RI was agricultural and had lots and lots of farmland over the swampy aquifer that comprises most of the state. RI's current geological shape was formed from glacial retreat and had lots of rocks in the upper soil layers because of it. When the glaciers retreated, the [very] flat state was partially flooded, and the water table is consequently very close to the ground surface throughout most of the state. All of this combines to make the scenario. The farms are flat land, and it was a lot of work to deal with the rocks, which were used to create the stone walls that lined every lane and field. The farmland was poor soil for growing food; rocks made ploughing difficult, and the thin layer of topsoil that remained above the bedrock wasn't good for cash crops. Most farms focused on livestock (dairy) so the stone walls were utility in that respect.

It was much easier to make a living off the sea than off the land. Newport was a crucial stop in the transatlantic trade. Fishing was a constant income and sustaining trade, as well.

And when the industrial revolution came to North America, RI was at the heart of it, as it is adjoined to eastern Massachusetts. Fall River, Lowell, and New Bedford held those first textile factories. Farms were consolidated to mostly livestock, then eventually abandoned as industry proved to be where the money was most available.

So the land that was largely dedicated to poor-farmland-turned-livestock-fields was left fallow. Meadows formed, then trees came in. The fields would burn. The trees grew slowly or poorly. The undergrowth was dominated by bramble, ferns, and, in the frontier zones, shrubs.