Lake Ontario - Geology

Geology

The lake basin was carved out of soft, weak Silurian-age rocks by the Wisconsin ice sheet during the last ice age. The action of the ice occurred along the pre-glacial Ontarian River valley which had approximately the same orientation as today's basin. Material that was pushed southward by the ice sheet left landforms such as drumlins, kames, and moraines, both on the modern land surface and the lake bottom, reorganizing the region's entire drainage system. As the ice sheet retreated toward the north, it still dammed the St. Lawrence valley outlet, so that the lake surface was at a higher level. This stage is known as Lake Iroquois. During that time the lake drained through present-day Syracuse, New York into the Mohawk River, thence to the Hudson River and the Atlantic. The shoreline created during this stage can be easily recognized by the (now dry) beaches and wave-cut hills 10 to 25 miles (15 to 40 km) from the present shoreline.

When the ice finally receded from the St. Lawrence valley, the outlet was below sea level, and for a short time the lake became a bay of the Atlantic Ocean, in association with the Champlain Sea. Gradually the land rebounded from the release of the weight of about 6,500 feet (2,000 m) of ice that had been stacked on it. It is still rebounding about 12 inches (30 cm) per century in the St. Lawrence area. Since the ice receded from the area last, the most rapid rebound still occurs there. This means that the lake bed is gradually tilting southward, inundating the south shore and turning river valleys into bays. Both north and south shores experience shoreline erosion, but the tilting amplifies this effect on the south shore, causing loss to property owners.