Complex sentence: However, when Theodore Roosevelt National Park was established in the 1940s, during construction, a few bands of horses were accidentally enclosed within the Park fence, and by 1960 these bands were the last remaining feral horses in North Dakota.
Split into simpler sentences: However, when Theodore Roosevelt National Park was established in the 1940s, during construction a few bands of horses were accidentally enclosed within the Park fence. By 1960 these bands were the last remaining feral horses in North Dakota.