravel to the coast and beyond, and to large deltas that supported lush forests (later converted to coal). Continue reading

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In the field: Windows into two billion year-old rocks

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My early geological education was very much New Zealand centered; the gamut of sedimentary, igneous and metamorphic rocks (there are no Precambrian rocks in New Zealand), in the context of a landmass (and attached submerged bits) still rent by active faults and erupting volcanoes. The timing was fortuitous. We learned at the cusp of the ‘new tectonics’, sea-floor spreading, and the morphing of continental drift into plate tectonics.  The fixists were a disappearing breed; now everything was on the move, attached in some way to one tectonic plate or another, rifted, drifted, and eventually subducted. Now, the rock formations, faults (particularly the Alpine Fault), and the volcanoes, were all connected in one, all-encompassing global, plate tectonic system.  Geologically active New Zealand had a place in this grand scheme.

Admittedly, not all our professors found it easy to teach these revolutionary ideas. We would be exhorted to go and read the latest journal papers, and come back with questions – I guess this gave the teachers time to read the articles themselves. But it was an exciting time, reading the claims and counterclaims. It really was a (Thomas Kuhn) paradigm shift.

Landing on the shores of Belcher Islands (Hudson Bay) was also something of a mind warp; from a country that straddles a plate boundary, has a volcanic rift zone in central North Island, and faces a subduction zone within a stone’s throw of the east coast, to a part of the Canadian Shield where not much has happened over the last two billion years.  Perhaps that’s a bit of an exaggeration, but this prolonged period of stasis had its advantages.  The rocks, despite being about 2000 million years old, are loaded with beautifully preserved structures and fossils.  They were not cooked by metamorphism during the time they spent being buried, nor fractured beyond recognition by tectonic forces. Basically, everything was intact. Stunning.

For someone interested in deciphering ancient sedimentary environments, being parachuted into the Belchers and being told to take the rocks apart, layer by layer, sequence by sequence, was initially a tad scary; an emotional response that quickly dissipated once the measuring, observation, and interpretations had begun. On finishing the work on one set of exposures, we couldn’t wait to get to the next, and the next.

If you were to stand all the Belcher strata in a single pile, it would be almost 9 km thick. But this pile was subsequently tipped on its side. Over the eons, the rocks were eroded by rivers and scraped by ice, fortuitous levellers that provided windows into each layer. Geologists are enticed to enter these portals, at least in their mind’s eye; the rewards are huge.  We can envisage times when there were broad platforms of limestone (now all converted to the mineral dolomite), that harboured a massive biomass of primitive algae, stromatolites of all shapes and sizes; layers as thin as a fingernail, and reefs 10s of metres high. The platforms were covered by warm, seas that shoaled into tidal flats and (deserted) beaches. Some areas infrequently inundated by high tides, became desiccated; there are remnants of minerals like gypsum and halite (common salt) that attest to salty seas. Walking over rocks like these kindles the imagination; a beach stroll, waves rolling in like they have done for billions of years, or parched landscapes exposed to the full effects of sunlight uninhibited by oxygen and the UV dampening effects of ozone (the incidence of UV light must have been intense). The experience is humbling.

However, idylls have a tendency to dissipate in the fog of time or, as was the case here, a smothering by erupting ash columns and lava flows. Now we get to walk across the tops of really ancient lava flows, around piles of pillow lavas, or along catastrophic pyroclastic flows of ash and pumice.  The earlier tropical paradise had been obliterated, but even in this volcanic brutality there is wonder.

Other strata tell of deep seas fed by turbulent mud flows cascading down an ancient submarine slope, and of sandy rivers turned red by iron oxidized by the gradually increasing levels of oxygen in the ancient atmosphere (deposits like this are commonly referred to as red beds). In every layer, every rock we looked at, there were mysteries waiting to be unravelled. A geologist cannot hope to solve all such questions, but finding a solution to even one of them is incredibly satisfying.

I spent a total of 5 months in the field during the 1976-77 summers. This was not the kind of location where, if I’d forgotten to do something, I could whip back for a couple of days to sort things out. Several of my student colleagues were doing similar kinds of research in remote parts of the country – field seasons were long. Once you had arrived, you were there for the duration. And despite the sense of excitement and discovery, it was always good to get back home.

