| Landform: |
Bedrock Geology:
A near-continuous exposure of the Silurian stratigraphy of the northern portion of the Niagara Escarpment occurs in the numerous rock bluffs of the escarpment and shoreline exposures in the study area. Bedrock is exposed, or relatively close to the surface, throughout the study area.
Queenston Formation (Upper Ordovician) - The Queenston Formation occurs as a low, recessively weathered outcrop belt at the shoreline beneath Boulder Bluff. Up to 3 m of grey-green, thin-bedded to blocky fractured, wet clayey shale occurs in sharp contact with the overlying dolostone of the Manitoulin Formation.
Manitoulin Formation (Cataract Group; Lower Silurian) - The upper surface-of the Manitoulin Formation is exposed in a bedding plane platform outcrop a few metres above lake level, underlying entirely the area known as Wingfield Basin. This secondary scarp, at over 1 km broad, is exceptionally well-developed at this locality. In the ANSI, the Manitoulin Formation is exposed in near-continuous outcrop from 3 m to 7 m thick above the shoreline of Cabot Head, from South Bluff to West Bluff. It consists of thin- to medium-bedded, ripple laminated, flaggy (crinidy) horizontally-fractured, buff-weathered, grey to buff-grey dolostone.
Intervening Units - The rock formations which occur stratigraphically between the Manitoulin Formation and the Amabel Formation are only sporadically exposed in the ANSI.
Amabel Formation (Albemarle Group; Middle Silurian) - The Amabel Formation is a reefy dolostone which forms the caprock of the northern portions of the Niagara Escarpment. It is subdivided into the lower Lions Head member and the upper Wiarton/Colpoy Bay member.
The Lions Head member forms a distinct outcrop belt below the brow of the escarpment. It is recognized by a bright buff weathered surface colouration, its thin to medium bedding and the vertical, conchoidal fracture pattern, which, when broken down, forms a flaggy rubble on top of the main talus slope. The unit is a very pale buff, grey-mottled, medium- to fine-crystalline, dense dolostone. The headlands of the promontories most exposed to the full impact of glacial and subglacial meltwater erosion have been stripped of their cover of Wiarton/Colpoy Bay member caprock. Thus the Lions Head member forms the brow of the escarpment at Rocky Bay, and along the leading edges (northeast-facing) of Middle, Boulder and South Bluffs overlooking Wingfield Basin. Only scattered remnants of the more massive Wiarton/Colpoy Bay member beds occur well behind the escarpment edge in these areas.
The Wiarton/Colpoy Bay member is subdivided into biohermal and non-biohermal lithofacies, with the non-biohermal lithofacies predominating in the vertical cliff faces below the brow of the escarpment in the ANSI. Both facies are typically thick- to massive-bedded, planar to undulatory parted, light grey to light grey-tan, variably mottled (typically blue-grey mottles), medium- to coarse-crystalline dolostones. The unit is very fossiliferous, with large macrofossils, primarily crinoids anid fragmental crinoidal material, commonly visible on weathered surfaces.
Glacial Geology:
The last glacial movement over the Bruce Peninsula was by the ice of the Georgian Bay lobe from the northeast, as deduced from scattered striae and other ice-direction indicators. The landscape of southern Ontario may also have been modified by powerful subglacial floods during late glacial time. Large-scale s-forms attributed to subglacial flood events occur sporadically along the brow of the escarpment and on the inland bedrock surfaces of the study area. In addition, direct evidence of glaciation in the area consists of a small moraine and clay till plain.
Some of the most spectacular large-scale s-forms are found on the escarpment at Cabot Head. These include the excellent development of stoss-side furrows, cavetto forms, rock drumlins and, notably, large sinuous channel forms. These predominantly linear forms are located at strategic points along the brow of the escarpment, generally "downflow" from a topographic notch or high. The most spectacular development of these large-scale s-forms occur in the Cabot Head ANSI west of the study area in St Edmunds Township, and in the study area at West Bluff, where sinuous channel forms with smooth, vertical and undercut walls several metres high isolate rock drumlins through the edge of the escarpment.
Postglacial Lakes and Shoreline Processes:
An important part of the post-glacial history at Cabot Head is the evidence left by lake levels which occupied the Georgian Bay basin. As the retreating ice cleared the northern portion of the Bruce Peninsula, about 11 000 EP, glacial Lake Algonquin, the north shore of which was in contact with the ice front, was established. It inundated most of the Bruce Peninsula, with the exception of a few islands formed by the highest points of the promontories along the eastern side of the peninsula. The elevation of Main Lake Algonquin in the Cabot Head area was about 250 m asl. Significant abandoned ("raised") shoreline features at this elevation occur on a number of "islands" which were isolated, surrounded by shallow offshore shoals, on the north shore of the Bruce Peninsula. Some of the most spectacular and ancient wave-cut shore features in the country were formed on these islands, including a series of shore stacks ("flowerpots") in various stages of development, on Boulder Bluff and South Bluff. These features now occur 66 m above Georgian Bay.
