When did coal deposits of the eastern US begin to form?

Coal deposits in the eastern United States began to form during the Carboniferous period, which lasted from about 359 to 299 million years ago. This period was characterized by lush, swampy environments that facilitated the accumulation of plant material, which eventually transformed into coal. The thick layers of vegetation, combined with geological processes over millions of years, led to the creation of significant coal deposits that we mine today.