As you walk northwards along the shore at Joggins Fossil Cliffs, you are walking backwards through time. As you look straight-on at the inclined rock strata exposed in the cliff face, the strata are rather like the pages of an open book, with the beginning of the story (or the chapter might be a better simile) told by the pages furthest to the left; and the the end of the chapter to the right. Each layer of rock has a different thickness and composition. Each stratum represents a different event or episode in geological history. Altogether, the Joggins Fossil Cliffs site comprises a 14.7 km coastal section with the majority of detailed stratigraphical and sedimentological studies being concentrated in about a 4 km section of the Joggins Formation.
Originally, the layers of rock were laid down in horizontal beds but, over time, earth movements have tipped them up so that now these are all lying at an angle of about 20 degrees. The conditions under which these sedimentary rock layers were laid down changed in a cyclical way. The location in which the sediments were formed was basically a coastal environment. Periodically the sea moved in and out. This meant that the place became either drier or wetter, more marine, more freshwater, or more terrestrial depending on the prevailing sea level.
When the sea level was high, the area was covered in shallow sea. This is when the limestone rocks were formed. When the sea level fell, the area become more terrestrial. The sea level changed according to such major phenomena as the movement of tectonic plates in the earth’s crust or due to eustatic land/sea level changes linked to the build-up or the thawing of ice sheets. The terrestrial environments were not always absolutely dry but were muddy swamp-like places with stands of vegetation including large tree-like plants.
The coastal strip was traversed by rivers that cut channels in the sediments and brought down large quantities of sand and other materials that could bury the coastal plants in times of flooding. This is where the sediments that formed the sandstone layers came from. In-filled river channels are preserved in the cliff face and are also visible at low tide as long lines of rock projecting from the surface of the shore as they extend out to sea. Mudstones and shales were also formed. Rapidly buried rotting vegetation and trees were sometimes converted under pressure to coal.
The rocks at Joggins belong to the Cumberland Group of the Langsettian in the Upper Carboniferous Pennsylvanian period. The rocks along the total 14.7 km stretch of cliffs are made up of four formations: from The Ragged Reef Formation, (the youngest to the south), then the Springfield Mines Formation, next the Joggins Formation itself, and then to the Little River Formation and Boss point Formation in the north. I was only able to walk from Dennis Point (SMF) to Coal Mine or Hardscrabble Point (JF) because of the state of tides and weather.
On my visit I found it impossible to work out which strata were which. However, since returning home and doing a bit of research, I have discovered that a sedimentological log has been recorded for the length of the Joggins Formation by Davies et al 2005. Each stratum was measured to the nearest centimetre and its composition, structure, and associated fossils described. This means that when I visit again, I will be well equipped to ‘label’ each stratum that I photograph – with a little help from the published diagrams, and hopefully an expert guide from the Joggins Fossil Cliffs Visitor Centre.
It has been possible to find out what some of the rock patterns, structures and textures represented. For example, there were many examples of preserved wave and current ripples patterns on beach stones and boulders as well as in bedrock layers:
Ironstone or siderite nodules formed of iron carbonate by precipitation in evaporating shallow water; and clay galls derived from dried mud polygons incorporated into sandstones are common.
Cross-bedding of sedimentary layers made by deposition from migrating streams and rivers; patterns of cracks in sun-baked muds; and conglomerates of various sorts with small pebbles and rock fragments brought down by running water and cemented into a matrix, were also seen.
Wrapped up within these layers of sedimentary rock are numerous, sometimes spectacular fossils, preserved in situ where they lived and died. The plant fossils include huge ‘tree’ trunks still rooted in their life position; fossil stems, roots, leaves, and diverse carbonised items of vegetational debris abound. Invertebrate fossils such as bivalved and gastropod molluscs are also found, as are pieces of giant millipede, dragonflies and whip spiders. Trace fossils of tracks left by Arthropleura millipedes and reptiles occur. In fact, the actual skeletons of small reptiles, such as Hylonomus lyelli – one of the first reptiles to evolve on earth, have been recovered from hollow tree trunks at this site. More about the fossils in the next post.
Calder, John (2012) The Joggins Fossil Cliffs: Coal Age Galapagos, Province of Nova Scotia, Department of Natural Resources, Crown Copyright, ISBN 978-1-55457-473-5.
Davies, S. J., Gibling, M. R., Rygel M. C., Calder, J. H., and Skilliter D. M. (2005) The Pennsylvanian Joggins Formation of Nova Scotia: sedimentological log and stratigraphic framework of the historic fossil cliffs, Atlantic Geology, 41, pp 115 – 142.
Joggins Fossil Institute, The Joggins Fossil Cliffs Field Guide.
Nova Scotia Department of Natural Resources, Nova Scotia Geological Highway Map.
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