Although southern Manitoba was covered with water during the Cretaceous, some plant fossils have been found. They consist of logs thought to have drifted into the area from the western shore of the ocean. One such log has tentatively been identified as a bald cypress, a tree now found only in sub-tropical regions of the earth. The presence of rings indicates seasonal changes in the climate at that time.

Amber is resin that oozed from injuries in prehistoric trees. When fresh, it was soft and sticky, much as fresh pine gum is today. Insects, spiders, and mites that flew or crawled onto it were trapped. In some cases, they became completely embedded in it by subsequent flows of resin. Similarly pollen grains, spores, and other matter blown into by the wind became preserved also. Sooner or later this hardened gum fell to the ground and became covered with sediment. Amber from Cedar Lake, Manitoba is by far the richest and biologically the most important Canadian amber known.