Final answer:
Between the 1840s and 1860s, a large number of people crossed the Great Plains and the Rocky Mountains to reach the Pacific Coast.
Step-by-step explanation:
Between the 1840s and 1860s, a large number of people crossed the Great Plains and the Rocky Mountains to reach the Pacific Coast. The primary motivations for this westward migration were the California Gold Rush of 1849 and the completion of the Transcontinental Railroad in 1869. Additionally, the Homestead Act of 1862 encouraged westward migration by offering free land to those willing to move west. Over 400,000 settlers followed the Oregon Trail during this period.