Field Trip to Ottawa Area 2009
Every year, the first-year students in our foundation course can look forward to a field trip to Ottawa and area. The goal of this trip is to introduce the students to some of the geology of the Ottawa region, including exposures of sedimentary, igneous and metamorphic rocks. The trip will help students formulate a suitable approach to geological field investigations, and direct their eye to large- and small-scale features that are exposed in the rocks. At each stop, there are lots of opportunities to discuss how geological observations provide the basis for inferring the origin and history of the rocks.
The Ottawa Valley lies in the northwestern part of the Ottawa-St. Lawrence River Lowland, and is underlain by Palaeozoic sedimentary rocks, specifically Ordovician (500 to 430 Ma) in age. These sedimentary rocks lie unconformably (on an erosion surface) on a basement of Precambrian igneous and metamorphic rocks that formed during the Grenville Orogeny (ca. 1.3 to 1.0 Ma). There are obvious differences in rock types, bedding characteristics, textures and fossil assemblages between Precambrian rocks at the first stop and Palaeozoic rocks at the subsequent two stops.
Palaeozoic strata are mostly horizontal on top of the Precambrian rocks except in the vicinity of faults where they are tilted. On campus, students can see an excellent example of this at the rapids in the Rideau River opposite the Herzberg and River Buildings. This fault (inactive) runs right under the River Building. In the Ottawa area, the thickness of sedimentary strata decreases from east to west from about 650m to 60m. These differences in thickness are primarily due to erosion of the units after faulting. Faulting has been related to subsidence of the basin in which these rocks were deposited, and later development of a northwest-trending structural depression that extends from the St. Lawrence River through Ottawa to the Lake Timiskaming area – the Ottawa-Bonnechere graben. Two prominent faults include the Eardley Fault (along the south side of Gatineau Park) that forms a boundary approximately 200m high, and the Gloucester Fault that cuts across the Carleton campus and is exposed in the Rideau River. Development of this graben re-activated older faults, and is thought to have formed around 115 million years ago. Small motions on the faults in the Ottawa area make earthquakes, but fortunately they are usually small (< magnitude 4) and only rarely cause damage.