How were ancient river valleys formed in Ordovician rocks in Laurentia?

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Silurian paleogeography — Silurian Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the middle part of the Silurian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

Ancient river valleys were eroded into Upper Ordovician marine shales on the Laurentian platform as a direct result of extreme, glacially induced sea-level drawdown during the late Ordovician Period.

This global fall in sea level, which reached a maximum of approximately 70 metres (about 230 feet), drained vast areas of former marine habitat, exposing the underlying rock to subaerial erosion by rivers. These valleys were later filled and buried when sea levels rose rapidly at the beginning of the Silurian Period, leading to the reflooding of continental platforms.1