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Newsletter No. 202

The Black Country Geological Society

August 2010

The Dudley Bug

Welcome

Hello and welcome to the August issue of the Dudley Bug. It's summer but sadly the weather isn’t that great at the moment! For those of you jetting off somewhere exotic, or to those staying a little closer to home and prefer the feel of home rocks below your feet, may we take this opportunity to wish you all a great summer! This issue we are going back to basics and giving you a quick introduction to clastic sediments. We are also giving you a guide on caring for your collections whether it may be a few bits you picked up recently or your large collection in the loft! And finally we welcome back the word search and give you on an update on the ‘Dudley Bug Geo-photo Competition!’ We hope you enjoy this issue!

Alison and Chris

Clastic Sediments

Clastic sedimentary rocks are formed by the amalgamation of small fragments of different minerals. Quartz is one of the most common minerals found in clastic sediments. The minerals have come from rocks which have been broken down through the process of weathering. The majority of clastic rocks form under water.

Fine grained clastic sediments make up between 45% and 55% of sedimentary successions. They vary in size from <4µm (clay) to 4-64µm (silt). Mud is a mixture of both clay and silt. Four main rock types make up the fine grained clastic group. These are mudstone, shale, argillite and slate. They are more commonly referred to as ‘mudrock’ between geologists. These four do have their differences. Mudstone tends to be blocky and non-fissile whereas shale is laminated and fissile (breaks along a plane). Argillite is indurated between shale and mudstone and finally slate is indurated so that it develops a cleavage.

Within a mudrock various structures can form. If the sediment was deposited in a pro-glacial lake then very fine laminations may form known as varves. But laminations can also occur in other, non-glacial environments such as estuaries and flood plains. Quite often a mudrock can have massive structures due to slumping, bioturbation, dewatering and various forms of flow.

The colour of mudrocks can give you a clue to their history. For example a dark grey, almost black mudrock will have a high amount of organic matter within it whereas a mottled grey mudrock may have been affected by bioturbation.

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Here in the West Midlands we can pick up strange bits of red, very fine grained rock which has a strange appearance. At first glance it looks like leaves or feeding trails. This is known as cone-in-cone structure. Geologists aren’t 100% certain what forms cone-in-cone because it only forms under certain conditions. It is believed that when a fine grained mudrock is put under high amounts of stress it will reach a point when it suddenly deforms. This movement creates small faults which form the strange surface seen on cone-in-cone. Another theory is that calcite grows in its fibrous form and forces the sediment in which it is located out of the way. ■

Clastic Rock Word Search

Can you find the following words?

  • Clastic

  • Clay

  • Cone-in-cone

  • Fine

  • Grained

  • Mudrock

  • Silt

  • Varves

Answers in the next issue of the Dudley Bug!

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