Pensyllchfania

Bryn Mawr Cymru 

Watching the Mare of Easttown (9/10 - excellent) threw up an interesting question. Why is there a place in the US with an obvious Welsh name? Bryn Mawr = Great Hill

Bryn Mawr Pen. USA

Quite different views. So I googled a bit, & found the story of The Welsh tract. From wiki:-

The Welsh Tract, also called the Welsh Barony, was a portion of the Province of Pennsylvania, a British colony in North America, settled largely by Welsh-speaking Quakers in the late 17th century. The region is located to the west of Philadelphia.

I thought this may be due to the presence of coal in the area, so:-

One of the earliest mentions that coal might be found in the Pennsylvania area dates back to 1698, when Gabriel Thomas wrote an account dedicated to William Penn. He notes the possibility of coal because the running water had the same coloring as the water in the coalmines in Wales

Also:-

Pennsylvania became a hub for Welsh miners, especially in the anthracite and bituminous coalfields. Many Welsh immigrants settled in the Scranton/Wilkes-Barre area and other parts of Northeastern Pennsylvania. 

The Welsh brought their cultural traditions, such as churches, fraternal orders, and a love for music and literature, to the mining communities. While their cultural institutions often faded within a generation, a distinct Welsh American identity developed.