The Dessau Chalk

Introduction
The Dessau Chalk is predominantly a chalky limestone to a well indurated chalk, with a number of well developed marl interbeds.  Through its nearly 28 meters of stratigraphic section, it varies in the degree to which it could be considered a classic chalk.  In the lower portion of the Formation, a highly distinctive lithologic change is not apparent between it and the underlying Jonah Formation.  An unconformity recognized approximately 1/3 above the base of the outcrop pictured in the following two pictures, separates these two units.  From this point the Dessau is most accurately described as a chalky limestone for the lower 8 - 8.5 meters of section.   A distinctive lithologic transition to a chalk occurs at this point.  This occurs near the top of the outcrop pictured below.  The upper portion of the Dessau contains two conspicuous layers characterized by the occurrence in great abundance of several types of oysters.  The lower of these is 5 meters in thickness (near the center of the chalky portion of the formation) and is dominated by great numbers of the oyster Pyrygia aucella.  Near the top of the Formation, another obvious increase in fossil abundance occurs.  Here two oysters, Exogyra laeviscula and Exogyra tigrina, become very abundant just below the irregular unconformity which separates the Dessau from the overlying Burditt Marl.

The lower Dessau Chalk

The unconformity which marks the Jonah-Dessau contact is present near the location where the slope is uncovered in the photo to the left below.  Here the white indurated chalk-limestone of the lower Dessau is visible.  The prominent, relatively thick, marly interbeds of the Dessau can be seen here.  Near the very top of this outcrop several distinctive marl seams produce a projecting layer which can be traced downstream.

Downstream...

the upper portion of the outcrop above can be tired to an outcrop exposing more of the section.  Here, along a moderately well exposed slope, the transition to the chalky facies of the Dessau can be seen.

The Chalky Facies...

can be seen more clearly at another location upstream.  Here the starkly white chalk of the Dessau is well exposed in an erosion gully.  About half way up the photo to the lower left, great numbers of Phrygia aucella occur quite abruptly.  This shell layer may be traced to another location, to move us higher in the section.

Tracing the Phrygia aucella layer...

Tracing dipping layers above Phrygia layer...

...to a nice exposure of the upper chalky facies of the Dessau.   Near the top of this outcrop two distinctive marl seams may be used to tie this section to another downstream, where the contact with the overlying Burditt Marl is visible.

"Lumps" of Exogyra ponderosa are found in the upper Dessau.

The two uppermost marl seams in the Dessau, refered to above, are seen in the two photos below. 

Directly overlying these seams, the layer characterized by great abundance of Exogyra laeviscula and Exogyra tirgrina is present.  This in turn is directly overlain by the unconformity separating the Dessau Chalk from the Burditt Marl.  This irregular surface is seen at approximately the position of the jacob staff (the wooden pole to you uninitiates) near the top left of the photograph below.

The following pictures also show the upper contact of the Dessau, at the approximate position of the break in slope seen below.