Chandos Lake, prior to 1937, was called Loon Lake, and much of South Bay is actually sitting on the Loon Lake Pluton.
A pluton is basically igneous molten rock (magma) that intrudes through the earth’s crust, but doesn’t quite get to the earth’s surface before it solidifies. Think of an underground volcano. This pluton is younger rock than the Canadian Shield, because it intruded after the shield was formed. The Loon Lake Pluton‘s age is estimated at 1065 +/- 13 Ma.
Over time, because of erosion and glaciation, portions of the pluton become exposed. Below is a suspected exposed portion of the Loon Lake Pluton, on the east side of South Bay, about 100 feet inland.
The geology is quite varied in this region, and has been well studied, as evidenced by the maps in the following papers….The Chandos Township Geology paper is the easier read.