Mulligan Île O'Connor Île Oscar-Béchamp Île Oster Île Paquin Île à Payne Îles Pelley Île Philemon Île Pigs Île aux Pins Île des Quinze Île Rainville Île des Rapides Île du Refuge Île Reid (Clarendon) Île Reid (L'Isle-aux-Allumettes) Île Rita Île à Ritté Île du Rocher Fendu Île à Rouleau Île à Roussin Île de Sable Petite île Sèche La Semelle Île Smith Île Snake Île des Soeurs Île Soulier Île Squelette Île Submergée Île Sunset Île Todd Île à Tom Île à Tom-Simon Île aux Tourtes Île Verte Île Victoria Île Wickens Île Winneway Île Woods Île Wight Île Young (Pontiac) Île Young (Gatineau) Île Yvette-Naubert Geology The Ottawa River lies in the Ottawa-Bonnechere Graben, which is a Mesozoic rift valley that formed 175 million years ago. Much of the river flows through the Canadian Shield, although lower areas flow through limestone plains and glacial deposits. As the glacial ice sheet began to retreat at the end of the last ice age, the Ottawa River valley, which, along with the St. Lawrence River valley and Lake Champlain, had been depressed to below sea level by the glacier's weight, filled with sea water. The resulting arm of the ocean is known as the Champlain Sea. Fossil remains of marine life dating 12 to 10 thousand years ago have been found in marine clay throughout the region. Sand deposits from this era have produced vast plains, often dominated by pine forests, as well as localized areas of sand dunes, such as Westmeath and Constance Bay. Clay deposits from this period have resulted in areas of poor drainage, large swamps, and peat bogs in some ancient channels of this river. Hence, the distribution of forests and wetlands is very much a product of these past glacial events. Large deposits of a material commonly known as Leda clay also formed. These deposits become highly unstable after heavy rains. Numerous landslides have occurred as a result. The former site of the town of Lemieux, Ontario collapsed into the South Nation River in 1993. The town's residents had previously been relocated because of the suspected instability of the earth in that location. As the land gradually rose again the sea coast retreated and the fresh water courses of today took shape. Following the demise of the Champlain Sea the Ottawa River Valley continued to drain the waters of the emerging Upper Great Lakes basin through