Scarborough Bluffs

The Bluffs were named the Scarborough Highlands in 1793 by Elizabeth Simcoe, wife of the Lieutenant Governor of Upper Canada, John Graves Simcoe. Elizabeth Simcoe named them for their similarity to limestone cliffs in her native Scarborough, England.

Sir Sandford Fleming first identified erosion as an issue at the bluffs when characterizing longshore drift in Lake Ontario. He identified the westward drift of sand from the bluffs as the major sediment source for the Toronto Islands spit system. In 1912 Professor A.P. Coleman of the University of Toronto calculated an average rate of erosion of the bluffs at 0.71 metres per year and recommended that a wide verge be left undeveloped at the top of the cliffs to accommodate at least 200 years of erosion. Just eight years later plans for cliff-top subdivisions were developed, indicating that this prescient advice was ignored.

Development along the Scarborough Bluffs proceeded in earnest in the 1940's, converting farmland to residential areas and by the early 1960's the area was heavily urbanized. This removed vegetation from the top of the bluffs, which naturally slows erosional processes, and hardened the water shed, increasing overland flow and focusing storm water into narrow, fast moving channels with much greater erosive power.

Between 1830 and 1930 material chunks of rock and other material was removed from Lake Ontario in a process known as "stonehooking". These rocks, which were used for construction material, protected the bluffs from wave action. Once removed there was nothing to buffer the impact of waves at the base of the cliffs, which led to increased toe erosion and oversteepening of the bluffs.

Geologically, Scarborough Bluffs and the nearby Don Valley Brickworks Park, represent approximately 135,000 years of glacial sedimentation at the end of the last ice age. Globally, very few old tills are preserved due to their generally unconsolidated nature and the destructive forces of moving ice sheets. These deposits, therefore, are world-class examples of preserved glacial sediments and are of significant interest to the scientific community.

The oldest unit that outcrops at Scarborough Bluffs is the Scarborough formation, deltaic sediments deposited at the mouth of a very large river. The lower Scarborough formation, layer 5 in the image on the right, comprises silt and clay deposited offshore in the pro-delta where energy in the lake was low enough for fine-grained particles to fall out of suspension. The upper Scarborough formation (layer 4) comprises cross-bedded sand deposited in a shallower deltaic environment. The Scarborough Formation overlies the Don Formation (not observed at the Bluffs), which was deposited during the Sangamon interglacial, and represents a return to a glacial climate and the onset of the Wisconsinan glaciation.

Layer 3 is the Sunnybrook Drift, a fine-grained silty clay till with a low pebble content. It contains dropstones and evidence of gouging by moving icebergs, and forms the dramatic cliffs at Bluffers Park.

The Thorncliffe Formation (layer 2 in the image to the right) is a glaciofluvial deposit of cross-bedded sand and silt that was deposited within lows in the underlying stratigraphy. It generally coarsens upwards, but is highly variable in composition, reflecting both glacial and pro-glacial deposits.

The uppermost layer is the Lake Iroquois sand (layer 1), which is a post-glacial lake deposit reflecting the shoreline of glacial lake Iroquois, a precursor to the modern Lake Ontario.

Given what you have learned about the stratigraphy and history of the Scarborough Bluffs, what do you think might cause issues in terms of coastal erosion? Take a few minutes here to jot down some notes on some of the potential causes and issues before moving on to the next section.

The Scarborough Bluffs comprise unconsolidated, interlayered coarser-grained deltaic sediments and fine-grained diamict (till). These have differing porosity and permeability and differing strengths. Some layers act as aquifers whereas others are confining layers, or aquitards. Heavy urbanization has led to the hardening of watersheds, and somewhat ironically, the implementation of erosion mitigation measures in parts of the bluffs section has led to a change in the sediment budget of other parts, resulting in increased erosion due to a change in sediment deposited through longshore drift.

Hardened watersheds - areas where vegetation has been removed and replaced with paved surfaces, buildings and roofs - focus storm water into narrow ravines and gullies where increased flow, especially during storm events, results in rapid downcutting through the Scarborough Bluffs sediments. A comparison of rural vs. urban hydrographs shows the difference in lag time between rainfall events, lag time and total discharge and indicates that runoff is significantly higher in hardened, urban environments.

Bellamy Ravine, which is one of the few access points to the base of the bluffs, is an excellent example of the erosive power of highly focused storm water. The ravine has been significantly enlarged since urbanization of the bluffs. There has been some attempt at shoring up the ravine, as evidenced by the limestone blocks in the image below, but it has been largely unsuccessful.

Residents have also been observed emptying swimming pools over the bluffs, and leaky pools, sewers, septic tanks, and water mains have contributed to erosion.

Two major slope forms are observed at Scarborough Bluffs - planar slopes and scalloped slopes, although parts of the slope along Marine Drive are undergoing rotational failure.

Planar slopes are steep, with a flat or stepped profile and a straight crest line. Planar slopes are undergoing parallel retreat either along vertical stress relief joints or through groundwater seepage and piping. Freeze-thaw processes, which cause an increase in pore water pressure in the cliffs in the winter, exacerbate erosional processes in the spring, when the sediment thaws and groundwater can escape. Talus deposited at the base of the slope is removed during summer storms. These planar slopes form predominantly in areas of sedimentary homogeneity, the best examples of which are at Bluffers Park.

Scalloped slopes form in areas of sedimentary heterogeneity, and have a distinctive evolution relating to groundwater movement. Scallops form when impermeable layers prevent the downward movement of groundwater, and instead lead to the formation of perched aquifers. Groundwater exits the bluffs at the contact between the sandy Thorncliffe Formation and the underlying Sunnybrook till, and at the contact between the Scarborough Sand and underlying Scarborough Clay.

Groundwater flow becomes heavily focused along these contacts to form cave-like pipes, which become enlarged during periods of spring melt. Eventually the weight of the overlying sediments collapses the pipes forming large erosional bowls, or scallops.

If erosion is to be prevented along the Scarborough Bluffs coastline then erosion mitigation measures need to be implemented. Currently about half the total length of the bluffs is protected, but the Toronto and Region Conservation Authority is working on remediating the remainder of the unprotected coastline with an anticipated completion date of 2030. Although the primary goal is to mitigate erosion and ensure public safety the plan also involves the development of recreational and cultural spaces and wildlife habitat.

Groynes and headland beaches, such as those in the image at the right, have considerably slowed erosion along the Meadowcliffe shoreline to the west of Bellamy Ravine. The image below shows the same area in 2010 before the installation of protective measures.

Not everyone believes that remediation measures should be implemented, however. Some would prefer that the bluffs remain natural and inaccessible, with only a few points where people can experience the bluffs. Some of this is driven by a lack of desire for public money to be spent protecting private property, while others consider the bluffs to be a scientific wonder, which should be preserved for scientific study. There have also been negative consequences to sediment budgets to the west of the bluffs particularly west of Bluffer's Park Marina. The development of the marina and other shoreline infrastructure has starved areas on the down-current side (the west) of sand.

Remediation efforts are primarily driven by public safety, of which there are many aspects. Firstly, erosion that continues at rates prior to the implementation of any kind of mitigation (real rates of 0-15 m per year) causes millions of dollars of damage to both privately owned and public infrastructure. Secondly, the bluffs attract thousands of visitors annually, but access to the base and to the beaches is limited to a few steep trails. Emergency services respond to about 100 calls per year for minor medical emergencies to cliff rescues, but access hampers their ability to assist people safely. Thirdly, landslides have the potential to flow up to 75 m from the base of the cliff putting visitors to the bluffs in danger in the event of a mass movement.