At
this site, you will be standing very near the point where the 45°
north parallel crosses the Door Peninsula, exactly
half-way from the equator to
the North Pole. There is a sign at the wayside, which describes the exact
location of the 45th parallel.
At this site you can closely examine the Silurian Dolomites that are visible throughout the Door Peninsula. Below is a detailed stratigraphic column
showing rock units found here.
Glacial erosion in the area left the relatively resistant dolomite outcropping at the surface throughout the peninsula. The Silurian rocks on the peninsula range in thickness from 0 to 500 ft thick in some places. The outcrop, particularly along the escarpment, and the deposits are generally covered by only a thin layer of unconsolidated soil material. Soil development in this area is generally less than 1 m thick throughout due to glacial erosion and removal of the majority of unconsolidated materials suitable for soil development (Stieglitz and Schuster, 1993).
A
general stratigraphic profile for the Door Peninsula consists of several rock
formations. First,
at the lowest position are the Maquoketa fossiliferous dolomites, limestones and
shales of Ordovician age. Next, the Neda Formation, thought to be of late
Ordovician age, appears to be inconsistently deposited throughout the Door
Peninsula. The Neda Formation is
thought to have been deposited during a transgression of the shoreline where
carbonates were deposited, or by a regression of the shoreline associated with
the deposition of orogenic muds. The
Silurian Dolomites, which consist of the Mayville, Byron and Hendricks,
Manistique, and Engadine Formations, were deposited during transgressional and
regressional phases which alternate throughout this time in geologic history.
The Mayville Dolomite, which is outcropping on the Peninsula is a
typically gray porous dolostone. The
Mayville Dolomite is the primary aquifer of the Door Peninsula.
The Byron Formation contains some fossils and reflects a shallow water
depositional environment of a regressional phase of the lake level.
The overlying Hendricks Dolomite appears to be a transitional phase
between the Byron and the overlying Manistique Formation.
The Hendricks Formation is densely laminated and interbedded with coarser
fossiliferous layers. The
correlation of the Byron and Hendricks Formations gives evidence of alternating
conditions between high and low lake levels.
The Manistique Formation overlies this transitional sequence and consists
of fossiliferous thinly bedded gray dolomite, containing some chert as well.
The youngest rocks outcropping on the Door Peninsula are thickly bedded
gray dolostones of the Engadine Dolomite, which outcrop on the eastern side of
the Peninsula. The Engadine
Formation is of late Silurian age and reflects an unconformity at the top
between what should be overlying Quaternary sedimentation eroded by subsequent
glaciation (Stieglitz and Schuster, 1993; Paull and Paull, 1998).
Photo by Alexa Teipel, 2002. Stratigraphic column from Stieglitz and Schuster, 1988.