Complex sentence: The "black blizzards" started in the Eastern states in 1930, affecting the agriculture from Maine to Arkansas, and by 1934 they had reached The Great Plains from North Dakota to Texas and from the Mississippi River Valley to the Rocky Mountains.
Split into simpler sentences: The "black blizzards" started in the Eastern states in 1930, affecting agriculture from Maine to Arkansas. By 1934 they had reached The Great Plains from North Dakota to Texas and from the Mississippi River Valley to the Rocky Mountains.