The rocks that form the Canadian Shield were formed about two billion years ago, when they were under water in the Precambrian era. All of P.E.I. has carboniferous cover including Pennsylanion-Permain sandston with coal deposits. The red sedimentary rocks of the late Pennsylvanian to Early Permian periods were deposited by rivers 300 million years ago as part of a large flat floodplain that covered Eastern Canada.
The rocks and soils that were formed above these ancient layers were removed by the scouring action of glaciers that covered northern North America in the several ice ages in the past 100,000 years. The Island is the result of three chunks of land ending up as one landmass, and today you can see a central section, with two "wings" to the sides, most noticeable at the 6 km wide isthmus at Summerside.
The Charlottetown Plain, Afton Plain, and Covehead Plain glacial deposits in the form of small hills or ridges called "kames" and winding ridges of sand and gravel known as "eskers" are geologic points of interest along the river. The mineral soils are developed from glacial till, glacial alluvial materials and marine, eolian and lacustrine sediments, derived mainly from the underlaying sedimentary rocks. A red to reddish brown glacial till loam covers the greater parts of the Island, usually about 20 inches thick. (Note to visitors: the rust in the red sand will stain clothing, so be cautious when sitting)
The sedimentation and lifting process along the Hillsborough River has resulted in the cliffs protecting the mouth of the Hillsborough and Charlottetown Harbour. The salt marshes and associated wetlands resulted from changes in sea level over the centuries. The tidal salt water marsh, fresh water marsh, uplands and bogs and the variations in between support a diversity of wildlife significant in the region. There are 57 rare vascular plants within the watershed, of which 25 are wetland species.
More history of Charlottetown