Five seismic reflectors in the basinal sedimentary section have been identified as representing erosional surfaces in shoaler regions of the lake basins and can be traced throughout most of the study area. The reflectors representing these erosional surfaces, and their conformable equivalents in the deep lake basins, are caused by intervals of coarser grain size in the sediments and are thought to represent lowering of lake levels prior to, and following, the Main Algonquin highstand (about 10.2 – 11.0 ka). These reflectors are used to define seismic sequences in the postglacial sedimentary fill of the basins. Two additional erosional surfaces, which closely follow the initial fall of the Main Algonquin lake level, are most clearly seen in thicker sections found in the northern part of the basins. The two oldest (pre-Main Algonquin) sequences appear to onlap highs and fill in lows within the basins, whereas the younger sequences (associated with the Main Algonquin through Mattawa times) tend to drape the topography and show little difference in thickness between the lows and their adjacent highs. The most recent sedimentary sequence represents deposition occurring since about 7.5 ka. It laps out against topographic highs and leaves older sequences exposed at the lake floor in many areas. The most pronounced erosional surfaces are associated with the Light Green and the Light Blue sequence boundaries. The Light Green sequence has a pair of coarser layers at its top, which are dated at 9.6 – 9.8 ka and 9.05 – 9.2 ka. The coarser interval at the top of the Light Blue sequence is dated at 7.45 – 7.80 ka. These layers are thought to mark maximum lake level lowstands and to represent the times of Ottawa–Marquette to latest Mattawa lake level fluctuation.