These are critically endangered microbial structures. Thrombolites-building micro-organisms resemble the earliest form of life on Earth. These organisms were the only known form of life from 3.5 billion to 650 million years ago. These are some of the earth's most primitive life forms. Thrombolites (meaning clotted structure) are large bun shaped Cambrian mounds weathering out of flat lying dolostones. They were the growth form of millions of tiny algae and bacteria. These structures are not exactly fossils, but they are evidence for biological activity. These unicellular critters have left a good size trace of their existance (sic) in the fossil record. Thrombo, meaning clotted, indicates an internal structure without lamination. The darker colored, more rounded boulder is a glacial erratic brought here during the Pleistocene glaciation. The furrows, that contain mud-cracked material and radiate from the centre and down the sides, may be drainage channels. These organisms are thought to have thrived in the tidal and subtidal zone of a warm, very salty sea, some being exposed at low tide, and covered at high tide, thus explaining the mud cracks. The larger one may be several communities that amalgamated as they grew.
When you look across the bay to the south, you can see another large colony of Thrombolites on the shore. You can walk out to the small point where there is a fish hut built on a collection of mounds. Here the Thrombolites are not standing as high, but they are more numerous.
These structures are very, very rare. One other place in which they grow is Hamelin Pool, Shark Bay, Western Australia.
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