Complex sentence: Clark & Edmonds, 1983 pp.51-52 Lack of food was a major factor, the Elk, the parties main source of food had retreated from their usual haunts into the mountains and they were now too poor to purchase enough food from neighboring tribes.
Split into simpler sentences: Clark & Edmonds, 1983 pp.51-52 Lack of food was a major factor. The elk, the party's main source of food, had retreated from their usual haunts into the mountains, and the party was now too poor to purchase enough food from neighboring tribes.