Scientists study the soil compaction (different layers of soil) and see what it was like living in a certain time and age. (the soil stacks, resulting in layers of different soil). To find out what is was like living in the time of jamestown, the scientists just have to dig in the area that they settled, and match soil to the time period. When it is matched, they can test the soil to find out if it was dry, a swamp, etc. From the data they collected, they then are able to find out if there was a drought in the area where Jamestown was settled
hope this helps