Not only is there in Virginia a lack of good roads, but the numerous streams have few or no bridges. In many cases where bridges have existed, one or the other of the contending armies has destroyed them to impede the march of its opponents. Streams which have an average depth of three or four feet are, however, generally without bridges, except where crossed by some turnpike, the common country roads mostly leading to fords. The famous Bull Run is an example. There were but two or three bridges over this stream in the space of country penetrated by the roads generally pursued by our army in advancing or retreating, and these have been several times destroyed and rebuilt. The stream varies from two to six feet in depth--the fords being at places of favorable depth, and where the bottom is gravelly and the banks sloping. Often such streams as this, and indeed smaller ones, become immensely swelled in volume by storms, so that a comparatively insignificant rivulet might greatly delay the march of an army, if means for quickly crossing should not be provided. The general depth of a ford which a large force, with its appurtenances, can safely cross, is about three feet, and even then the bottom should be good and the current gentle. With a greater depth of water, the men are likely to wet their cartridge boxes, or be swept off their feet. There is a small stream about three miles from Alexandria, crossing the Little River turnpike, which has never been bridged, and which was once so suddenly swollen by rain that all the artillery and wagons of a corps were obliged to wait about twelve hours for its subsidence. The mules of some wagons driven into it were swept away. Fords, unless of the best bottom, are rendered impassable after a small portion of the wagons and artillery of an army have crossed them--the gravel being cut through into the underlying clay, and the banks converted into sloughs by the dripping of water from the animals and wheels.