The Geologic History of Cave Point

The Geologic History of Cave Point. . , How old are the rocks? , The rocks exposed at Cave Point are from the Silurian Period, a geologic time period which lasted from approximately 435 million years ago to 410 million years ago. , Where can the same bedrock be found? , The Silurian aged rocks in Wisconsin and throughout the Great Lakes region are represented by a series of dolomite layers. These layers are more resistant to erosion relative to other similar-aged rocks such as shale and sandstone that may be found in other areas of the state. The rocks in this layer are exposed as a line of bluffs extending throughout the Great Lakes. This line of bluffs is referred to as the Niagara Escarpment. The same series of dolomite is exposed in the Eastern Great Lakes at Niagara Falls. , How were the rocks formed? , During the Silurian Period, the area that is now Cave Point was located near the earth's equator. Coral reefs thrived in what was then a tropical sea. Over the millions of years throughout the Silurian Period, the remains of the marine plants, animals and calcareous sands were deposited and accumulated. These materials and the conditions that existed, followed by millions of years of compaction and chemical alteration, resulted in the formation of dolomite. This is the type of bedrock exposed at Cave Point. , How were the cliffs formed? , Approximately 410 million years after the Silurian Period, the most recent glaciation of the northern hemisphere took place as massive lobes of continental ice sheets advanced and retreated several times over the Upper Midwest. As the ice covered and uncovered the area, a series of ancient lakes came and went. As a result of scouring by these ice lobes and the drainage system they created, the modern Great Lakes were formed. This glaciation was responsible for much of the erosion that accentuated areas such as Door County and many of the prominent rock outcroppings in the Midwest. As the scouring action of the ice lobes removed the softer rocks and sediments, the more resistant dolomite remained to be attacked by thousands of years of erosion through wave activity. The jagged cliffs and sea caves you see here are examples of those wave-cut features. , Why does Cave Point look like it does today? , There are many forces at work in this geologically active area. The most obvious is not very hard to witness. As you look out into Lake Michigan you can see the waves crashing against the cliffs exposed here at Cave Point. The continuous act of erosion is slowly removing the bedrock where water and land meet. As water enters crevices and other openings in the rocks it collects and freezes in the winter. As the water freezes it expands and enlarges the openings until gravity does its part to remove the large sections. Water also erodes in a less visible but possibly more significant way termed dissolution, the chemical breakdown of the bedrock as a result of its reaction with water. Erosional processes have been changing the way Cave Point looks for millions of years, and will continue to do so in the future., [photo captions] , . Representation of a Silurian Reef , . Modern reefs such as those seen near the equator are similar to Silurian reefs formed near the equator 425 million years ago. , . Photography courtesy of Harmann’s Studio