Map layers associated with the Marcellus Shale in the interactive mapping system include various Onondaga map layers. This is confusing to many users--i.e., why don't we use maps showing the Marcellus structure? The answer is simple--because we don't yet have those specific map layers for the Marcellus Shale. In order to provide this geological information, we are using map layers created for adjacent stratigraphic units to provide approximately similar information as for the Marcellus. Structure Map Layers - The Onondaga Limestone and equivalent units underlie the Marcellus Shale. The top of the Onondaga Limestone and equivalents is approximately equal to the base of the Marcellus Shale. So, a map showing the structure on the top of the Onondaga Limestone provides essentially the same information as a structure map on the base of the Marcellus Shale.