This period begins about 65 million years ago and ends about 3 million years back. The continents continue to spread. Mammals continue to grow in size and number and diversity. Birds are flourishing. The world is warm and humid as far north as what is now the Arctic sea.
Palms, laurels, vines, citrus plants are flourishing. We have oaks and beeches. Our water lilies are again doing well.
But like most periods the climate changes. Warm weather gives way to colder weather. Ice sheets form at the poles dropping down the level of the ocean. Tropical jungles are retreating and temperate forests of oaks and maples are gaining ground.
Our first plains and prairies appear soon followed by our first grasses. Not long after grass eating mammals appear. This era is the age of grasses.
Now the world gets drier, forests begin to decline like the jungles did a bit earlier. The world is both colder and drier and grasses take this opportunity to expand out.