Grand Manan Island, New Brunswick: July, 2015
Walking on the seashore, I am often struck by the diversity that can exist in a very small area. Certainly you can observe a range of features and life forms if you walk in a forest or across a grassland, but on the shore the diversity effects are magnified and multiplied by the juxtaposition of land, air and sea. Physical forces above and below tide line act upon the water, sediment, and rock; life forms respond to this complex and dynamic system with their own complexity and dynamism. On a summer morning the beautiful sands of Seal Cove beach may seem like a peaceful, idyllic place for a walk, but even at a time like this the change is constant, and if you look you are bound to be surprised.
© Graham Young, 2015
For information about the Seal Cove National Historic Site, see here.
It has been suggested that our current time interval is different from all the times that preceded it, that human activities are dramatically affecting the Earth’s environment, atmosphere, and oceans. Geologists have long known the time since the end of the last ice age, about 12,000 years ago, as the Holocene Epoch, but about fifteen years ago it was proposed that we have passed from the Holocene into a new interval, the Anthropocene Epoch. The Anthropocene has not been accepted as a formal geological term, and its concept is still somewhat fuzzy. For instance, it does not yet have an agreed start date; it is most often considered to have begun around the start of the Industrial Revolution in the late 1700s, but other suggestions are that it began at the dawn of agriculture about 12,000 BP (in which case it would be virtually synonymous with the Holocene), or even with the first use of atomic weapons in 1945.
It is not easy to find much information about what the geological signature of the Anthropocene might be.* A lot is written about the magnitude of human effects on the modern environment, and geochemists have made many suggestions about characteristic chemical anomalies that can be seen in sediments and glacial ice cores, but how are we actually affecting geology that might be observed with the naked eye? A geological age should be readily recognizable in the sediment and/or rock record. The following are a few thoughts on things that geologists might be considering in some distant future . . .
A few weeks ago I was working in my parents’ garden in the Maritimes. It was a rainy spring day, too wet to do much digging, so I drove to the garden centre and bought a few bags of crushed rock.** Returning to the house, I spent much of the afternoon alternating between happily trundling an empty wheelbarrow down the path through the trees, and somewhat less happily pushing a barrow full of gravel upslope through the drizzle. This sort of activity is wonderful for opening the mind, and as I spread and tramped the gravel, a thought entered that empty space between my ears: I am making an unconformity.
Which, surely, I was. The hill at Fredericton is underlain by grey and brown Upper Carboniferous sandstones between 330 and 300 million years old, which are thinly clad by soil and trees many places, and which yield the abundant fieldstone that rises to the surface with every spring melt. The sandstone is an excellent material for drystone walls, but apparently it doesn’t work well as an aggregate, and the most common crushed rock in that area is quite different: fine-grained, mid-grey material that breaks into sharp-edged pieces. When we were children who invariably scraped our knees by falling on gravel paths, we referred to this rock as “slate.” I am pretty sure that much of it comes from the quarry at Springhill, just up river from Fredericton, in which case it is geologically a wacke belonging to the Burtts Corner Formation, of mid Silurian age (about 435-420 million years old).
The basic principle of superposition tells us that sedimentary strata are younger than those they overlie, since sediment is generally deposited on top of the Earth’s surface. If sedimentation was continuous (or appears to have been virtually continuous), then the contact between a stratum and that below it can be said to be conformable. If there was a break in sedimentation that can be identified as a gap in the time/rock record, then we call that an unconformity. A rock unit will be older than that beneath it only if the area was subsequently subjected to immense geological forces that resulted in folding or thrust faulting.
A Silurian rock that has been quarried, crushed, transported, bagged, and spread on a path has to be considered as a “new” geological material. All of the value-added work of the quarrying industry has acted as a reset button on the geological age of this material. It may contain microfossils of Silurian age, but the human redeposition has made it an Anthropocene (or Holocene) deposit, here lying unconformably above the Carboniferous sandstone.
