-.Eh:, entire upper valley of Potomac stretches along the Blue Ridge, from south-west to north east, 160 miles from the sources of the Shenandoah to those of Conecocheague. It is 75 miles where broadest, between the Alleghany at the source of Potomac to the Blue Ridge, opposite the sources of Hedgman river; but the mean width is only about 50 miles; area 8000 square miles.
By reference to the tables of relative height, it will be seen, that the low water level in the river, at its passage through Blue Ridge, is 238 feet above the level of tide water in Potomac. The lowest elevation that can be given to any part of the arable soil of Upper Potomac valley, is at least 350 feet, or a full equivalent to a degree of latitude, as regards mean temperature. Under the head of the Appalachian chains, the rapid rise of their re spective vallies were shown, and we may, to save reference, observe here, that the valley of the South Branch, and which is continued northwardly across the valley of Juniata, and southwardly in the higher valley of James river, has its lowest point of de pression between Susquchannah and New river at Cutnberland, where the common water level is 578 feet above the Chesapeake tides. \Ve may there fore safely assume 600 feet as the lowest arable sur face of the South Branch of Potomac. Where the Ohio and Chesapeake canal is to pass the Alleghany mountain, between the sources of Little Will creek and Casselman river, or the extreme eastern branch of Voughingany river, the summit level is 1903 feet by actual admeasurement. The extreme highest peaks or ridges of Alleghany exceed 2800 feet; and we are safe in stating a difference of 1200 feet between arable land of the lowest part of the Upper Potomac valley, and the most elevated farms along the higher mountain vallies; and as many of these farms are actually on mountain table lands, the rapid change of temperature experienced in a few miles is fully accounted for from relative height.
Passing the Blue Ridge the Potomac Notts 15 miles S.E. by E. to the influx of Monocacy, and passage of the main stream through the South East Nlountain. At this latter pass the water level is still 224 feet above tide water; and similar to its inflections below Cumberland, and below its pas sage through the Kittatinny, the Potomac bends 5 or 6 miles to the south•westward, after passing South East Mountain, and thence very gradually curves to a course of S.E. which it maintains 30 miles to its final descent over the outer primitive ledge, into tide water at Georgetown. Below the head of tide water in Potomac, occurs one of those river resemblances which demonstrate the extent of mountain influence. Like the Delaware below
Trenton, and the Upper Chesapeake below the in flux of Susquehannah, the Potomac, after meeting the tide, first inflects along the base of the primitive ledge, by a course south of west upwards of 20 miles, and thence, by a very extensive curve, re gains a south-eastern direction. It is in the above noted great curve that the Potomac so nearly ap proaches to the Rappahannoc, as we have shown under the head of the latter river.
From its leaving Blue Ridge to where it finally quits the primitive ledge, between Charles county in Maryland, and Stafford in Virginia, the Potomac receives from the left Cotoctin, Monocacy, Seneca, Rock, East Branch, Piscataway, and Mattawoman; and from the right, Goose creek and Occoquan, with many of lesser note from each shore.
The Monocacy valley is a part of that between Blue Ridge and South East Mountain, the main or middle stream flowing, a little south of west, along the mountain valley, having interlocking sources with those of Conewago and Codorus branches of Susquehannah, comprising the far greater part of Frederick county, in Maryland, and the southern part of Adams, in Pennsylvania; length 50 miles, mean breadth 20, and area 1000 square miles.
Below the influx of Monocacy, no stream above the size of a large creek enters Potomac from Ma ryland. Rock creek, entering between Washington and Georgetown, and the North East Branch, both rise near Mechanicsville, in Montgomery county, and within twenty miles from the city. The latter opens to the width of a river, and forms the har bour at the United States navy yard, and gains im portance from this circumstance; but its extreme head is not quite as far remote from their common recipient as is that of Rock creek. Indeed, from the source of the Patuxent, in South East Moun tain, to its outlet into Chesapeake, the distance be tween its channel and that of Potomac in few places amounts to twenty miles, and does not average fifteen. On the opposite side to the wallies of Mon cacy, Rock creek, and North East Branch, there is a small section of Virginia drained into Potomac. This section, embracing the northern part of Fau quier, and all Loudon, Fairfax, and King William counties, is watered by Goose creek, Occoquan, and some smaller streams. But, as noticed under the head of Rappahannoc, the valley of Potomac be comes restricted to a narrow slip below tide water in the former. The whole of that part of the basin of Potomac below the Blue Ridge, from the source of Monocacy to Smith's Point, on the right side of the mouth of Potomac river, is in length 165 miles; the mean breadth 30; and area 4950 square miles.