As lake levels rapidly dropped, evidence of lower lake stages was left in the form of cobble and shingle beaches in the Gillies Lake area and between the bluffs overlooking Wingfield Basin. These were probably formed in the Penetang and Cedar Point lake stages. After the low Hough stage in Georgian Bay basin was achieved, during which large offshore areas below the escarpment were exposed and forested, about 10,200 - 10,000 BP, the Nipissing transgression was initiated about 8400-8200 BP. It culminated in the Nipissing stage, about 5500-3800 BP, at a present elevation of about 194 m asl, which left little evidence in the Cabot Head area, with the exception of faint beach traces. As this lake level dropped to present levels, the Algoma stage, at about 185 m is recognized. It is probably slightly older than 3000 BP. Most of the abandoned shingle beach berms at Wingfield Basin are attributed to this level.
Only faint traces of post-Algonquin lake stages exist along the cliffs and between the promontories of the study area. The best of these consists of a spit complex enclosing Wingfield Basin. This spit complex consists of multiple, abandoned (raised) and modern storm berms composed of flaggy shingle. The spits are situated on a flat bedrock scarp of thin-bedded, well-jointed Manitoulin Formation dolostone from which the shingle is derived. The spit complex consists of storm berms of flaggy cobbles to pebbles. Unvegetated berms up to 2 m above lake level represent the modern processes active on the spit complex. Three or four berm levels in the modern zone reflect increasing storm strength events with height above the lake. Above the unvegetated beach area, at least 3 or 4 abandoned, raised, beach berms, now vegetated with cedar and mosses, occur to about 10 m above current lake levels. The beach berms at Wingfield Basin occur at a maximum present elevation of about 187 m asl, the level of the Algoma stage isobase in this area. Thus the oldest beaches were formed about 3000 BP. The spit complex at Wingfield Basin represents a Recent feature, reflecting the late emergence of the source outcrop and platform above present lake level. The shape of these beaches echo the modem beaches, suggesting that the spit-forming process has been ongoing at Wingfield Basin since the time of emergence of the platform.
Karst Processes and Features:
In the Cabot Head area, the surface rock consists of the fossiliferous dolomitic Amabel Formation and is ideal for the development of surface karst features. The karst landforms at Cabot Head developed since the last glaciation ended, over the 10 000 years. The ANSI incorporates some of the best-developed karst features on carbonate bedrock in Ontario, and possibly Canada. They consist primarily of the small-scale surface forms known collectively as karren. Of these, the most common are kluftkarren and solution pitting.
Where solution has exploited a jointed surface to create a pattern of grooves and isolated blocks, a surface called carbonate pavement, or in the case at Cabot Head, a dolostone pavement, is formed. Pavements are best developed on flatlying or gently dipping bedrock surfaces where water cannot drain quickly. Existing joints become widened by solution, forming grikes, isolating "floating" slabs of dolostone, called clints. Other karren forms are commonly imprinted on this surface. Clint-and-grike pavement is the most common karst form in the study area, and has been described as the most significant karst pavement in Ontario, and probably one of the best-developed dolostone pavements in Canada.
Mass Wasting:
The edge of the Niagara Escarpment throughout its length is punctuated by various degrees of erosion, notable by remnant outliers, crevice caves, monolithic promontories, and massive slopes of talus. In the Cabot Head area, the most striking postglacial features are massive, forested to bare talus slopes below the brow of the escarpment. The talus slopes at Cabot Head constitute some of the most massive talus slopes along the length of the Niagara Escarpment.
Where the Wiarton/Colpoy Bay member forms the caprock of the escarpment, the talus slopes are generally composed of huge, open-matrix blocks near the base and middle portions, with smaller blocks and rubble, from the Lions Head member, near the top. On the promontories overlooking Wingfield Basin, where the Lions Head member forms the brow of the escarpment, huge talus slopes almost masking the escarpment face consist of finer rubble from this more fractured unit. These massive scree slopes of cobble- to boulder-sized debris are unique to the escarpment.
