Complex sentence: Appreciation of the outdoors was not the only activity in the valley, however; logging was also a boom industry, and during the first half of the 20th century, most of the lower slopes of the surrounding mountains were cleared of old growth.
Split into simpler sentences: Appreciation of the outdoors was not the only activity in the valley, however. Logging was also a boom industry, and during the first half of the 20th century, most of the lower slopes of the surrounding mountains were cleared of old growth.