Geological Survey: 5K

Welcome to the Cretaceous Interior Seaway, 80 million years ago, in what will become Colorado. This inland sea is 3,200km long and 970km wide, and can be up to 760m deep. Herds of duck-billed hadrosaurs live along the shoreline.

You're running west, and that rumble you feel is mountains being formed. Far to the west is a subduction zone, where an oceanic tectonic plate is being ground beneath a continental plate. But this oceanic plate is very bouyant, and is scraping against the continental plate, compressing and rumpling it to form mountain ranges. This is why western North America is so mountainous. If you look behind you, more mountains are closing off the seaway and tilting it up, and the water is draining away.

You've reached 2k and run through nearly 40 million years of time. You're at the top of the newly formed Rocky mountains. They're hidden underneath older rock at first, but are slowly exposed as the older rock erodes. You follow a stream of water as it joins up with other streams to become the Colorado River. The Colorado Plateau that it runs through is still gaining elevation quickly, so the water cuts down through the rock instead of spreading outwards.

You're run 3k and covered another 20 million years. The crust here is under tension, being pulled apart. This creates large cracks, and eventually forms parallel mountain ranges with deep basins between them. The Colorado River cuts through these basins and ranges. Small volcanoes dot the area where the crust is thin enough for magma to break through.

You've finished 4k and run forward another 13 million years. Far to the west, a process called 'rifting' has created the Gulf of California. This has made the sea level even lower so the Colorado River is cutting down even more. All rivers want to go down to sea level. You've moved into the Pleistocene ice age which is much cooler and wetter, and has added even more water to the Colorado River. More water means the river has cut the canyon wider and deeper.

You've reached 5k and are now at the mouth of the Grand Canyon in the modern day! What you see in the canyon is nearly two billion years of exposed Earth history. But this is only the start of the story of the Grand Canyon.