If you have read this far, you are probably thinking, “So what? Why is it at all significant that you spread a few bags of gravel on a path in the rain?” Considered by itself, this isn’t very important at all, but it is a very small example of what we humans are doing all the time. We are nothing if not industrious. Much of our industry produces results that seem fabulous to us, but that will leave virtually no trace when we are gone; I challenge you to imagine how Facebook or Twitter or The Beatles will produce any geological record. But some of our works will leave evidence long after our species has departed.
Think of how many tonnes of concrete, asphalt, and glass are required to produce one city of modest size. Consider the volume of aggregate that has been quarried for harbours and airports, the amount of iron that has been incorporated into vehicles and bridge spans, the quantity of clay that has been baked into brick for city houses. As buildings and roads are demolished to make way for newer ones, the materials are buried and incorporated into the Earth. The hundreds of years through which we have behaved in this way this may seem long to us, but they are really the blink of an eye in geological terms.
The burial of these materials could be considered as a single depositional event, generating an immense human-made stratigraphic horizon that extends around our planet. They have all been re-worked and redeposited, and most lie incongruously above the natural surfaces that they hide. On floodplains or lakebeds the human deposits may rest on top of silts only decades to centuries old, but even there the dividing line is crisp, obvious, and mappable. In many other places the bricks, concrete, and asphalt lie directly on much older surfaces, such as that Carboniferous sandstone.
Considering how much of the world is now occupied by our activities, and what a proportion we have paved over in one way or another, it is not unreasonable to talk about an Anthropocene Unconformity. And when we take the city deposits, and add to them the mine waste, and the garbage dumps, and the smelting slag, and all the refuse that will eventually fall out of the Pacific trash vortex and be redeposited on the deep ocean floor, and the general debris left in places as difficult to reach as Mount Everest, then maybe we should even call it the Great Anthropocene Unconformity.***
Although the horizon composed of specifically human-made and human-modified materials would be huge, it would also be patchy. To really understand the scale of the Anthropocene Unconformity, we will need to consider secondary factors that also affect the geological record. In various regions, the erosion due to farming and logging may result in considerable sediment being redeposited in places where there wasn’t sediment before. In some places, fluid injection into deep wells is causing earthquakes to become common where they were previously rare or nonexistent, so we might expect to see an increase in earthquake induced deposits such as landslides and seismites.
But the really big player in the Anthropocene depositional story will probably be sea level rise. Various estimates suggest that global sea level will rise by 20 centimetres to two metres by the year 2100, and possibly by 4-6 metres in the coming centuries. It has been calculated that 26,000 square kilometres of land would be inundated by a sea level rise of just 66 centimetres, so clearly the loss of land would be immense if a rise of several metres takes place.
In the sediment and rock record, what is the signature of a sea level rise? There is considerable variation, of course, but the stratigraphic record of sea level rise (or transgression) is generally relatively abrupt, with land surfaces or shallow-water deposits being quite sharply replaced by sediment deposited in substantially deeper water. A marine flooding surface, evidence of the sea moving across a formerly eroded subaerial surface, is a classic form of unconformity. Future scientists looking at the sedimentary evidence of the third millennium sea level rise would expect to see river valleys filled with marine sediment, coastal sand dunes replaced by shallow shelf deposits, and drowned reefs and barrier islands covered with deeper water sediments.
Locations currently occupied by low-lying coastal cities such as Miami will be the best places for field studies of the Anthropocene. In Miami, a huge mass of concrete, glass, and other debris, mixed with sediment resulting from human-caused erosion, will rest almost directly on top of porous limestone dating back to the Pleistocene Epoch. The top of these anthropogenic materials will form a flooding surface, overlain by subsequent marine deposits. The completeness of this record might permit the suggestion that Miami Beach could make a very good type locality for the base of the Anthropocene.
