When the Appalachian Mountains were formed, the bedrock of Ohio was bowed up into a low arch known as the Cincinnati Arch. Over the next 200 million years, where the arch stood highest across the western half of the state, the forces of erosion cut deepest all the way to limestone bedrock. By the time the Ice Age began about two million years ago, what is now western Ohio had been eroded away to a relatively flat plain offering little resistance to the advancing glacial ice. Consequently, most of the hills or landforms in the Till Plains are glacially deposited rather than bedrock as in the eastern half of our state where the more erosion resistant sandstone bedrock hills slowed and stopped the spread of glacial ice. Within the Till Plains, bedrock is typically covered by a thick blanket of glacially deposited soil known as till, which reshaped and smoothed the Till Plains into a comparatively smooth rolling landscape. Only in Logan County, remains an isolated massive bedrock hill known as the Bellefontaine Outlier that rises 1,549 feet, the highest point in Ohio. The very fertile glacial soils of the Till Plains provide some of our richest farmland. During pre-settlement times, these same soils supported our most extensive tall grass prairie remnants, surrounded by forests dominated mostly by Beech and Sugar Maple trees. Today approximately 95% of the Till Plains are farmland and urban development. Below the Bellefontaine Outlier, fast-flowing meltwater from the glacier filled valleys south of the ice front with sand and gravel. Today, these buried valleys of porous outwash materials provide some of the most extensive groundwater sources in the state. In a select few places within West-central Ohio, where ground water emerges at the surface, fens or alkaline bogs such as Cedar Bog and Prairie Road Fen nature preserves can be found. Some of Ohio’s rarest and most unusual plants, living relicts of the Ice Age, grow in fens.