Saturday, December 24, 2011
What on Earth: Volume 7 2011
In natural science, the frontier of the unknown is never far away. One of the frontiers lies below our feet. What lies below the surface of the ground? How did the University campus get to be so hilly?
In May, 1976, a hole was drilled by the Department of Earth Sciences drilling rig beside the Chemistry Building, in order to get a glimpse into the unknown below us. Although when the various buildings were constructed exposures and borings were made, these generally extended to only shallow depths of 3 to 10 metres and deposits still farther down remained concealed and unknown. A few test holes had also been put down to test water supplies but no soil samples had been obtained from these holes. From these, however, we estimated that the depth to rock was about 30 metres under the Chemistry Building, and so we made plans to get a continuous core sampling of the materials down into bedrock. The sequence of deposits encountered in the hole reveals part of the geological history of the area. Together with other available information, an interesting story can be put together.
The story of the bedrock
The solid rock encountered directly below the campus is green shale and brown dolomite of the Salina Formation. Southeast of us, near Paris, this same rock formation is exposed in the banks of the Grand and Speed Rivers. Paris got its name from gypsum deposits formerly mined from the Salina Formation (for “Plaster of Paris”). At Goderich, rock salt is obtained from the same formation. These evaporite minerals tell us that the Salina Formation was formed in a shallow arm of the sea that then covered much of the eastern United States. The age of these rocks is about 400 million years. Green shale of the Salina Formation was penetrated for about 6 metres by the 1976 boring.
We know from rocks laid down elsewhere that a long span of time is unrecorded by the rock succession at this location. Presumably some rocks were formed, but were later eroded away. The traces of old stream valleys cut into the rock surface are revealed by water well records, which show that various “buried valleys”, completely filled and concealed by much younger glacial deposits, cross the area. The missing interval here spans the period from 400 million years ago, to several tens of thousands of years ago.
The story of the glacial deposits
The deposits overlying and concealing the bedrock are mainly derived from glaciations. More specifically, they are a mixture of all sizes of material from clay to boulders, called till, laid down directly by the ice itself. The several layers of different kinds of till represent several advances of the ice over the area. In all, five till layers were encountered in the borehole, as shown in the accompanying sketch. At a depth of about 42 metres, in the lowest till layer, a small piece of spruce wood was found.
This tells us that the ice which deposited the till overrode the vegetation which grew earlier in the area. We don’t know the age of the wood (it is too small for radiocarbon dating) but it could be 50,000 to 100,000 years old.
We know that the middle layer of till (Catfish Creek Till) was laid down by a strong ice advance about 18,000 years ago, which covered all of Ontario and the northern States. Melting back about 16,000 years ago temporarily uncovered this area, but it was covered by ice again about 15,000 years ago. About this time melt-waters from the glaciers were building up large accumulations of gravel and sand, forming the Waterloo sandhills. We are situated on the eastern edge of these rolling sandhills and fine sand of this origin is found exposed in the slopes south of Columbia Street. Ice flowing from the Georgian Bay area and Lake Ontario area both covered this vicinity one time or another near 14,000 years ago and then finally retreated. The next ice readvance from the east only reached the east edge of Waterloo but melt-waters eroded into the older deposits and gave birth to the various branches of Laurel Creek. Remnants of the Tavistock Till sheet, brought here by the last advance of the Georgian Bay ice are to be found on the tops of the higher areas near St. Paul’s College, along Columbia Street and at the Graduate House and Minota Hagey Residence.
Thorough the thousands of years following, the valleys have deepened as Laurel Creek continued to cut down. In some places erosion was hindered by the glacial deposits and the streams became sluggish and swampy. The climate became warmer, and the spruce forest was succeeded by pine forest, then later by the hardwoods.
Erosion continues today along Laurel Creek however. During construction of the Village residences, disturbed soil around the excavations was washed into the stream channel by heavy rains. This sediment was carried down to the pond at Conrad Grebel College, where it accumulated in a delta. As the delta grew rapidly, it had to be dredged out several times by dragline to prevent filing of the pond. With cessation of construction, and landscaping, sediment load in the stream has greatly diminished and a more stable pond environment has resulted.
Further information on the local geology may be gained from the following:
Chapman, L.J. and Putnam, D.F., 1966, The physiography of southern Ontario; 2nd edition, University of Toronto Press. 386 p.
Isherwood, A.E., 1976, Quaternary geology and soil conditions, University of Waterloo campus; M.Sc. project report. Dept. of Earth Sciences. 59p.
Karrow, P.F., 1971, Quaternary geology of the Stratford-Conestoga area, Ontario; Geol. Surv. Can. Paper 70-34. 11p.