The Colorado River has been forming the Grand Canyon in Arizona for at least 6 million years. During this time, the river has slowly caused erosion of the rock. As the rock eroded, soil eroded as well, and plant populations have had to adapt to live in increasingly rocky conditions. The river continues to deepen and erode the canyon at a rate of a few milimeters per year. What geologic principle is this an example of?

the abdomen (less formally called the belly, stomach, tummy or midriff) constitutes the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. the region occupied by the abdomen is termed the abdominal cavity. the stomach secretes acid and enzymes that digest food. ridges of muscle tissue called rugae line the stomach. the stomach muscles contract periodically, churning food to enhance digestion. the pyloric sphincter is a muscular valve that opens to allow food to pass from the stomach to the small intestine.