I know the people after Europeans arrived consisted of the Lenape, Shawnee, Iroquois and that the people that were hear prior are typically referred to as the Monogahela people.
My understanding is that when settlers arrive these people were gone and no one really knows why, some speculate disease arrived before settlers there or other tribes ran them out or absorbed them.
Does anyone know if we know much else? Being from the area I've had an interest in the people that were there before me, especially since we occasionally find flint not native to the area and in some cases arrow heads and tools.
"Circular Villages of the Monongahela Tradition" by Bernard Means is a 2007 is a good overview of this poorly understood culture. Bernard Means also publishes extensively in specialized archaeology journals, so if you have access to J-STOR, you will find a treasure trove of information about the Monongahela Culture, written by him.
Mary Butler wrote several articles about the WPA funded excavations of Monongahela sites, during the 1930s. Some of the sites they worked on in or near Meyersdale PA were visited again, more recently, when US 219 was being turned into four-lane highway that bypassed Meyersdale. Penn DOT published a book for grade school children that is a good introduction to the Monongahela Culture, based on what was found during the US 219 project at Meyersdale.
http://www.academia.edu/409895/Protohistoric_Monongahela_trade_relations_evidence_from_the_Foley_farm_phase_glass_beads
This web-site goes into great detail about the most interesting, late Monongahela site. Foley Farm has evidence of glass beads, showing that between 1600 and 1635 AD, the Monongahela Culture had indirect contact with Europeans and that French and Dutch glass beads were finding their way into Monongahela Culture sites, more than 100 years before any written sources described the region. Conrad Weiser, in 1748, was the first Englishman to write about his travels in western PA and the Monongahela Culture was long gone by then.
In addition to the sources davratta suggested (thanks for that, btw. Added Circular Villages to my Amazon wishlist and have a library copy on the way), I thought I'd summarize some additional information. Just to keep things simple for now, I'll only be focusing on the Monongahela culture. If we wanted to we could go way back in time for a discussion of human history in southwestern Pennsylvania, which has one of the oldest archaeological sites on the continent.
The average Monongahela town might be home to 75-150 people, living in houses arranged (as you might have guessed from the title of Means' book) in a circular pattern. Their homes were usual circular as well and about 20 feet in diameter, occasionally with "petal-like" rooms adjoining the main central room. These side rooms seem to have been for storage mainly or possibly for family sweat-houses. However, in at least one Monongahela town, the houses were rectangular, ranging from 8x16 feet to 20x30 feet. At another site, a large 30-foot diameter circular building was found, possibly a council house, religious structure, or both. Their towns were almost always surrounded by a palisade wall.
The center of the town was an open plaza, around which all the homes were placed. These plazas seem to have been kept clean and were likely swept clear of debris regularly. A central fire seems to have burn in this plaza in some Monongahela towns, while others seem to have had a pole. A pole in the middle of a central plaza is a trait they shared with their Fort Ancient neighbors to the west and the Mississippian cultures with whom they were contemporaries. The Monongahela also seem to have picked up the chunkey game from the Mississippians, while the Fort Ancients didn't.
The Monongahela were heavily dependent on maize agriculture, with other crops like beans and squash being a comparatively small part of their diet. Like many other Eastern Woodland cultures, the Monongahela stored their crops in pits that were protected from the elements by a bark-and-grass lining. These were found within homes, in side rooms, or elsewhere in town. They also gathered wild plants and hunted--though some sites on the Monongahela River itself seem to have relied very little on hunting in favor of fishing.
As for their "disappearance," European diseases are thought to have played a part but their not the sole cause. Their heavily reliance on maize, coupled with several years for drought conditions, has also been implicated. Weakened by disease and short on supplies, they were easy targets for the more powerful Seneca. Under pressure from the Seneca, the Monongahela dispersed and were absorbed into neighboring nations (including the Seneca).
The Monongahela are often identified as an Iroquoian-speaking nation. If so, it seems likely that their the same people as the historic Massawomecks--an Algonquian name for an Iroquoian-speaking people that frequently raided the Chesapeake Bay in the early 1600s, coming across the Appalachians and down the Potomac. The the Monongahela, the Massawomeck appear to have been an important link in the exchange of goods from Chesapeake Bay (acquiring those goods through both raiding and trading). The Massawomecks disappear from history about the same time the Monongahela do from archaeology; the last contemporary mention of the Massawomecks is from 1634. Europeans did encounter the Massawomeck, first as enemies and later as trading partners. Early Massawomeck-European (usually English) trade took place at the Great Falls of the Potomac. From there, it was a 5-10 days' journey to the Massawomeck's homeland, a trip that a few Englishmen claim to have made. Of course, it's not as clear cut as I'm perhaps making it sound here. Several colonial accounts indicate that the Massawomeck lived on the Great Lakes and within or near France's claims to Canada. This might be the results of confusion regarding exactly how far away French Canada was from English Virginia and attempts to reconcile John Smith's early account of the Massawomeck living on a saltwater sea with later information. You can see where Smith originally thought the Massaswomeck's lived on his map of Virginia, in the upper right hand corner. He put a large body of water right were the Appalachian Mountains are. He would later have this error pointed out to him by the Powhatans and correct it, but other colonial writers (notably Strachey) continued to repeat the mistake.
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