For any geologists considering Earth in the distant future, the Anthropocene Unconformity may well be some of the best evidence of our presence here: a sharp and nearly horizontal base to the varied mass of Anthropocene deposits that overlie it, recognizable in many places around the planet. Thinking about this a bit more, I just hope that the top of the Anthropocene deposits is not also defined by a crisp surface that can be traced globally. It would be particularly bad if that upper contact is marked by an ash horizon, a boundary clay, or an anomaly rich in Caesium-135.
* The great thing about publishing a blog post from a position of some ignorance is that readers will point you toward what you should have known before you published the post. My friend Roy Plotnick suggested I should look at this interesting paper:
Zalasiewicz, J., Waters, C. N., and Williams, M., 2014, Human bioturbation, and the subterranean landscape of the Anthropocene: Anthropocene, v. 6, no. 0, p. 3-9.
This is in a volume titled “Landscapes in the Anthropocene: State of the art and future directions”, which contains quite a few worthwhile publications on related topics.
** Here in Manitoba we would have called it “quarter down”, as it consists of particles of mixed size that are smaller than about 1/4 inch across. In New Brunswick, I think it was referred to as “crusher dust”. It is fascinating how variable some of these terms are, even within Canada.
*** I have searched the Internet, and it seems that the term Anthropocene Unconformity has only been used in a local sense, and Great Anthropocene Unconformity has never been used before. It seems like a very useful concept, and I would be interested to know if there is some comparable term already out there.
© Graham Young, 2015
Mélange: A mixture; a medley; odds and ends; a motley assortment of things, . . .
“In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix.”
However you look at it, St. James Church in Lower Jemseg, New Brunswick, is a mélange. Its architecture is a mixture of features and influences that somehow combine to make a charming and coherent building. Geologically, it can also be considered a mixture, though of course it is one produced by human agents rather than the immense forces that generate a mélange under natural conditions.
Even visitors with no knowledge of geology will immediately recognize that materials in the sturdy outer walls were derived from a variety of sources. Below and beside the windows, large blocks of dark purple sandstone contrast with various paler shades in the smaller blocks. The buttresses are armoured with wedge-split granitic rock, while rounded granite fieldstone can be seen in places in the walls. And then there is that soft, pale carved stone around the windows and the doorway. What are all these stones, and how did they get here?
The stones were pulled together through human expediency and opportunity. Jemseg, on the low-lying east bank of the lower Saint John River valley, is not a place endowed with wonderful bedrock, but the local geology is varied. Some of the stone came from nearby sources, but a bit of it travelled what might be called an unreasonable distance.
Constructed in New Brunswick’s great burst Anglican church construction in the latter part of the 19th century, St. James Church dates from 1887 and was consecrated in 1889 (general information about the church can be found here). This stone building is quite different from most of the churches built in this phase, which are wooden and “gothic”. Its form is also crudely gothic, but leavened with a dose of what might well be Scandinavian influence, particularly in the shape of the tower.
Although this church may seem like an old structure to those of us who live in western Canada, it is relatively recent in the history of Jemseg, a place that was first settled by Europeans in 1659 and that served as the capital of Acadia in 1690-91, some 325 years ago. Settlement occurred so early here because the junction of the Jemseg and Saint John Rivers was desirable as a trading post location. Both waterways are readily navigated, and Jemseg is higher than the marshy islands and moose pasture immediately adjacent to the rivers.
And although 1659 seems like the dawn of time in terms of European settlement in Canada, the stones that form the church are of course far older; the oldest stone is literally as old as the hills that form the west side of the valley some 20 kilometres below Jemseg. This stone is the remarkably tough and beautiful granite that forms the edges of the buttresses and corners of the church. It shows remarkably little wear from its long exposure to the elements there.
The granite quarried in the Hampstead area from the 1830s up to the mid 20th century is commonly referred to as Hampstead Granite or Spoon Island Granite. It was used most commonly for monuments and grindstones, but this church demonstrates that it could also be incorporated into structures (though I suspect that it was rather difficult to work with in this role). Geologically, the Hampstead Granite is part of the Early Devonian Evandale Granodiorite (roughly 400 million years old), an intrusion of 20 square kilometres that consists of “light grey to pink, medium-grained, equigranular, hornblende-biotite granodiorite varying to monzogranite” (Government of New Brunswick industrial minerals summary).
