PENINSULA STATE PARK

EAGLE BLUFF OBSERVATION TOWER

As you look southeastward from the top of the tower you will see Eagle Harbor, the town of Ephraim, and, on a clear day, across the Peninsula to Lake Michigan.  As you look eastward from the top of the tower, you will see the dolomite bluffs at Sister Bay about 3 miles in the distance, and the bluffs at Ellison Bay about 8 miles in the distance.  Ellison Bluff County Park is also visible, where the dolomite bluffs will be closely observed. As you look northward from the top of the tower, you will see the bluffs at Nicolet Bay and Horseshoe Island.  These are both part of Peninsula State Park.  Horseshoe Island lies about 1 mile north of Nicolet Bay, and is named for its distinctive “horseshoe” shape.  The island’s first owner was a Norwegian immigrant by the name of Ole Larson, who reportedly traded a few pounds of tobacco for it in the 1850’s.  The Folda family purchased the island from Mr. Larson for $500 in 1888 and established an impressive summer lodge on the island that they called “Engelmar”.  The Folda’s attempted to sell their establishment to the State of Wisconsin for $5,000 in 1909 when the park was established, but they reportedly never received payment.  Today, the island is open to visitors who are adventurous enough to get to the island by boat, which is the only means of access.  Visitors can take the Engelmar Trail, a one-mile rustic path that meanders the circumference of the island. A pit toilet is available for visitors.  Visitors are expected to pack out their own garbage.  Camping and fires are prohibited on the island (Peninsula State Park website, 2000).

TENNISON BAY BOAT LAUNCH

This site offers a close-up look at the Silurian dolomite rocks that form the Peninsula and the fossils found in them. 

The rocks that form the Niagara Escarpment were formed during the Silurian period of geologic time, between about 425 and 405 Ma.  At that time in geologic history, the North American continent was located very near the earth’s equator, and the global climate was much warmer than that of today.  A shallow, warm, salt-water sea covered much of the North American continent during that time, including the area presently identified as the state of Wisconsin.  Many different ancient life forms, such as corals and brachiopods thrived in this broad, tropical sea.  Enormous coral reefs formed in which these life forms lived and died.  Upon death, the broken and decayed remains of these creatures accumulated in the many nooks and hollows of the coral reefs.  A constant settling of microscopic particles filtered through the ocean waters, filling every space with fine material.  The sands that formed were made of microscopic particles of this calcareous material.  Far from the reefs a white, limey mud covered the ocean floor.  The rocks that make up the Niagara Escarpment were formed as this calcareous material collected on the sea floor and was compacted for over 25 million years.  The thickness of these rocks reached more than 800 feet in some areas.  Initially, limestone formed in the clean Silurian waters, and then was chemically altered and compacted to form the dolomite that is now exposed on the Green Bay side of the Door Peninsula (Langolis and Njaa, 1989)

Specifically, the myriad of fossils of ancient organisms that lived in the Silurian sea that covered this area of North America are found primarily in the Schoolcraft and Cordell Dolomite formations, which together form the Manistique Group in the stratigraphic column of the Door Peninsula. 

The Schoolcraft Dolomite directly overlies the Hendricks Formation of the Burnt Bluff Group.  Kluessendorf and Mikulic (nd) describe the Schoolcraft as consisting of thick- and uneven-bedded, coarsely crystalline, cherty dolomite containing silicified corals and pentamerid brachiopods. Pentamerid brachiopods are also common in the Cordell Dolomite, which directly overlies the Schoolcraft Formation.  It is described by Kluessendorf and Mikulic (nd) as a dark gray, thin- bedded, coarsely crystalline dolomite, containing abundant silicified fossils, primarily of Pentamerid brachiopods.  Also very common in the Cordell Dolomite are the tabulate corals, Favosites, Halysites, and Syringospora.  These corals have been found to occur in colonies as much as three feet in diameter and one foot high, and in association with other tabulate corals, horn corals, bryzoans, crinoids, cephalopods, and trilobites.

The fact that the Cordell Dolomite contains such a wide variety and abundance of fossils of these ancient organisms is highly indicative of the existence of a reef-like environment at the time that the limestones that would eventually become the Schoolcraft and Cordell Dolomites were deposited and formed (Kluessendorf and Mikulic, nd).  

These Silurian-aged fossils tend to be erosion-resistant because they have been silicified, and so they get separated out in cobbles and boulders as the surrounding dolomite groundmass is removed by chemical weathering.  As a result, numerous specimens of these fossils, especially the corals and brachiopods, can be found on almost any gravel or beach deposit on the Peninsula (Niagara Escarpment Commission).