5-7 July 2013 Engineering Group Field Trip – Engineering Geology of the Solway Firth

Led by Dr Alan Gibbs

This year’s fieldtrip marked a departure from past excursions: a new convener, a new leader, and a new country. The trip started for some with pre-dinner swim at the Cairndale Hotel and Leisure Club in Dumfries. Following a sumptuous four-course meal, we gathered together for an introductory lecture from Alan. In a twist on the classic dictum, Alan’s talk “The Past is the Key to the Present” led us through the history of the Solway Firth and its wider context, using little more than a geological map to show us how events in the Carboniferous continue to shape the modern landscape and geology.

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Standing on the shore of the Solway Firth the following morning, with the hills of the Lake District rising through the haze, it was easy to imagine standing in the same place 400 million years earlier gazing across the closing Iapetus Ocean. We had started out at the eastern end of Gillfoot Bay and Alan challenged us to a simple task: “follow a layer of the rock until your feet get wet”. We duly did so and those paying attention avoided wet feet by following the layer around a tight angular fold. “How did that get here?” came the question, and so began a day of challenging geometry and enlightening insights. Showing how the rapidly changing nature of the rocks – from limestones to sandstones and even coals – directly reflected the modern environments – estuarine channels and sandbanks, to onshore mudflats – Alan began to elucidate the meaning of “the past is the key to the present”, interspersing geological truths with tales of pirates and witches. Following the coast, we were shown how fluid escape features in the rocks indicated the original vertical, as well as a range of slump structures and fossil organisms.

We drove on to Rockcliffe, observing the path of an esker en route, and hiking south for lunch on the cliff top at the edge of Auchencairn bay. We then descended to the shore to observe one of the principle basin-bounding faults, the North Solway fault. The first point of interest is that the cliff itself is the exhumed fault zone: the past is again the key to the present. The second point is that the fault is not a simple linear surface; in places it has been exploited by porphyry dykes and it is offset at intervals, the nature of which would become apparent later. Where exposed, the fault surface displays a range of puzzling features. Conglomerate containing locally derived angular and rounded cobbles infilling cracks in the porphyry and Silurian country rock, indicating that the modern ground surface closely parallels the Carboniferous ground surface, an inference confirmed by the nature and dip of the basin-fill sediments adjacent to the fault scarp. These again consist of angular conglomerates dipping steeply seaward, with minor syn-sedimentary faults, dewatering structures, and repeated marine transgressions typical of active normal faulting.

A further hike east past Gutcher’s Isle, where Gutcher attempted to leap across to the stack to impress a farmer’s daughter, with fatal consequences, led us to one of the most geologically spectacular sights of the trip. Here, the porphyry dyke traces a relay ramp between two sections of the North Solway Fault, sandwiching a section of Silurian sediments on the ramp. The path of the dyke beautifully illustrates how an apparently simple regional structure can result in local complexity; it was not hard to imagine the difficulties that would result from encountering such a change in a series of boreholes on site.

The Sunday morning once again started with a long coach journey west, to the far side of Auchencairn bay, along a spectacular cliff top path and down to a complex coastal section. Once again, the cliffs bound both the ancient and modern basins and a view along them, and hence along the basin margin, reveals the several offsets similar to those at Gutcher’s Isle, itself visible across Auchencairn bay. Traversing along the shoreline, footwall conglomerates are evident, interbedded with finer-grained marine sediments, again indicating the discontinuous nature of fault slip. The fault zone is brecciated as at Rockcliffe but here there are kinematic indicators for oblique slip. A steep scramble up the cliff led to lunch, and a return to study the interbedded sediments. The engineering implications of their variability, particularly in relation to water flow, is only too apparent.

The day ended with a visit to a former copper mine, its 40 m shaft covered only by a boulder. Mineralisation is common in these tectonic environments and was economically important in the area throughout Britain’s industrial heyday and may soon be so again, given the continued rise in demand for copper, gold and other metals.

We returned to Dumfries in time to see Murray victorious at Wimbledon and celebrated a challenging, insightful and thoroughly enjoyable trip.

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