Captain William Clark was the primary cartographer for the Corps of Discovery. With very little training prior to the expedition, he created maps based on field sketches, celestial readings, and compass brings that were an invaluable contribution to knowledge. Not only did they dispel the myth of the Northwest Passage, they also recorded previously uncharted lands and paved the way for westward expansion and opportunity. Several creeks in the Yellowstone Valley were named after Corps of Discovery members or dramatic incidents. Private Gibson was thrown from his horse and pierced his thigh on a snag. The wound, two inches deep, left him in great pain. Clark named a creek "Thy Snag'd Creek." Using mostly a surveyor's compass, field observations and estimates, Clark drew nearly 5,000 mils of new mapping. Amazingly, his measurements were only off by a mere 5%.