Friday, January 31, 2020

The Michigan Puddingstone

Steven Wade Veatch

Michigan’s puddingstones are intriguing rocks that look like a glob of pudding stuffed with raisins, nuts and bits of cranberries. These white rocks, with small red, brown, purple and black pebbles, are not a Michigan product. During the last ice age, they hitched a ride into Michigan on an ice sheet and got off in the southern part of the state when the ice melted.

Fig. 1. An unpolished puddingstone from Michigan. Some contain trace amounts of gold and diamonds. These rocks are commonly found just after farmers plow their fields in Michigan.  Puddingstones were brought to Michigan by Ice Age glaciers. Jo Beckwith Specimen.  Photo by S.W. 
Puddingstones went through several steps in their formation (in what is now part of Ontario in Canada), before they went on their journey to Michigan. First, a network of rapidly flowing streams tumbled red and coffee-brown jasper, funeral-black chert, hematite and quartz in their churning water. Next, the streams deposited the material as sedimentary fill in eroded troughs and as alluvial fans, when the streams reduced their velocity and scattered the colorful pebbles onto mounds of sand (Lowey, 1985; Baumann et al. 2001). 

Then, the sand and pebbles hardened beneath the Earth’s surface and, over time, formed sedimentary rocks known as conglomerates (Slawson, 1933).  Later, intense heat and pressure metamorphosed the matrix of sand into a light-colored, coarse-grained, sugary-textured quartzite that tightly held the pebbles (Schaetzl, n.d.).  These geological forces formed the puddingstones around 2.3 billion years ago.

Today, geologists recognize these conglomerates as part of the Lorrain Quartzite of the Cobalt Series (Door and Eschman, 1970). This rock formation occurs as thick beds at Saint Joseph Island in Northern Ontario, Canada. The conglomerates also are found by the Saint Mary's River north of the Bruce Mines. This area is located 65 km (40 miles) east of Sault Sainte Marie in Ontario.
Puddingstones traveled south during the last ice age with the immense Laurentide Ice Sheet as it flowed at a glacial pace down from Canada. This ice plucked the puddingstones from the underlying bedrock, carried them hundreds of kilometers, and delivered those rocks to Michigan about 24,000 years ago.
This slowly advancing ice plowed across the landscape for thousands of years until rising temperatures, brought on by a climatic shift, ended their movement in Michigan. As the glacial ice melted, it deposited glacial till that contained the puddingstones. 

Today, farmers in the southern part of Michigan find puddingstones after spring plowing.  Since tightly cemented puddingstones can be cut and polished, they are in demand by Michigan artists and crafters, who make jewelry and ornaments out of them.  Puddingstones are commonly found as garden decorations that adorn Michigan homes and farms. People also collect and display puddingstones for their striking colors and appearance. 

Fig. 2. Since puddingstones are so hard, they take a nice polish as seen in this example. 
Steven Veatch specimen. Photo by S.W. Veatch.

In fact, as grandparents and parents take children outside to hunt for puddingstones, they pass an interest in puddingstones and geology down through generations of Michigan families. The tradition of looking for these goes back to the settlement of Michigan, and there is no sign of this interest ending anytime soon. 

References cited:

Baumann, S. D., J. T. Arrospide, and A. E. Wolosyzn, 2011, Preliminary Redefinition of the Cobalt Group (Huronian Supergroup), in the Southern Geologic Province, Ontario, Canada. Midwest Institute of Geosciences and Engineering, Chicago, Illinois, USA.

Door, J. A. and Eschman, D., 1970, Geology of Michigan: Ann Arbor, The University of Michigan Press.

Lowey, G.W., 1985, Stratigraphy and Sedimentology of the Lorrain Formation, Huronian Supergroup (Aphebian), Between Sault Ste. Marie and Elliot Lake, Ontario, and Implications for Stratiform Gold Mineralization, Open File Report no. 1154. Geological Survey of Canada, Ottawa, Canada.

Schaetzl, R. J. (n.d.), Geography of Michigan and the Great Lakes Region. Retrieved, from on January 22, 2020.

Slawson, C. B., 1933, The Jasper Conglomerate, an Index of Drift Dispersion. The Journal of Geology, Vol. 41, No. 5, p. 546–52.

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