Humberside Geologist no. 15

Whitby and the Cleveland Dyke

Hull Geological Society Field Trip for Yorkshire Geology Month

Sunday 3rd June 2007, led by Paul Hildreth B.Sc.

Only 5 people, and the leader, turned up for this excursion on what was a beautiful sunny day. Members of the select group had travelled from Hull, the Isle of Axholme and Wakefield.

Spring low tides gave us the opportunity to spend a comfortable two hours on the wave-cut platform between East Pier and Saltwick Nab. The leader indicated the mudstone-dominant rocks on the foreshore and the occurrence of ammonites, belemnites and the small nut-shaped bivalve Dacromya ovum. These indicated a marine environment and were deposited during the early part of the Jurassic period. Some time was spent looking for different types of ammonite (Dactylioceras and Hildoceras).

Further east, a brief study of the cliff section revealed the Dogger, about 10 metres above the base, overlain by thick sandstone deposits. In one place it was clear that a river had channelled its way through surrounding sandstone ‘flats’ scouring out a new course down to, and in places through, the Dogger. The group discussed the change in environmental conditions that these sandstones (Saltwick Formation) represented, an advancing delta. Fallen blocks of sandstone were examined; many showed cross-bedding, slump structures and ripples. Some beds were iron-rich (sideritic), others rich in plant fragments (?Whitby Plant Bed).

The group then moved along to Long Bight. Here it was noted that we had crossed the hinge of a syncline. The Dogger is well exposed as a gently dipping ledge and is easily accessible. The junction of the Dogger with underlying Whitby Mudstones was clear showing a pebbly base and borings into the lower beds, a classic disconformity. Some derived ammonites were present as ‘pebbles’ on blocks of fallen and overturned Dogger.

The group returned to Whitby harbour where the leader pointed out the sandstone cliffs at Khyber Pass on the west side. The group members were asked to recall where the Dogger had occurred in the East Cliff section and were informed that on the West Cliff it is at low water mark. The difference in level (about 16m) is accounted for by the Whitby Fault which is now occupied by the estuary of the River Esk.

After lunch the group travelled inland to Duckscar Quarry, Egton Bridge. Amongst the thick vegetation and aroma of wild garlic, the leader pointed out the silty sandstones of the Middle Jurassic country rocks then indicated the presence of much harder, more rounded blocks. Fresh specimens revealed a black, crystalline fine-grained igneous rock – the Cleveland Dyke – which had been removed for roadstone.

The group moved on to its final locality on the moors above Goathland avoiding the busy traffic bound for Adensfield! Here trenches scar the moorland surface where dyke rock has been removed. It was finally noted that the trend of the excavations (120 - 300°) is precisely the same as that of the dyke at Egton Bridge.

Paul Hildreth


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