The field observations stopped at the second road crossing on Horsepen Run. Three corrugated metal culverts, approximately the same diameter as the culverts downstream, discharge the upstream water into the study area. The tributaries that were included in the study generally mimicked the local characteristics exhibited by Horsepen Run. The western tributary, which is on the upstream end of the study area, tends to meander more and carries less bedload than the Horsepen Run. It has several falls and pools where bedrock is exposed.
Some banks are high, steep and very eroded.
Shallow soils and rock outcrops contribute to the toppling of trees that grow along the tributary. This unit contains relatively young granitoid dikes that have been folded and crosscut. An upright syncline is mapped striking across the gneiss, approximately perpendicular to the course of Horsepen Run. Above the gneiss, the Quantico Formation is mapped. This unit is often referred to as the Quantico Slate.