The Colorado River and other erosional forces sculpted the southern edge of the Colorado Plateau to form the Grand Canyon, revealing a beautiful sequence of rock layers that serve as windows into time. Grand Canyon's geologic story reveals itself in three chapters: Sedimentation, Uplift, and Erosion. Chapter One- Sedimentation. Environments as diverse as desert and ocean once existed to provide sand, mud, silt, and other sediments to form the sedimentary rock layers in the Grand Canyon. The fossils of the Kaibab Limestone tell of life in a warm shallow sea. Crinoids, or animals that look like sea lilies, are among the most commonly found fossils. Sponges and brachiopods are also plentiful. The only layer deposited by wind, the Coconino Sandstone is a remnant of an ancient sandy desert. Tracks of reptiles that pre-date dinosaurs are found in the cross bedding of the preserved dunes. The Hermit Shale gives evidence of a swampy environment. Chapter Two - Uplift. Geologic changes that occur slowly on our human time scale, dramatically alter landscapes. Plate tectonics, the theory that the earth is covered by a crust that folds, crumples, and lifts, explains why a rock formed at sea level can exist at over 8000 feet. The Colorado Plateau was lifted gently enough to preserved the order of the rocks, allowing them to be studied as chapters in a book. The Kaibab Plateau, an upswell within the Colorado Plateau, distinguishes the North Rim at 8000 feet, from the area surrounding it. The North Rim is more than 1000 feet higher than the South Rim, causing it to receive more precipitation. Hence, the wetter Kaibab Plateau is a forested island standing above the surrounding plateau and canyon. Chapter Three - Erosion. All rivers erode landscapes. The Colorado River, with its steep gradient, high sediment load, and roaring floods, carves canyons. It falls more than 13,000 feet in its journey from the Rocky Mountains to the sea. Within the Grand Canyon alone, it falls 2000 feet in 277 river miles. Rocks are picked up, ground, and carried, giving the river the texture of liquid sandpaper, eating away at anything in its course. In 1889, John Wesley Powell called the river "too thick to drink and too thin to plow," its sediment was that great. Over six million years, this constant erosion has carved down into the Colorado Plateau, exposing the rocks of the Grand Canyon. The North Rim is almost twice as far from the river as the South Rim because this rim is eroding away more rapidly. Precipitation flows toward the rim, carving at a rate twice that of the South Rim. John Wesley Powell was a geologist and explorer of the American West. He is famous for the 1869 Powell Geographic Expedition, a three-month river trip down the Green and Colorado Rivers that included the first passage of European Americans through the Grand Canyon. Powerful spiritual ties connect people and place. It is believed that the first humans laid eyes on the Grand Canyon as far back as 10,000 years ago. These people, referred to as the Archaic Culture, were nomadic, hunter-gatherers. The Ancestral Puebloan (Anasazi) people are believed to be descendants of the Archaic Culture. They lived on both rims and the inner canyon and thrived with what nature offered them - hunting, gathering, and growing crops such beans, corn , and squash. The were adept farmers, using check dams and irrigation systems to divert snow melt and limited rainfall. The Grand Canyon remains a place of spiritual and cultural significance to modern cultures. Some modern day Pueblo nations consider the Grand Canyon to be their place of emergence into this world. The Paiutes also have a rich heritage on the North Rim and the Kaibab Plateau, hunting and gathering in this area for over 1000 years. Kaibab is actually a Paiute word meaning "mountain lying down." The Navajo people arrived in the Grand Canyon from the north, following a nomadic migration pattern. It is believed that they moved to the Grand Canyon area around 400 years ago.