I’ve been fascinated with stones ever since I was a little girl. These days, I love to collect them and spread them around my garden landscape. After moving into the region bordering the Van Buren and Allegan counties of Southwest Michigan, I became fascinated with the Septarian nodules “Lightening Stones” lying on the neighboring beaches of Lake Michigan! I have written about them along with approximately forty other beach stones found on these beaches providing photos, identifications and interesting facts in two separate photo essays HERE and HERE in case you are interested. Follow along below for photos and explanation about Septarian nodule’s formation and photos of Lake Michigan’s Pier Cove Beach and and sand formations along the Pier Cove Creek where it empties into the lake!
Formation of Septarian Nodules (Mystery Solved)
How did they form? Septarian nodules, often called “Lightning Stones” or “Turtle Stones” formed around 50-70 million years ago during the Cretaceous Period in shallow, subtropical sea beds.
The process forming Septarian nodule concretions (hard, compact, often spherical or oval-shaped mass) are a bit complex, but easy enough to understand when explained clearly.
Septarian brown stone concreations were created on ancient shallow seabeds when organic matter of mud, clay, and minerals such as calcium carbonate, arogonite and other minerals formed a mud ball. The mudballs became altered with an outer shell often composed of limestone which is made from coral skeletons, seashells and other dead sealife. When the seabed dried, the mudballs eventually dried, hardened somewhat and formed cracks or fissures (septaria) where mineral-rich water containing calcite (yellow) and aragonite (brown) crystalized inside the brownish grey limestone or mudstone parent material which often altered the color to reddish-brown.
As the mud balls continued to harden, water continued to infiltrate the cracked veins, septaria, and continued to deposited the yellowish-white calcite mineral composed mostly from seashells and corals. In time, the calcite mineral crystalized inside the veins. Other minerals can also infiltrate the veins such as barite, pyrite, or even chalcedony. Veins are highly variable in shape and volume
Septarian concretions did not form in Michigan during the Cretaceous period between 50-70 million years ago, but rather were transported from southern regions of the United States that were covered under shallow seas during the period, possibly Missouri. During the last Ice Age around 10,000 to 15,000 years ago, or prior Ice Ages since the Cretaceous period, massive glaciers, often a mile thick, scraped up the bedrock from southern regions and transported these rocks, depositing them across Michigan as the ice melted. This explains why they are found in local geological deposits including West Michigan lake shores in areas like Fennville, South Haven, and Van Buren Park.
- Calcite (CaCO₃): Fills the cracks or veins, creating yellowish or brownish areas.
- Aragonite: A less stable form of calcium carbonate, also present in some septarians.
- Barite: A secondary mineral that sometimes forms in the veins.
- Limestone/Mudstone: Forms the outer shell or parent material.
Interesting Photos of Snow Melt Sand Formations
With the rapid snow melt, unusual patterns in the sand created by the wind and frozen snow-cover are revealed. Much of the sand formations hold their shape temporarily due to the continue cold temperatures along the steep banks of the creek and the shoreline beaches of Lake Michigan.
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