During the Cretaceous Period , flowering plants appeared on Earth for the first time. During the middle Cretaceous, mountain ranges like the Sierra Nevadas and the Alps were formed by heavy tectonic and volcanic activity.
Scientists disagree about what happened.
Others think it was caused by volcanic activity or by climate change. Until we gather more evidence, no one can say for certain why the dinosaur days came to an end. It's one of the greatest mysteries of science.