The OML crosses back into Howard County from Carroll County via this bridge over the Patapsco. Here, we're so far west that the river is little more than a stream. For access, drive east along the dirt road that connects to Watersville Road on the north side of the tracks. The source of the Patapsco River, which the OML faithfully follows all the way from Baltimore, is located just south of Mt. Airy. Rain falling here on the east side of town finds its way to Baltimore harbor.
Rain falling on the wide side of town flows toward the Potomac River and Washington.
So, this area is a watershed divide. The best route the surveyors could find in 1830 was one that circled around a hill. At these locations, known as Planes, horses were employed to assist the early, weak locomotives up the hills. Later steam power was tried. Eventually, the power of the locomotives increased such that they were able to negotiate the steep plans by themselves.