UNDERGROUND VISIT – The Boulby mine was sunk in the 1960s/70s to exploit potash and other mineral deposits found during oil exploration in 1939. The deposits date from the Permian 290-245 million years ago when the Zechstein Sea covered much of Northern Europe from NE England to Germany. This tropical sea (the UK at the time was at a similar latitude to the current Sahara Desert) had a restricted access to greater oceans – similar to the Mediterranean Sea – which
resulted in high salinity and the deposition of evaporates.
The area was subject to repeated cycles of evaporation and inundation forming beds of evaporates with the least soluble at the bottom and most soluble at the top of the sequence. The last period of inundation led to the deposition of a layer of poorly consolidated clay marl which directly overlies the potash deposit.