I'm a pennsy native, and the combination of slope and shale is daunting- my parents added the third S- shade. the archangel is the ONLY think that grows on their property besides mayapples...but I got my original plugs from a lady who had it in full sun as a groundcover for when the deer ate everything else.

groundcover roses, lavendar and artemesia also make good second-string choices, once the others have established themselves...

and I wouldn't worry about the weeds too much- my mom got over them when she retired, and just has me scythe things off at knee height round about july (going over there this weekend, the rain's been helping things along)

wish I were closer (I'm out on the delaware) I'd come give you a hand!

Table Mountain Pine Grows naturally that sort of ecotype (almost exlusivly actually) and has low sweeping limbs that prevent the rain from washing the soil away. its pretty and has great wildlife value, and would probably help stabilize the soil better than just low-growing weeds.

Other attractive shrubby plants that would go well with table mountain pine are: Bear oak (Quercus ilicifolia), mapleleaf viburnum (Viburnum acerifolium), and low sweet blueberry (Vaccinium angustifolium) are the most important stand components in the northern part of the range of Table Mountain pine and all are very attractive to humans and wldlife alike, unlike tha groutweed and crown vetch, wich would be reletevly unappealing to wildlife, and those plants invade and destroy existing wildlife habitat.
Yoy could try little bluestem as gass, tough it takes a few years before it really gets top growth going- it invests most of its early energy into root growth.

the only place it doesn't seem to be a total pest?

low bush blueberry is a great idea, though- especially if there are younguns in the area who'd care to harvest them for a share of the spoils ;)

a low-growing sumac? ooh! yum! glorious ! MUCH prettier than crown vetch ;)!

Sometimes you have to pick from a short list of plant alternatives. Perhaps I am wrong about crown vetch, but I am seeing it advance at an alarming rate. Have seen it overtake groundcovers, invade grass, continue up a hill of shrubs. I am beginning to think it is the worst weed I have ever had to deal with. ooh! yummy!

Has anyone suggested sedum? There are many varieties in various heights and would make a lovely pattern.

They say they're well rated by GardenWatchdog-

There are also other low spreading groundcover junipers that have yellow and blue foliage to give a tapastry effect. This is assuming you want to plant it and leave it and not be out tinkering in it as they are prickly.

You can also interplant with heathers, Calluna vulgaris.
They are evergreen, love good drainage and sun and bloom and are hardy to zone 4 mostly. Ericas aren't all hardy, you want Callunas.