The subsurface geology in the Toronto area, in which much underground con-struction has taken place in the past 30 years, is, in general, fairly simple, consisting of almost flat-lying Ordovician sediments overlaid by a variable thickness of Quater-nary drift deposits.

The Ordovician rocks provide an interbedded sequence of shales, siltstones and limestones; and the Quaternary deposits, which range in age from Illinoian to Late Wisconsinan, consist of several tills interbedded with water-bearing sands and lacustrine clays and silts.

Construction difficulties arising from the local geological conditions are not great, but unexpected encounters with excess ground water do occur. The presence in the bedrock of high horizontal stresses at shallow depths can cause difficulties, especially to the completed structure, unless the stress condition is recognized, designs are modified, and construction procedures are adapted to the unusual stress environment.