|GEOLOGY OF MISSOURI CAVES|
The Silurian Period of the Paleozoic Era occurred between 408 and 438 million years ago. The Silurian world was very different from the world of today. North America was shaped much differently and was located so that the equator passed through it. Much of North America was covered by a warm, shallow sea, called the Silurian Sea. This sea held coral reefs and marine wildlife. Some fossils can still be found today.
Limestone was deposited in the sea, layer upon layer, which geologists estimate began to form 450 million years ago, and is known as the Mississippian layer. The second layer is called the Jefferson and the third layer is the Gasconade Dolomite. It was primarily in this third layer that Missouri caves were formed.
Before the first big upheaval 42 to 46 million years ago, this shallow sea consisted of 60 feet of water over about 200 feet of silt. Caves began to form after this upheaval occurred. In all, six ice ages have occurred over North America at widely spaced intervals of geologic time.
Since the first big upheaval, there have been three tremendous earthquakes. Geological evidence indicates that each quake was lower in magnitude than the previous quake. According to historians, this more "recent" period would have taken place about 25 million years ago. We had our first earth upheaval 42 to 46 million years ago and our second upheaval 16 to 17 million years ago. The second quake predated the last ice age by several hundred years. The last, or third of these tremendous quakes was the New Madrid Earthquake in 1811-1812. It was the strongest recorded earthquake in the history of the North American continent but geological evidence indicates that it was lower in magnitude than the previous two quakes. The New Madrid earthquake began December 16, 1811 at a magnitude of 8.2. On January 23, 1812, a second shock of magnitude 8.1 occurred and was followed on February 7, 1812 by a magnitude 8.3 shock. Two hundred damaging aftershocks followed. The New Madrid earthquake was so violent the Mississippi River was forced to run backward for a period of time and changed it's course when it started to run toward the ocean again. Evidence of three earthquakes can be seen in Jacob's Cave.Evidence in Jacob's Cave shows there were formations broken from the ceiling of the cave by the truckloads during the second earthquake. The last ice age deposited two inches of clay on top of the broken formations.
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