Geologic Map of US

This is part of the Geologic Map of the US, published by the USGS.  Most of Canada is underlaiin by the PreCambrian rocks of the Canadian Shield.  The Adirondacks (Adk's) are similar rocks of the same age.  Connecting the two is a region called the Frontenac Axis.  The ancient (1.1 billion years old) metamorphic rocks of the Frontenac Axis are responsible for the Thousand Islands as well as the Indian River Lakes, which include Sixberry.

View the region of the Frontenac Axis on the NY State Geologic Map here 

View a close-up of the region around Sixberry and Millsite Lake here.