Holding Golden Cap in your hand
There are 90 million years of time and several thousand metres of rock missing between these two fused layers. The grey rock is Blue Lias, which is Lower Jurassic in age, about 200 million years old. The sand above it is Gault/Upper Greensand, which is just 110 million years old.
This is the Great Unconformity which marks an erosion event in the middle of the Cretaceous period. Throughout the Triassic, Jurassic and early Cretaceous, sedimentary rocks formed layer upon layer. Then, about 110 million years ago, earth movements tilted the rocks to the east. Like a see-saw, in the east those rocks tilted down, but in the west they rose up and became exposed to erosion. Several thousand metres of rock were eroded away before the sandstone, a product of that very erosion, was laid down on top.The sand grains look like they may have come from the Otter Sandstone and there are also clasts (rounded pebbles of mud) that look like Mercia Mudstone. Both are Triassic age.
The second image shows borings on the surface of the limestone probably made by animals when erosion stopped and the sea deepened enough for them to live briefly, before being buried when sediments started to accumulate on the sea bed once again.
Golden Cap – the famous Dorset landmark – is a giant version of this specimen. The lower, grey cliffs, are Lower Jurassic in age, and the sandstone above is the Upper Greensand.
Find out more on the rocks and pseudofossils pages.