Granitic intrusions are bodies of rock that cooled slowly from the molten state, often associated with the melting of crust that occurred as the Earth’s crust was deformed near plate boundaries. In this particular case the granite was formed near the meeting of two tectonic zones, the Gander and Avalon zones, during the growth of the Appalachian Mountains. The Early Devonian in this region is associated with closure of an ocean during the Acadian Orogeny, one of the several intervals of intense deformation, plutonic (deep) igneous activity, and metamorphism that resulted in the development of the Appalachians (see here for a substantial discussion of the Acadian Orogeny in this region, a topic far too complex to be explained using my meagre knowledge of structural geology and tectonics).
Following a mountain-building interval, the new steep slopes and high elevations are subject to greatly increased rates of erosion and sediment transport. In this region, the Devonian orogenic deformation was followed by a major interval of sediment deposition during the Carboniferous, roughly 360 to 300 million years ago. Carboniferous sedimentary rocks, many of them formed in rivers, swamps, and floodplains, cover eastern New Brunswick and the rest of the Carboniferous Maritimes Basin.
Most stone in the walls of the Jemseg church is clearly Carboniferous clastic rock: broadly speaking it is “sandstone”, though much of it is “dirty sandstone” that could be more properly called arkose or even greywacke. I am not sure what formation was its source, though from its very mixed quality it is likely the most local of the stones incorporated into this structure. The bedrock underlying the village would be of this sort, so some of the stone could have even been quarried here, or perhaps across the river where the land is a bit higher. The variety of stone, however, suggests that much of it was collected as loose pieces. Since the pieces are angular they were not transported by water or ice; they are regolith that had been frost-wedged from the bedrock beneath.
It is unfortunate that the builders of the church did not cast slightly farther afield for sandstone: the Rainsford Sandstone (Minto Formation) used in the walls of Christ Church Cathedral in Fredericton has excellent colour and consistency, and the remarkably tough Grindstone Island Sandstone (Boss Point Formation) used in the cathedral’s buttresses is among the best sandstones anywhere. The former was quarried on what is now the Fredericton Golf Club, while the latter is from the upper end of the Bay of Fundy; clearly both of these stones would have been a bit dear for the builders of this parish church.
What the sandstone in St. James Church lacks in construction quality, it makes up in geological interest and variety. Walking around the building on a sunny day, you can pick out a great range of features in the buff, golden, chocolate brown, brick-coloured, or rust-red stone: angled crossbeds from the interior of a river sandbar, rusty ironstones that might be associated with overbank flooding on an ancient floodplain, microfaults that cut across the stratification, and a dirty sandstone with admixture of angular and rounded rock fragments from those older granites.
Although the church’s builders were content with local sandstones, the fine stone that makes the window arches and other carved features has come from very far away, and travelled here by a circuitous route. This is Caen stone, a yellowish limestone of Jurassic age (about 167 million years old) that formed on ancient seafloors in what is now northern France. Caen stone is fine-grained, oolitic, and of very consistent texture. As it is readily carved, it has been used in French buildings for many centuries, and also can be seen in major structures elsewhere such as the Tower of London, Canterbury Cathedral, and the Old South Church in Boston.
In St. James Church, why does such an elite material appear directly adjacent to the lowly local sandstone? Certainly it would have cost a substantial amount to ship this stone from France to New Brunswick near the end of the age of sail, but that cost was not borne by the builders of this church. Rather, the stone window frames and door arch here are “remnants”: this Caen stone was part of a large batch shipped to Fredericton several decades earlier for the construction of the cathedral, and presumably the leftover stone had been in storage waiting for just this sort of use. So it was shipped downriver the 50 kilometres from Fredericton, for a fraction of the cost of receiving stone from the other side of the Atlantic Ocean.
