Imagine yourself standing in Frick Park 300 million years ago, long before Frick Park ever existed, long before people even existed. You're in the middle of a dense, hot jungle. Ferns the size of trees tower above you. Dragonflies with wingspans of almost two feet zoom by. At this point in time Pittsburgh was located just south of the equator (somewhere along the way continental drift had to go and mess it up for us) and the weather was hot. Probably even hotter than tropical countries are today.
If you stood there for a few million years, you would see enormous plants and animals die, fall to the ground and decompose. Sometimes sea water would rise and cover you.
When the land was underwater, a myriad of sea creatures were busy living and dying and falling to the bottom of the ocean. Small sharks navigated the water. Brachiopods—clam-like creatures—anchored themselves to the ocean floor. Crinoids (also known as sea lilies) were also fixed to the ground. These flower-like animals were made up of a stalk with many narrow, finger-like appendages at one end. These "fingers," completely covered in sticky hairs, would catch small organisms which fed the animal. Small corals were also prevalent. Coral needs sunlight to survive and did very well in the warm shallow tropical sea of the time. The dead shells of these sea creatures formed the limestone deposits that we see today. If you look closely at a piece of limestone you can see small, circular pockmarks and some tiny horn-shaped figures that could easily be taken for rock weathering, but these are actually fossils from a tropical Pittsburgh.
Sometimes the sea would ebb and leave the jungle to take over. Amphibians lived at the water's edge. Ten-inch centipedes, spiders, and cockroaches scuttled over the ground. Those decomposing plants would build up, layer upon layer, creating the pressure needed to turn all of the rotting matter into coal seams. This age is called the Pennsylvanian Period because it was during this pre-historic time period that the foundation for Pennsylvania's coal and steel was literally being laid down in the form of coal seams.
Skip foward in time to 2 million years ago. The Appalachian Mountains, once Himalayan in size, have been worn by millions of years of erosion and glaciers have started to cover the land. Pittsburgh, just south of the glacial line, was still as flat as Kansas. Unlike mountain ranges formed by movements under the earth's crust, Pittsburgh's "hills" were formed by water erosion. Rivers and streams cut through the area, making deep grooves in the landscape and creating the cliffs and steep ledges that make driving in the city today such an adventure.
All of this takes an inordinate amount of time. Geological time runs on an immense scale. Centuries are like days and the span of a human life could happen in the blink of an eye. The valleys and hills that you see in Frick Park have been shaped by erosion for close to one million years.
So the next time you take a jaunt through Frick Park, take a moment to look at the cliffs that surround you and think about how many innumerable years of water erosion and climate changes went into forming them.