At the base of West Bluff, a blocky talus rests on a submerged bedrock shelf, probably a subordinate scarp of the Manitoulin or Queenston Formation. This talus is subject to the direct action of storm waves in Georgian Bay. This has resulted in an unusual landform of huge shoreline boulders marked by the immature development of caves, hollows, stacks and other shoreline erosional features. The boulders protect the base of the escarpment cliff at this locality from erosion. Where the base of the escarpment lies close to the shoreline of the lake, scattered blocks and larger rubble of Amabel Formation dolostone also litter the nearshore shelf area.
Small-scale crevice caves, caused by the calving of competent blocks of bedrock from the edge of the escarpment on a softer shale substrate, are located at the brow of the Manitoulin scarp along the north side of Wingfield Basin. The crevices are 3-6 m deep and a few metres wide at the top.
Gillies Lake:
Gillies Lake is an anomaly on the Bruce Peninsula. Most of the lake is typically shallow, being less than 10 m deep. At its southeastern end however, the basin is over 30 m deep, with a mostly submerged cliff along its eastern shoreline. It is generally thought to represent a features related to karst processes acting on significant regional structural trends in the bedrock. It has also been suggested that the basin represents a collapsed karst-formed cave. The present study suggests that scouring by powerful subglacial meltwater flood events represent a likely mode of formation of the lake. However, the origins of the Gillies Lake basin require further research along several lines of investigation before they can be fully understood. [Kor 1994]
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| Representation: |
1) The features related to karst processes at Cabot Head are Provincially significant, and possibly nationally significant. The immature, postglacial surface karst features and processes documented at Cabot Head are considered to be the best in Ontario, and compare favourably with national examples. A large portion of the original ANSI in St. Edmunds Township was acquired by Parks Canada in part for this reason.
2) The abandoned and raised ancestral islands of glacial Lake Algonquin at Cabot Head, and their shoreline features, primarily shore stacks and cobble beaches, are considered provincially significant, and possibly nationally significant. The grouping of shore stacks around a small Algonquin island on Boulder Bluff, some possibly collapsed, are a unique element of the history of the lake, and probably represent the oldest such features in the province. The features contribute significant elements to the lacustrine landform/process theme of the Earth Science Framework.
3) The sculpted bedrock features related to subglacial fluvial erosion in the ANSI are provincially significant. The large-scale s-forms which are found at the brow of the escarpment represent another dimension to the occurrence of these forms. The occurrence and geological significance of s-forms and their interpretation as indicators of catastrophic subglacial flood events are still controversial. Their availability for study in a variety of settings and styles is necessary to add to our future knowledge of the processes responsible for their formation. The protection of the Cabot Head occurrence complements the representation of s-form occurrences elsewhere, notably in French River Provincial Park (Kor et al. 1991) and Lion’s Head Provincial Nature Reserve and ANSI (Kor 1992a). The features contribute very significant elements to lateglacial time themes (as yet undetermined), and to glacial and fluvial landform/process themes of the Earth Science Framework.
4) The shingle spit complex which encloses Wingfield Basin, one of the largest such features to occur along the shoreline of the Canadian Great Lakes, is provincially significant. Its occurrence is related to a complex combination of setting (on a flat, very broad bedding plane outcrop), location (at the head of a promontory), and, source (the flaggy dolostone of the Manitoulin Formation). The occurrence of several generations of beaches, some raised above present storm levels, on the shingle spit complex provides evidence that conditions on Georgian Bay in the recent past were similar to those of today. The beach complexes contribute elements to the lacustrine landform/process theme of the Earth Science Framework.
5) Gillies Lake is an unusual feature, the origin of which is still not known. Its anomalous depth, unusual position above the brow of the escarpment, and the apparent lack of debris within the basin suggests that the lake is not a karst-related feature, as commonly held. Rather, it is here interpreted to have been formed largely through the scouring action of catastrophic subglacial meltwater flood events. The anomalous orientation of its basin suggests that it is the product of more than one such event. The preservation of features related to possible multiple subglacial flood events is unique to the knowledge of these events to date. Gillies Lake may represent a very significant research tool in future studies of these events. The Gillies Lake basin and its surrounding shoreline areas are considered provincially significant until its origins are better understood. The features associated with the basin represent potentially valuable research tools in the deciphering of the regional glacial history of the area.
6) The type area outcrop of the Dyer Bay Formation is provincially significant. The outcrop, at lake level, shows a variety of surface forms, is very accessible, and has high research and interpretive potential. It occupies a shoreline location predominantly within the Crown high water reserve. Its location outside the Cabot Head ANSI, on the only access road to the ANSI, provides a possible staging point for the interpretation of the geological features of the ANSI. [Kor 1994]
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