As is the case for some other wonderful carving stones, Caen stone’s softness limits its ability to stand up to long-term weather exposure in tough climates. In Jemseg this stone has clearly suffered badly over the years, to the extent that some of it has been patched and much of it has been painted.
For the final piece of the geological story, a close examination of the church’s walls reveals a few interlopers: between the angular blocks of sandstone are occasional pieces of rounded sandstone and cobbles of grey or pinkish granite. These materials are similar to the other sandstone and granite in the walls, but geologically they have travelled through yet another process. They have been to “finishing school”, in the form of erosion, transport, and redeposition.
Some time in the relatively recent past these cobbles and small boulders were picked up from the bedrock where they had resided for several hundred million years. Perhaps they were transported by glacial ice, or maybe bounced around in a river system for a while, before being deposited in a place where people would gather them, such as in a gravel pit or in one of New Brunswick’s famously stony fields (the old story is that farmers often complained about “growing rocks”, as each spring the snow melt revealed a new crop of stones that had been elevated to the surface by the frost).
This blog post started out as a simple set of photos of the stone in this lovely church, but like the rocks in the fields it also grew. That is the fascinating thing, when you start to look into the geology and history of many old stone buildings: the linkages radiate outward in every direction, in ever-expanding circles. It is remarkable that a small church can store so much of the past, but all stone buildings, whatever their size, hold within them many stories – the history of the people who built them, the history of the people who quarried and transported the stones, and the geological pasts of the stones themselves.
In addition to the variety of sources linked in the text above, I also consulted print publications including:
Gregg Finley and Lynn Wigginton, 1995, On Earth as it is in Heaven: Gothic Revival Churches of Victorian New Brunswick, Goose Lane Editions.
William A. Parks, 1914, Report on the Building and Ornamental Stones of Canada, Volume II, Maritime Provinces, Canada Department of Mines.
If you wish to find it, St. James Church is located along NB Route 715, at 45°47’9.29″N, 66° 5’42.37″W.
© Graham Young, 2015
The Churchill part of the Hudson Bay Lowlands is replete with strange and obscure little corners. One of the strangest is this “playground that time forgot,” hidden away behind trees and boulders near the shore road between town and the airport. I assume that it was built back in army days and served for the entertainment of children who lived on the base. It is in the same area as the “golf course,” which always seemed a humorous name since this area of bog, boulders, and stunted black spruce is as far from a golf course as you can get, this side of the continental slope.
You might note that my photos of this playground are limited in their viewpoint and angles; these were constrained because I was shooting from the vehicle. When we arrived here we had just been surprised by an immense polar bear as we photographed roches moutonnées along the road, and it didn’t seem to be worth pursuing better photos if it meant that I might serve as the primary protein for a polar bear’s picnic.
Although it feels abandoned, someone seems to be maintaining this place at least in a basic way, so maybe it receives the occasional visit from children when the bears are absent. Or maybe the bears like a good seesaw.
The ancient teeter-totter and roundabout would, in all likelihood, not be considered acceptable in a city playground that adheres to modern safety standards. But given the frequent visits from Ursus maritimus, these are probably relatively minor issues if you are considering the overall safety of this particular pleasure park.
Happy New Year!
© Graham Young, 2015
Silurian Stromatolites in the Grand Rapids Uplands
On the long drive northward from Winnipeg to Grand Rapids, I always look forward to seeing one area not far beyond where the road rises onto the Grand Rapids Uplands. After the monotony of the “great bog” north of St. Martin Junction, the curves and slopes make a very welcome change. When I first visited Grand Rapids 20 years ago, this area had been freshly burned; it was a blackened waste similar to the one that followed the larger fire north of Grand Rapids in 2008.
