Unconformities are key surfaces in sedimentary basins - they represent gaps in time, periods of erosion and during which sediment was not deposited. Probably, the most famous one is Hutton's unconformity at Siccar Point (East of Edinburgh, Figure 1), where almost vertical Silurian sediments are overlain by gently dipping Late Devonian conglomerates and sandstones. It is known as the birthplace of modern geology (Figure 1), so theres is a lot of material on the internet. So here we'll look at a less well known but equally spectacular angular unconformity, further N at Arbroath (red arrow, Figure 1).
Its at the far end of the esplanade at Whiting Ness (NO 659 411), next to the car park. Best view of the outcrop (Figure 2) is from the foreshore but take care as the rocks are slippery. The red colour of the rocks is due to oxidation, the climate being hot and dry (desert). The unconformity separates the pebbly Upper Devonian Sandstones from the Lower Devonian Sandstones (Ramos & Friend, 1982); the Lower Devonian was uplifted, eroded and subjected to compressional activity during the Middle Devonian. Large structures such as the Sidlaw Hills (anticline) and Strathmore Syncline just S of the Highland Boundary Fault (Figure 3, 4), formed during the Middle Devonian (Trewin & Thirlwall, 2002). Sediment deposition in the Midland Valley resumed in the Upper Devonian by way of slow moving braided or meandering rivers. Along the Arbroath coastline, the Upper Devonian sands are very pebbly and conglomeratic in character.
Figure 1. Location of Scottish unconformities @ Siccar point and Arbroath (red arrow).
Figure 2 Angular unconformity (dashed white line) between near horizontal Upper Devonian (pebbly) sandstones and seaward dipping Lower Devonian Sandstones, Whiting Ness, Arbroath.
Figure 3. Steeply dipping beds of the northern limb of the strathmore Syncline, Just South of where the Highland Boundary Fault (at Stonehaven) enters the Sea.
Figure 4. Strathmore Syncline and Sidlaw Anticline within the Midland Valley of Scotland (From Tanner, 2008). Photo in figure 3 taken at eastern (coastal) extremity of the Strathmore Syncline at Stonehaven.
References
Ramos, A & Friend, P.F. 1982. Upper Old Red Sandstone sedimentation near the unconformity at Arbroath. Scottish Journal of Geology, 18, (4), 297 - 315.
Tanner, G. 2008. Tectonic Significance of the Highland Boundary Fault, Scotland. Journal of the Geological Society, 165, 915-921.
Trewin, N.H, & Thirlwall, M.F. 2002. Old Red Sandstone. p213 - 249, "The Geology of Scotland" 4th ed (edited by N.H. Trewin).