The dyke can be viewed from the western end of the North Pier but to see it close up it is necessary to go on to the shore which is very rocky and strewn with boulders. Access is available from the end of the North Pier but care needs to be taken when walking on the shore and an eye kept out for the in-coming tide.
Depending upon the weather conditions, the pier may be closed and it may be inadvisable to go on to the shore. If this is the case, or the sea is very rough, the cache should NOT be attempted.
A geological dyke is a long mass of igneous rock that intrudes fissures through the structure of adjacent rock. This implies that the dyke is always younger than the rock that it is cutting through. The sub vertical basaltic Tynemouth Dyke cuts through a shore platform of sandstone and is one of a number cutting through the area that were formed during the Tertiary Period about 58 million years ago. These dykes were created by volcanic lava flowing through to the surface as a result of crustal tension centred in the area of what is now the Isle of Mull in western Scotland. The coarse dark red mottling of the buff coloured Carboniferous sandstone of the shore platform and lower cliff was caused by desert weathering during the Permian Period (290-248 million years ago).
The cliff behind the dyke demonstrates an example of an unconformity where the contact surface between younger and older rocks represents a missing interval in the geological record. An unconformity is usually produced by the erosion of deposited continuous strata followed by renewed deposition. In this example we have the very conspicuously cross-bedded Permian Yellow Sands lying unconformably on the Carboniferous (coal measure) levels.
Also of interest at this site are the boulders on the shore. Many are of Scandinavian schist and gneiss and are thought to have been deposited following the last Ice Age 10-15 thousand years ago. Many could also have been brought to this area as ballast on ships entering the river Tyne.
Tasks:
1. This is an optional task. Take a photograph of the dyke which includes yourself and/or your GPS.
2. What is the approximate width and direction of the dyke?
3. Approximately how thick are the Permian Yellow Sands?
4. What words might appropriately complete the following definition of an unconformity? “A boundary separating two or more rocks of markedly [---------] ages marking a [---] in the geological record.”
By all means include your photograph with your log but not your other answers which should be emailed to the cache owner.
This EarthCache has been placed with the co-operation and permission of Natural England and North Tyneside Council.