Table 1. Rock types and their nonzero-yield water wells in the Blue Ridge Province of Loudoun County, Virginia, sorted by geologic age and alphabetically. (Geologic abbreviations are from Southworth and others, 1999. Rock types are condensed from Southworth and others, 1999.) Geologic abbreviation Age Rock type Area (km2) Number of wells Mean yield (gpm) Jurassic Jd Diabase dikes and sheets 3.0 16 12.81 Cambrian Ca Antietam quartzite 1.6 31 15.22 Ccp Carbonaceous phyllite 0.6 7 17.61 Ch Harpers Formation phyllite and metasiltstone. 15.7 120 7.80 Cl Loudoun, phyllite 3.0 1 4.00 Ct Tomstown Dolomite 0.6 6 3.50 Cw Weverton Formation quartzite 5.8 23 7.43 Cwl Weverton Formation, lower member 10.6 11 17.68 Cwm Weverton Formation middle member, quartzite 10.9 11 26.09 Cwu Weverton Formation upper member quartzite and quartz pebble conglomerate 3.0 1 12.00 Late Proterozoic Zc Catoctin Formation metabasalt 191.0 724 11.72 Zcm Catoctin Formation marble 2.2 4 21.38 Zcp Catoctin Formation quartz muscovite phyllite 7.5 13 23.46 Zcr Catoctin Formation metatuff 0.9 11 10.54 Zcs Catoctin Formation metasandstone 0.3 1 30.00 Zfa Fauquier Formation meta-arkose 6.4 17 14.98 Zfs Fauquier Formation metamudstone 2.1 7 20.29 Zmd Metadiabase dikes 50.1 259 16.96 Zrd Metarhyolite dikes 1.2 7 7.57 Zrr Granite of the Robertson River Intrusive Suite 3.5 2 18.50 Zsm Swift Run Formation marble 0.2 1 8.00 Zsp Swift Run Formation marble, slate, and phyllite 9.1 68 19.47 Zss Swift Run Formation metagraywacke 8.4 57 24.82 Middle Proterozoic Ybg Biotite granite gneiss 12.1 74 16.09 Yc Charnockite 4.6 16 11.17 Yg Leucocratic metagranite 38.7 265 12.76 Ygt Garnetiferous metagranite 104.4 603 16.34 Yhm Hornblende monzogranite gneiss 43.7 210 14.73 Ylg Layered granite gneiss 49.8 160 15.98 Ymb Marshall metagranite, biotitic 146.1 868 16.72 Ymc Marshall metagranite, coarse grained 20.7 197 22.74 Yml Pink leucocratic metagranite 27.5 167 21.10 Yn Metanorite 6.3 29 10.92 Yp Paragneiss 6.9 49 12.66 Ypg Porphyroblastic granite gneiss 27.0 79 13.88 Totals 825.5 4,115 15.52