drove herds of horses to the Sierra, where horse meat became a major new food source. Exploration by European Americans Although there were Spanish missions, pueblos (towns), presidios (forts), and ranchos along the coast of California, no Spanish explorers visited the Sierra Nevada. The first European Americans to visit the mountains were amongst a group led by fur trapper Jedediah Smith, crossing north of the Yosemite area in May 1827, at Ebbetts Pass. A group of trappers led by mountain man Joseph Reddeford Walker may have seen Yosemite Valley in the autumn of 1833. Walker approached a valley rim as he led his party across the Sierra Nevada, but he did not enter it. A member of the group, Zenas Leonard, wrote in his journal that streams from the valley rim dropped "from one lofty precipice to another, until they are exhausted in rain below. Some of these precipices appeared to us to be more than a mile high." The Walker party probably visited either the Tuolumne or Merced Groves of giant sequoia, becoming the first non-indigenous people to see the giant trees, but journals relating to the Walker party were destroyed in 1839, in a print shop fire in Philadelphia. The part of the Sierra Nevada where the park is located was long considered to be a physical barrier to European American settlers, traders, trappers, and travelers. That situation changed in 1848 after gold was discovered in the foothills west of the range. Travel and trade activity dramatically increased in the area during the ensuing California Gold Rush. Resources depended upon by local Native Americans were depleted or destroyed, and disease brought by the newcomers spread rapidly through indigenous populations. Extermination of native culture became a policy of the United States Government. The first confirmed sighting of Yosemite Valley by a non-indigenous person occurred on October 18, 1849, by William P. Abrams and a companion. Abrams accurately described some landmarks, but it is uncertain whether he or his companion actually entered the valley. In 1850, one of three brothers, Joseph, William, or Nathan Screech, became the first confirmed non-indigenous person to enter Hetch Hetchy Valley. Joseph Screech returned two years later and spoke with the Native Americans living there, asking them what the name of a grass-covered seed meal was and was told, "hatch hatchy." The surveying crew of Allexey W. Von Schmidt conducted the first systematic traverse of