Read the passage below and answer the questions that follow

Development of the Great Lakes

Before the Pleistocene Epoch, there were no lakes in the region of the present-day Great Lakes that were comparable to them in size, because the Pliocene streams of that area drained northward. Lake Superior is located in a structural basin of Precambrian rocks, and most of the other lakes are situated on the sedimentary sequences that probably were low in the Pliocene topography. As Pleistocene ice sheets moved into the region, these earlier valleys were enlarged by scouring and were depressed under the weight of the ice mass. The lakes began to form in the last stages of the Wisconsin glaciation began to r melt and retreat. Melt- waters became trapped between the margins of the ice sheet and the high ground along the drainage divide of the Mississippi basin. During the first part of the history of the lakes, they actually drained into the Mississippi, but later, as the ice retreated farther, a new cutlet, was opened so the waters from the southern margin of the ice front drained westward across Michigan.

The sequence of events known to have taken place in the development of the Great Lakes is extremely complex. The following are among the most important factors that have contributed to the course of events in this region:

1. During glaciation the weight of the ice sheet caused the continent to be depressed. As the ice wasted away the land rose. Thus the old shorelines have become titled by this upward rise of the central part of the Canadian Shield
2. The ice front did not remain steady during the evolution of the lakes. It oscillated back and forth, advancing and retreating in different places at
3. The shape of the lakes has naturally been determined in part by the configuration of the land surface wasted. that has become exposed as the ice sheet
4. With each advance of the ice sheet, morainal debris was pushed into new positions and partially removed from older ones, thus changing the outlets and barriers of the lake waters.

3In spite of these complicating factors, the history of the lakes has been largely worked out. It is possible to trace the position of the lakes by means of mapping the beaches, wave-cut cliffs, sand dunes, bars, and other shoreline features formed at each stage in the lakes' history. Along these old shorelines it is possible to find outlets that were used at the time each lake occupied each position. It is not possible to outline exactly the position of each lake at every time during the evolution of the lakes, because the story has not been one of a gradually receding body of water. Some of the more recent lake shorelines cross and obliterate earlier shoreline features. In addition, most of the earlier lakes were marginal to the ice so no shoreline features are found where the beaches ran into the ice sheet.

The author's conclusions in the passage are most influenced by which of the following assumptions