American prairies began forming 25 to 50 million years ago after the Rocky Mountains were pushed up into existence. This new mountain range reduced the flow of moist Pacific air moving toward the east. Over the centuries, moisture-dependent woodlands were replaced with drought-resistant plants like prairie grasses. Ranging in height from inches in shortgrass prairies in the West to over ten feet in tallgrass prairies in the East, these prairies covered more than one million square miles. Prairies were thought to have moved into western Ohio approximately four to eight thousand years ago during a climatic warming trend. Eventually, cooler and wetter conditions returned, and by the 1700's only small pockets of Ohio prairie, known as outliers, stayed behind. Early settlers would have encountered scattered prairies ranging in size from several acres to many square miles. Today, prairies are one of our more threatened ecosystems, with less than one-tenth of one percent remaining.