Park in the lot on the east side of Belair Road. Ride north on the road, across the bridge and over Little Gunpowder River. The white-blazed trailhead is just over the bridge on the right. The trail initially follows closely along the river, with a few tricky sections. There are many scenic spots along the way to photograph the river and even take a quick dip.
From there, the trail will climb away from the river and up into the forest. There is a long, sometimes steep, and very rocky climb. This is followed by repeated downhills and uphills before returning to the river. About a mile in from the trailhead, there is a fallen tree that completely blocks the trail. Once you get around this hazard (watch out for poison ivy), you arrive at the main stream crossing. You'll need to hike your bike across.
Stay on the Little Gunpowder Trail: Bel Air to Jericho Bridge along the river until you reach a large field marking the entrance to the Jerusalem Mill Village. Cross Jerusalem Road and catch the Horse Trail on the other side of the pedestrian bridge. Hop on the yellow-blazed Horse Trail out to the Jericho Bridge and then return loop around to the Village.
Once you cross Jerusalem Road, ride along edge of field and enter the blue-blazed Jerusalem Mill Trail on the left. It will be the third trail entrance. Don't worry if you pick the wrong one, the white trail will cross the blue trail again quickly. This trail is another climb through the forest away from the river. Lots of rocky, rooty technical sections along this entire trail. Some short, steep climbs with rocks and roots.
When the blue trail meets the white trail again, it is a bit confusing. Make sure you head the correct way on the white trail. You want to be sure to continue in the forest. There are still a lot of ups and downs ahead. If you come upon the river quickly, then you are headed the wrong way. You'll reach the stream crossing and downed tree on your way out. Ride the white trail back to the Bel Air Road bridge and parking lot.
If you're looking for a little bit more, instead of heading back to the parking lot, go down the stairs and under the bridge. From there, you can catch the Little Gunpowder Trail: Bel Air Rd. to Harford Rd. This loop will give you two more miles of rocky, root-covered trail along the river.