The ever-changing appearance of the burn is also a great relief to the monotony, as over the years we have been able to watch saplings grow into trees, scorched trunks slowly replaced by a new forest. The regenerating plants attract wildlife, and we have seen deer, foxes, and a variety of birds there. One fall day, we even had a lynx pad across the empty Highway 6 in front of us – a rare sight indeed!
The reality of constant landscape change was brought home last autumn as I again drove over the southern end of the Uplands, this time with Dave Rudkin and Michael Cuggy. We were surprised to see that, in a place where there had been roadwork in recent years, the ditch had since been stripped completely bare by local floodwaters. What had been gravel the previous summer was now a gleaming white dolostone pavement. It didn’t appear to extend very far but it did look interesting, and we made a mental note of the location as we zoomed past. Somewhere to stop on the return drive from William Lake, if time and weather permitted.
Remarkably, both time and weather did permit on this occasion. After parking on the shoulder, we stepped down into the beautiful dry ditch. And there we saw a world of stromatolites – nothing but fossilized microbial mat structures covering the full extent of the bedding plane. How could it be that an ancient seafloor was entirely covered with cyanobacteria, those single-celled organisms that used to be called blue-green algae? Aren’t stromatolites supposed to be rare in any rocks that date from after the end of the Precambrian, their growth limited by gastropod grazing and seafloor burrowing by other creatures? And where are the snails and the other varied marine invertebrates, which can be seen in so many of the Paleozoic limestones elsewhere in this region?
Like the nearby burn, the ditch itself also tells part of our Earth’s story of ever-changing environments. This dolostone exposure may be small in area, but it represents a very interesting interval in geological time. In Manitoba’s sedimentary succession, stromatolites are very widespread in some parts of the Early Silurian Interlake Group. Some of the stromatolite-rich intervals were traditionally assigned to the Inwood Formation, but it has since been recognized that stratigraphic correlation of this unit is difficult, and former “Inwood” rocks are generally placed in the Moose Lake and Atikameg formations.*
Regardless of the technical correlation issues, the blooming of Early Silurian stromatolites in the Manitoba part of the Williston Basin was probably related to global patterns of extinction and evolution. The Early Silurian was the time immediately after the Late Ordovician mass extinction, the first of the “big five” extinctions in the history of life. During an interval about 443-445 million years ago, many families of marine life became extinct, some of them members of groups familar to fossil collectors: trilobites, brachiopods, and bryozoans. This extinction was associated with a glaciation on the South Pole, in a place that is now the Sahara Desert (I kid you not; you could look this up!). In Manitoba, fabulous Ordovician life forms such as the giant coral colonies in Tyndall Stone, and our beloved Churchill giant trilobite Isotelus rex, were replaced in the Early Silurian by . . . not much at all.
The post-extinction faunas around here are, as might be expected, not very diverse. But their lack of diversity can be compensated by remarkable abundance, such as in the nearly monospecific assemblages of the brachiopod Virgiana decussata, which can be seen near Churchill and along the riverbank near Grand Rapids. These stromatolites are also remarkably abundant: they were clearly the dominant life form in this area’s warm sea when the sediments we see were being laid down. Perhaps these stromatolites did so well because the water was too salty or too hot for gastropods and other invertebrates to thrive, but stromatolite blooms also occurred in other places following mass extinctions, to the extent that their abundance may actually be a diagnostic character for marine ecosystems that have been greatly disturbed.
Cyanobacteria are always with us, and maybe all it takes is the removal of grazers for them to return to Precambrian-like dominance. Which makes me wonder: how will the stromatolites do during the next few millennia? Not too well, I hope.
But enough of these morbid thoughts. In the present day, Dave, Michael, and I admire the ditch stromatolites. At first glance these are monotonous, but upon inspection they show considerable variation in size, shape, and surface texture. We snap a few photos and then it is back into the truck, wheels crunching on the gravel as we accelerate onto pavement and toward the “great bog.” Homeward.
* See Bezys and McCabe, 1996, Lower to Middle Paleozoic Stratigraphy of Southwestern Manitoba.