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In the field: from one extreme to another

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Have you ever looked at some locale on a map or photograph and thought “that looks like an intriguing place to work”, only to find, sometime later that you are smack in the middle of that same spot?  Time-warp? Some god’s lap?

I was preparing to travel to Canada. The plan was to do a PhD, and because I had not long completed a Masters thesis on geologically very young sedimentary deposits, had entertained the idea that research on very old rocks would add a kind of symmetry to my geological outlook – from one end of the geological time-scale to the other.  In preparation, I borrowed The Geology of Canada, a weighty tome, and homed in on the Precambrian system (basically everything older than 540 million years).  What caught my attention were some squiggly-shaped islands about 150km off the southeast coast of Hudson Bay; the Belcher Islands.  Their shape belied some interesting geological structures, and the strata a mix of sedimentary and volcanic rocks about 2 billion years old. What a neat place to work, although I envisioned the islands to be treed.

I arrived in Ottawa (early January, 1976) to minus 25oC and snow; I had never seen so much snow. I thought it quite beautiful, which elicited wry comments from the Ottawans I was meeting who were sick of shovelling driveways and digging vehicles out of snow drifts. Destination – Carleton University. My supervisor was to be Alan Donaldson, well-known in Precambrian geology circles. Following the introductions, he announced that my project, unless I had some objection, was to focus on the Belcher Islands.  LOL. I was to spend 5 months there in total over the summers of 1976-77.  The social environment, the weather, and the geology were remarkable.

Getting to the islands was a milk run: a drive to Montreal airport, a flight to Moosonee near the southern shores of James Bay (northern Ontario), a very noisy DC3 leg to Umiujaq (Quebec) where we picked up field equipment (kindly loaned to us by the Geological Survey of Canada), then Twin Otter across the 150 km to Sanikiluaq, the sole village on Belcher Islands. We were able to stay in a small house owned by the Hudson’s Bay Company for the two days needed to sort gear, buy food, and make sure the two inflatable Zodiacs and outboards worked. My assistants (John McEwan in 1976, Mike Ware in 1877) and I always used two boats as a safety measure (and for visitors).

The seas around the islands are mostly ice-free during the summer months, but the water is still only a few degrees above freezing, and the air close to the water cold. Even in the summer, we had to bundle up with wet-gear, fleeces, and life jackets (I was told the life jackets were necessary for insurance purposes – so they could retrieve the bodies). The islands and intervening channels are also elongated north, so that wave set-up could change drastically any time there was a wind shift.  We were caught out a few times with unfavourable seas, but there was always somewhere to shelter.

Belcher Islands are mostly held together by a thick volcanic unit that creates more or less linear coastlines. The strata were folded, like a series of waves, into simple anticlines and synclines, such that the package of sedimentary rocks is exposed in the anticlines, while the synclines are drowned by major channels and inlets.  The terrain is subdued with low relief – the islands were scraped clean by the Laurentide Ice Sheet during the Last Glaciation.

Our base camp was to be in a small, relatively sheltered inlet along the western shore of Tukurak Island (one of the largest and easternmost island).  It was a 3-4 hour journey, depending on weather.  This is the site of an abandoned Hudson’s Bay post. It was also the favoured summer holiday spot for local Inuit, primarily because it is close to their source of soapstone.  Belcher Island soapstone has an enviable reputation amongst northern communities, because of its uniform, deep green colour, and general lack of fractures that would render carving difficult. Whenever we were in base camp, we would watch the elders carving, and teaching their younger folk the same skills. They would also bring us bannock and Arctic Char. And there was never a shortage of Inuit kids around, checking in, telling stories, or simply hanging out. We would spend 4-5 days away from base camp, returning to stock up and cache samples. Time in base camp was always a delight.

Belcher Islands sit well below the Arctic Circle at 56oN (latitude), and yet the landscape is typically Arctic. The northern Canada tree-line is located south of Hudson Bay, such that the Islands have a typical Arctic flora (especially wild flowers), and no trees – so much for my earlier, wistful image of the place.

