Geology of Port Hope area
The Port Hope area is divided into two physiographic regions by the Lake Iroquois shoreline and raised beaches. In the northwest, drumlinized uplands, underlain by Bowmanville till and draped by glaciolacustrine sediments, are truncated by the Iroquois shoreline. These drumlins likely record a catastrophically released subglacial meltwater erosional event directed toward the south-southwest or they record an ice erosion event. The southern margin of a flat-topped delta is present in the northwest. The remainder of the area is the low-relief Lake Iroquois plain. This plain is covered by glacial lake sediments which in-fill the underlaying surface leaving the tops of SSW-oriented drumlins as till islands. Both regions are dissected by creeks that have their headwaters in the Oak Ridges Moraine or drumlinized uplands to the north and discharge into lake Ontario.
Bedrock Geology, Topography and Drift Thickness:
Bedrock is exposed along the Lake Ontario shoreline east of Cobourg, and along the Ganaraska River. Middle Ordovician, Simcoe, or Trenton Group (Lindsay formation) limestone mostly subcrops below Quaternary drift. The bedrock surface slopes gently southeastward toward Lake Ontario, but exhibits several southeast-oriented bedrock valleys. The Ganaraska River appears to follow one of these bedrock valleys. Drift thickness is highly variable, being controlled by bedrock topography and post-depositional erosion.
Older deposits:
The oldest exposed sediments are the clayery Port Hope till and the glaciolacustrine sands, silts and clays of the Clarke beds, identified in exposures along the Lake Ontario bluffs. Only the Clarke beds are mapped at the land surface, at the shore around Port Hope and east of Port Granby. The Clarke beds were deposited in a proglacial lake, prior to the main Late Wisconsinan ice advance responsible for much of the surficial geology in the Port Hope area.
Till:
The main Late Wisconsinan ice re-advanced from the north depositing a stony, silty sand to sandy silt diamicton, locally known as the Bowmanville till. This till functions as a regional aquitard and is up to 10m thick at the Lake Ontario bluffs. Carbonate and Precambrian pebbles are dominant in this till, and it exhibits a wide range of sedimentary styles from lodgement to shear structure to debris flow. The Bowmanville till is in the same stratigraphic position and has similar characteristics to the Newmarket Till north of the Oak Ridges Moraine; extending below the Oak ridges Moraine, these tills are likely correlative. The main Lake Wisconsinan ice that deposited the Bowmanville till retreated northward, separating from downwasting ice in the Lake Ontario basin.
Glacial Lake Deposits:
As the ice downwasted and retreated out of the Lake Ontario basis, Glacial Lake Iroquois was impounded within the basis to a level of 165 – 170 m asl in this area. At its margin, this lake eroded into the till uplands, and longshore currents reworked their sediments into gravelly beaches and baymouth bars. Nearshore deposits are dominantly sand and gravel. Offshore deposits occur in topographic lows that coincide with bedrock lows, and include massive to laminated silt and clay. Elsewhere in the offshore zone, wave-washed Bowmanville till is exposed. It is likely that stronger currents in these slightly elevated positions prevented deposition of fine-grained lake sediments. Post-Iroquois, lower lake levels (80 – 150 masl) are locally recorded by wave-eroded scarps (washing limits), gravel beaches and nearshore sands.
Logging requirements:
To log this earthcache:
Take the trail to the posted coordinates where you will find an information plaque. All answers can be found on the plaque.
Please send me your answers within 4 days of posting your found log. If there is more than one cacher in your party, include the names in your group. Only one person needs to send me the group answers.
Questions:
- How many years ago was the bottom layer (limestone bedrock) formed?
- When did the Wisconsin Glaciation period end?
- Name the four physiographic regions in this area?
- According to the sign, what is the “icing on the cake” for this region?
- From the sign, what layer is found above the Weathered Bedrock?
References:
Minsistry of Natural Resources Canada - Surficial geology of the Port hope area, NTS 30M/16, southern Ontario
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