This is an accessible EarthCache on the North East coast in the Seaburn district of Sunderland.
If you visit the posted coordinates you will be able to locate the 'Cannonball Rocks'.
These intriguing geological features are spherical rocks formed from limestone that resemble cannonballs.
Limestone formation
Limestone is a sedimentary rock. It forms predominantly on the sea floor where material rich in calcium carbonate accumulates.
The Magnesium Limestone seen here was formed 260 million years ago. It was deposited in a shallow inland sea called the Zechstein Sea.
The limestone originated as soft lime mud, shells and other fragments of the organisms living in the water ; when these sediments were buried under others, they were compacted, the water squeezed out of them and, in some cases they were physically or chemically changed by the pressure.
Later, mineralised waters percolate through and minerals precipitateinto the pore spaces, cementing everything together to form a rock.
Limestone concretions
Concretions are solid, rounded masses of mineral matter that occur in sedimentary rock. Concretions have a different composition from the rock in which they are found. An example is a limestone concretion found in a bed of shale.
Concretions form after the material around them has been deposited. The concretion hardens into solid rock while the surrounding bed still is somewhat soft. The concretions formed in the limestone after it was deposited and are made up of the mineral Calcite.
Sunderland's cannonballs
The coast of north east England between South Shields and Sunderland provides an almost 10 kilometre long continuous exposure of Magnesian Limestone. These unusual rocks have attracted attention since the earliest days of geological science.
Perhaps best known of these is the ‘cannonball limestone’, so called from the presence of almost perfectly spherical concretions which range in size from a few millimetres across to ‘cannonballs’ over 0.3 m in diameter.
These 'Cannonball Rocks' are exposed in the cliffs from Trow Point to north of Seaham, and in quarries inland, most spectacularly at Fulwell.
The task
1. At the posted coordinates you will find numerous examples of the cannonball limestone formations.
Describe the texture of the rocks and there shape and sizes.
What is the largest you can see ? Why do you think they differ in size ?
2. Using the information within the listing, what kind of environment do you think the cannonball rocks were formed in ?
3. In which geological period were the cannonball rocks formed ?
4. OPTIONAL
Take a photo of yourself with some of the cannonball rocks in the background.
.