© Graham Young, 2014
Churchill River, Manitoba: August 24, 2014
Heading south from Churchill, the helicopter follows the river for the first 30 kilometres above its mouth. Then we head off overland, taking a straight-line shortcut instead of duplicating the river’s long dogleg. “Overland” is, perhaps, a bit of a misnomer, since the surface we pass over must be at least 20 percent open water, with much of the rest consisting of bog and moss, but as far as I know “overtundra” and “overmuskeg” are not words. Anyway, we fly low across this strangely-coloured otherworldly landscape for many kilometres, before rejoining the river’s course.
Coming over the steep bank, we can see that the river is still broad, but it is very different from the estuary you see at Churchill. Here the water rushes over its bed, with many treacherous shallows, boulders, and long stretches of rapids. After a stop to survey the cliffs beside Bad Cache Rapids, we are picked up again for the hop to Portage Chute. One hundred and twenty-five kilometres from the Town of Churchill, this will be the farthest we go during our several days of helicopter work in August, 2014. Around the steep rapids of Portage Chute we can see no landing place (the name “Portage Chute” apparently means “falls requiring a portage”), but a little way downstream a flat platform of bedrock extends from the cliffs on the river’s northwest bank. It is a perfect natural helicopter landing pad, and Frank quickly sets down. Once he gives us the all clear signal, we pile out and gather the packs, tools, and shotgun (never forget the shotgun, as there is still a risk of meeting polar bears even this far inland).
Portage Chute represents the beginning of Ordovician geology for this part of the Hudson Bay Lowland. South and west of here, everything is Precambrian for hundreds to thousands of kilometres. In fact, the granitic rock on which the helicopter rests is Precambrian in age, while the limestone cliffs beside us are Ordovician (Portage Chute Formation, Bad Cache Rapids Group, Katian [Upper Ordovician, in the range of 450 million years old]). We are standing on a great unconformity, one of the most spectacular geological contacts in the world. Our feet are on a surface that was eroded for more than a billion years, starting in a time when there was no complex life on this planet, while our hands can reach out and touch bedrock that was deposited as carbonate sediment on an ancient tropical seafloor during a time when marine life was reaching the peak of its first great diversification.
It is a magical place for a geologist, but it is also spectacularly beautiful, and for a few minutes I just stand there drinking it in. Not only is the exposure of the unconformity here almost too wonderful for the average geologist to believe, but it extends as far down river as I can see. In fact, Precambrian rock makes up the riverbed all the way to Bad Cache Rapids, some 20 kilometres away, while the Ordovician cliffs extend much farther than that!
Those Ordovician cliffs are also special for a paleontologist, because their mottled limestones hold wonderful examples of the fossil assemblage that has been sometimes called the “Arctic Ordovician fauna.” This biota includes a great variety of groups, and it is notably defined by receptaculitids, large gastropods such as Maclurina manitobensis, and cephalopods. In just an hour or so on the site, I am able to collect very good examples of all of these, and also corals (Calapoecia, Catenipora, Manipora, Palaeophyllum), brachiopods, and parts of trilobites; a rich haul!
The Arctic Ordovician fauna is widespread in central and northern North America; its best known occurrence is in the Tyndall Stone (Red River Formation, Selkirk Member) quarried at Garson, Manitoba, but it also occurs in places as widespread as New Mexico, Montana, the Canadian Arctic islands, and Greenland. The name “Arctic Ordovician fauna” was proposed by Dr. August Foerste in the 1930s as a shorthand for those Ordovician fossils in central North America that do not belong to the “Richmondian fauna” typical of the Cincinnati Arch area, and the concept was expanded upon by Dr. Sam Nelson in the 1950s.
