mudstone - hand/display specimen of a hard green Lower Cambrian mudstone from the Harkless Formation
This mudstone is part of the Lower Cambrian Harkless Formation, from the White Mountains of California. It is hard, so it will not crumble during student examination as will younger mudstones. With greater compaction this would have become a shale, which, instead of being blocky, has the clays mashed into more or less parallel laminations, allowing it to break into thin plates when exposed to weathering - or to a geologist's hammer.
Students should know the basic sedimentary rocks in order of increasing grain size, from mudstone and shale, siltstone, sandstone to conglomerate. They should know that a conglomerate composed of shells is a coquina.
Both mudstone and shale are composed of silt and clay-sized particles too small to see with the naked eye. Both are categorized as "mudrocks."
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