Physiography of Lake Clear

This Did You Know story describes some of the reasons for the attractive physical features of the lake, many of which have been portrayed by famous artists such as A.Y. Jackson.

Big Rock, 1961, by A.Y. Jackson

The physiography of the area reflects profound geological events that can be traced back for more than one billion years.

A view of Lake Clear from Ryan Mountain

The high hills adjacent to the lake mark the trend of the ancient Petawawa fault along the western side of a rift valley running 435 miles from Montreal to near Sudbury. This rift valley was formed when the Earth's crust subsided about 1/2 mile (805m) between the Mattawa Fault (which follows the Ottawa River) and the Petawawa Fault.

Just south of the Petawawa Fault, Foymount's Ryan Mountain at 1716' (523m) is the highest peak in Renfrew County.

No Ordovician strata has been found in the Lake Clear area

(nor should there be any, since the area south of the Fault is within the uplifted block of Precambrian rock)

The Petawawa and the Mattawa Faults are two of many normal faults that define the Ottawa Valley. The Ottawa Rift Valley, also referred to as the Ottawa-Bonnechere Graben, was formed more than 570 million years ago during the Precambrian Eon when the first mult-celled animals began to appear. Lake Clear is in this ancient rift valley which almost resulted in the North American continent splitting apart along the Ottawa River with the result that today there could have been an ocean rather than a river lapping along the shores of Renfrew County. The cliffs around the lake provide visible reminders of the earthquakes and faulting occurring at that time. (see page 28, Nature in Your Neighbourhood Guide for additional information)

4th Chute

During the Ordovician geologic time Period, 510 to 440 mya age, when North America was under shallow seas, the first corals, primitive fishes and a variety of other sea life emerged which today can be found as fossils in the rocks around the 4th chute and elsewhere along the Bonnechere River (refer to: Ordovician geologic time period and at the Bonnechere Caves).

This erratic boulder along Manning Rd. was transported southward by the continental ice sheet during Pleistocene glaciation

During the Pleistocene Epoch (2.5 mya to 12,000 years ago) glaciers covered the area completely, leaving a significant imprint on today's landscape in the form of sandy soil, marine clay and large boulders.

The margins of ancient glacial and post-glacial lakes are recorded by successive raised, step-like beaches and wave-cut terraces at various elevations. The sand and gravel around the Sand Rd. waste transfer depot is an example of glacial sand deposits.

Outline of Lake Champlain at its maximum state, when the front of the continental ice sheet had retreated to the approximate position shown, just north of the present-day Ottawa River. Sites at which skeletal remains of whales have been recovered are shown by numbers (refer to Champlain Sea fossils

After the glacial retreat 12,000 years ago the Atlantic Ocean flooded the Ottawa-Bonnechere Graben forming the Champlain Sea. The unconsolidated marine sedimentary deposits of shell-rich sand and clay are related to this period. Remnants of salt from the sea still affects some of the wells in the region.

Additional information on glaciers and the Champlain Sea's effect on the Ottawa Valley can be found on McElroy's Glaciation of the Ottawa Valley.