Interestingly, Nelson carried out his early field research in 1950-51 along this same river we are visiting today, and his publications list a great variety of fossils from this very site.* This place is so inaccessible that I doubt anyone has done serious fossil collecting here since Nelson’s visit more than sixty years ago, which probably explains why there are so many fossils for us to find! With another hour or two working through these scree slopes, I’m sure we could easily equal his total of 46 species from the upper part of the Portage Chute, without even bothering to go down river to other sites such as Bad Cache Rapids. Still, the hour of collecting has given me a very good set of samples to take back to the Manitoba Museum, and perhaps we will return next summer for another visit.
Listening to the deep rumble of the water as it rushes past over the rapids, I consider Nelson’s traverse of this river with considerable admiration, since he did the great majority of it by canoe or on foot (he had to walk along this particular stretch of rapids). But now it is time to wrap up my fossils, and get them labelled and bagged before they go into the back of the AStar. Our fieldwork is ridiculously easy by comparison with Nelson’s, and I especially appreciate the padded seat as I nestle in by the window for our return to Churchill, with thoughts toward plans for a shower and a hot meal back at the Churchill Northern Studies Centre.
Still, miles to go, and all that tundra, river, and coast to contemplate. Will we see any bears this evening?
* See Nelson, S.J., 1963, Ordovician paleontology of the northern Hudson Bay Lowland, Geological Society of America Memoir 90, 152 p.
© Graham Young, 2014
There is really no way of knowing what the media, scientific or otherwise, will grab onto.
A week ago, at the Geological Society of America meeting in Vancouver, I presented a descriptive talk that may well have been the simplest I had ever given to a scientific audience. We had just listened to a series of presentations, many of them by students and postdocs, which incorporated considerable amounts of “big science”: sophisticated imaging techniques, chemical analyses of fossil preservation, or multivariate statistical studies of large numbers of specimens.
When I got up in front of the same audience, I was a bit worried because I realized that my talk could have been just as easily presented in 1914 as 2014: I was describing a single specimen, illustrated with photographs. Nineteenth century Natural History, really. But it was such a strange specimen that it seemed worth presenting, and as it turned out, the reporter from Science News thought so too (here is his short article, freshly out).
Those of you who visit this page occasionally will know that I am, perhaps, obsessed with the topic of jellyfish fossilization. In addition to ongoing detailed work on Ordovician-age jellies from the William Lake site in Manitoba, I have been collaborating with my colleague James Hagadorn of the Denver Museum to figure out the global fossil record of medusae.
Jellyfish are rarely fossilized; many things have been published as fossil jellies, but few of those actually are preserved medusae. So we have been working through the world literature of all papers published describing “fossil jellyfish.” As we have studied the literature, James and I have determined which museums hold collections that need to be examined, and when the opportunity arises we will go and spend a day or two on a collections visit. When necessary, we will borrow material for further study: some of the fossils are easily interpreted, but others are problematic.
The presentation in Vancouver was about one of the specimens we had found in the huge collections of the Field Museum in Chicago. Preserved in a slab of the Carboniferous Mecca Shale from Indiana, it looked like a blob of pure white quartz sand surrounded by thinly bedded black shale. Except this was a sand blob with tentacles.
Spending many days with the specimen, photographing it in every possible way (double polarized photography is our friend!), we were able to recognize many features that allowed us to identify it as a chirodropid cubozoan (a group of box jellyfish still abundant in modern oceans). It is the same age as the box jellies in the well-known Mazon Creek Lagerstätte of Illinois, and it is very similar to the Mazon Creek box jelly Anthracomedusa turnbulli, but preserved in a very different way. We explained its unusual preservation in this manner: A body of pure quartz sand is very unusual in the middle of a black shale bed; this resulted from sediment rafting by the jellyfish, a process analogous to ice rafting. The medusa apparently stranded on a beach, ingested sand as it attempted to free itself, and then was washed or rafted into a lagoon where it was buried in anoxic mud.
The scientific manuscript describing this remarkable fossil is almost complete. Maybe this publicity will motivate me to get it out the door and move on to the next batch of “fossil jellyfish”!
© Graham Young, 2014