MINERAL OF THE MONTH
August 2016 - Puddingstone
In central Ontario and eastern Upper Peninsula of Michigan, in the area around northern Lake Huron, rockhounds can find puddingstones. These jasper conglomerates formed during the Huronian Period of the Proterozoic Era around a billion years ago. Erosional forces were dramatic at the time since vegetation had not yet evolved, so exposed rock quickly wore down. Sediments from this erosion were deposited on the bottoms of large bodies of water including sand as well as red jasper pebbles and other small stones. Over time the weight of the piled sediments caused the components to cement together forming large masses of conglomerate rock. The conglomerate masses subsequently eroded and broke apart. Some of the loose fragments and puddingstones were distributed around the area by the Labrador ice sheet.
The Puddingstone was named by British settlers who lived along the St. Mary’s River and Northern Lake Huron. They believed it looked like Christmas pudding. Puddingstones range in size from a small pebble to boulders larger than trucks!
Puddingstone conglomerate rocks are made up of a mixture of different, irregular sized grains and pebbles held together by a finer matrix, usually formed from quartz sand. The sedimentary rock is formed in river channels and may contain various minerals such as chromite, corundum, platinum, diamond, gold, sapphire, and zircon. The rounded imbedded pebbles have a sharp contrast in color to the rock’s matrix. In addition to the puddingstone rocks found in Michigan and Ontario, there are different types of puddingstone, with different composition, origin, and geographical distribution found in:
Hertfordshire puddingstone, principally found in Hertfordshire, England Bearfort Mountain puddingstone, is a purple puddingstone found in northern New Jersey.
Roxbury puddingstone, principally found in and around Boston, Massachusetts.
Plumstead Common, located southeast of London, England, has a fine example of puddingstone, probably left behind after the last Ice Age.
James G. Kelley, http://www.drummondislandchamber.com/index.php
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