The weather alternated between gorgeous, with light winds and clear skies, and abysmal. On more than one occasion we returned to camp from a day’s work to find tents down and sleeping bags soaked.  High winds also prevented longer excursions with the boats, unless we were riding through sheltered channels and inlets.  With the boats, there was always one eye kept on the weather.

During the first couple of weeks in 1976, Bill Morris (Geological Survey of Canada) had joined us to sample rocks for geophysical measurements (looking for ancient magnetic poles). The day he was to leave base camp (and fly out of Sanikiluaq) was particularly inclement. He insisted on attempting the trip, but instead of using our inflatable boats, I decided to rent one of the larger, sturdier, Inuit canoes with twin outboard motors (I was the only one with boating experience). We ventured out of the sheltered inlet, into the maelstrom – at least that’s what it looked like from the perspective of our small craft. I doubt we got any further than 50m from the inlet entrance; a lull in the waves, a quick decision to about-face, a beeline back to calmer waters, and the colour returned to the faces of my two passengers.

“Guess I’m going to miss my flight”. We all new he probably would have missed it, even if we had continued. Back to base camp to drain what was left of a bottle of scotch, and cogitate on an earlier field season on a warm New Zealand Pacific coast.

This is the first blog on my Belcher Islands episode

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In the field: Beaches, sand dunes, and shellfish for lunch

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Final exams over, a moment of tomfoolery, and I found myself disconsolate in a hospital bed, recovering from an operation on a dislocated knee (no such thing as key-hole surgery back then).  I had completed my BSc (November,1972) and was contemplating doing a Masters on something sedimentological. A visit from one of my professors, a period of impatient recovery, then loaded the aging Morris Minor and headed north almost as far as it is possible to drive in New Zealand. This was to be my first bona fide field project.

My field area incorporated a 22km stretch of glorious coastline that, at its northern extent, culminated in a sandspit (Kokota Spit) and Parengarenga Harbour. The task was to decipher the sand deposits, from the very recent to maybe a million or so years old. This narrow strip of land is part of the much larger Aupouri Peninsula, founded on sandy bars and spits that connect small islands and headlands of harder, older igneous and sedimentary rock. Tasman Sea washes the west flank of Aupouri, and the Pacific the east; the two seas meet at Cape Reinga, the place where Maori spirits begin their new journey. Continue reading

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A 40 Million Year Old Forest, Looking Like it Formed Yesterday

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How Geologists Interpret Ancient Environments; The Stunning Preservation of an Arctic Fossil Forest

Arctic mapIn 1985, my field assistant and I were examining sedimentary rocks on central Axel Heiberg I. in the Canadian Arctic.  The project was part of a broader science program being run by the Geological Survey of Canada. I had surmised, from some of my earlier work that the deposits here had formed in response to tectonic upheaval in the region about 40 to 45 million years ago (a geological time called the middle Eocene); we were on the look-out for additional information to assess this hypothesis.  Our helicopter had dropped us off at the base of a gentle ridge, known as Geodetic Hills. Continue reading

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The Ancient Earth 7. The Art of the Stromatolite

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Algae, Fossil Slime and Organic Precambrian Art

Stromatolites are the earliest physical life forms on earth; they were the precursors to pretty well everything you see living today. There may be indications of earlier life forms preserved as chemical signatures, but as fossils go, something you can see and touch, stromatolites are it. The oldest stromatolites known are from Western Australia – about 3400 million years old. These ancient structures were built by primitive algae and bacteria, aka cyanobacteria, sometimes referred to as blue-green algae. Clearly life had already evolved to something quite complex by 3400 million years ago.

Continue reading

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From Sand to Stone = Sandstone; A Remarkable Transformation

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How Geologists Interpret Ancient Environments. 5 Being Buried and Loving it

 

Jakes sand

You are at the beach, by the river, in the garden; you walk through soft sand, squish through mud, pull weeds and sow seed in soil. They’re all soft, squishable, digable.   But throughout the 3400 million years of our Earth’s known sedimentary record, countless millions of times, these same deposits have hardened to rock. Continue reading

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