Volcanic Formations
All volcanic formations in Missouri formed when the St. Francois Mountains formed 1.485 billion years ago. You can read more about that here. These formations are very rare and very unique. Below are a few areas that I have highlighted that have geological significance.
I will also say that I am NOT a geologist, just someone who has a slight fascination with rocks.
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Elephant Rocks
Elephant Rocks formed due to erosion of granite deposits resulting in the distinctive boulder formations resembling elephants seen today. More info about the park here.
Silver Mines Basaltic Dike
This is one of the most well known dikes within the St. Francois Mountain. It is located near the Dam at Silver Mines Recreation Area. A dike is when magna forces its way into a crack in existing rock and cools.
Shut-In Creek Quartz
This creek is located 750ft. beneath Bell Mountain and 550ft. below Lindsey Mountain. An old mineshaft can be seen in the western side of the creek. A vein of quartz runs along and into the mineshaft.
Hughes Mountain
Hughes Mountain is a formation called "columnar jointing". Volcanic ash deposits cooled and cracked similar in hexagonal shapes, similar to how mud cracks when it drys. More info on the conservation area here.
Millstream Gardens Dikes
At first I thought these dikes were snakes sunbathing on the rocks. Upon further inspection I realized they were small dikes that had formed in a zig-zag formation.
Lower Rock Creek Dike
This dike is most likely composed of basalt and is slightly smaller than the dike at Silver Mines. However, this dike has been seemingly broken in half due to external pressures surrounding the rock.