Close up of Frontenac Axis

In this close up, you can see that Sixberry Lake is located near the SW edge of the Frontenac Axis, where the sedimentary rocks start to show up on top of the metamorphic rocks.  There are two geologic formations, the Potsdam Sandstone directly on top of the metamorphic rock, and the Theresa Formation of limestone and Dolomite on top of the Potsdam.  As the metamorphic rocks were metamorphosed by the Grenville Orogieny, about 1,100 million years ago, and the Potsdam is about 520 million years old, the contact between them represents a missing stratigraphic interval of about 700 million years!  This contact is visible at many places around Sixberry Lake.

Another thing to notice on this map is that the shapes and orientations of Millsite, Sixberry, and Lake of the Woods reflect the fabric in the metamorphic rock.  Just as the Thousand Islands resulted from the variations in susceptibility to weathering and erosion of the various types of metamorphic rock within the Frontenac Axis, the Indian River Lakes reflect the trends in the old metamorphic rock.