Joppa Shore Geology Walk
Figure 1. Adapted and annotated Google Earth image of the rock platform exposed at low tide at Joppa, Edinburgh. (Note text and figures in legend have been taken from the Joppa Shore leaflet produced by the Lothian & Borders RIGS Group).
Late on Tuesday afternoon, (15th September) I had the pleasure to visit one of Edinburgh's Regionally Important Geological Sites - Joppa Shore. The rock layers here are stacked like pages of a book, perpendicular to the shoreline (see above), and show the continuous sedimentation of changing depositional environments during part of the Carboniferous Period (approximately ~320-300 million years ago). My aim was to hunt for loose fossils along the shoreline, as you will read with limited success. NOTE: Joppa Shore is both classified as Regionally Important Geological Site (RIGS) and a Site of Special Scientific Interest (SSSI) so please do not hammer the bedrock. Fossil material can be found in loose material (if you are lucky).
Upper Limestone Formation
Figure 2. Dipping strata of the Upper Limestone Formation (NT 31789 73408).
The first rocks you encounter heading east from Portobello beach are those of the Upper Limestone Formation.
They are the oldest rocks exposed at Joppa and represent the deposition of fine grained muds and silts in a shallow marine environment (Figure 4.).
The top of this formation (aka the Castlecary Limestone Unit) is apparently fossiliferous however, I was unable to locate this particular unit or find any marine fossils in it.
Discoveries in the Upper Limestone Formation
Figure 3. Carbonaceous fossil plant material within a loose, grey mudstone cobble (NT 31772 73448).
Figure 4. Intercalated rusty orange sideritic ironstones within dark grey-blue horizontally laminated mudstone (NT 31810 73421)
(Scale ~13 cm).
The only 'fossil' material that I was able to find was within a loose, mudstone cobble at (NT 31772 73448). It consists of carbonaceous fossil plant material (see above) which was likely washed into the marine environment from a nearby land source.
At NT 31810 73421 (see left), the top of the Upper Limestone is well exposed. It consists of alternating strata - rusty, orange sideritic ironstones within a predominately dark grey-blue mudstone. These alternations show variations in sediment input within in the shallow marine environment. The presence of ironstones may suggest periods of increased flux of weathered, iron rich terrestrial material entering the marine environment.
Overall, the Upper Limestone Formation at Joppa shore can be interpreted as a marginal shallow marine environment with variations in the amount of sediment supply sourced by nearby land sources.
Figure 5. Dipping strata of the Passage Formation
(NT 31789 73408).
Overlying the rocks of Upper Limestone Formation are those of the Passage Formation.
These rocks are predominately composed of sandstones and mudstones and represent a different depositional environment - terrestrial river channels and floodplains.
This particular formation records an number of different geological features which are documented in Figure 1.
Discoveries in the Passage Formation
Figure 6. Mottled red palaeosol (fossil soil horizon) with reduced white rhizoliths (root traces) (NT 32033 73411) (Scale ~13 cm).
Figure 7. (left) shows a fault (red dashed line) at (NT 32085 73457) displacing an upper sandstone layer against a lower palaeosol horizon. Figure 8. (right) shows the same fault displacing a bedding plane (white dashed line) at (NT 32089 73451) (Scale ~13 cm).
Figure 9. Cross bedded sandstone at (NT 32144 73420) showing palaeocurrent towards the right (Scale ~13 cm).
The prevalence of red mudstones with mottled roots within the Passage Formation (Figure 6.) are evidence of well drained, fossil soil horizons.
This alongside cross bedded sandstones (Figure 9.) is evidence of deposition on a river floodplain environment under a somewhat arid climate regime.
These sedimentary rocks later experienced tilting and faulting which are responsible for the displacements observed in local rock layers (Figures 7 and 8).
Lower Coal Measures
Figure 10. Dipping strata of the Lower Coal Measures (NT 32200 73378).
Preceding the Passage Formation are the rocks of the Lower Coal Measures.
These rocks are predominately sandstones with rarer thin coal seams and represent a water logged, river floodplain depositional environment.
On my particular visit to Joppa shore, this formation was more poorly exposed and lacked any significant sedimentological variation.