Recently a question arose on these pages regarding the history of the Nyack Brook. The issue was whether the brook may have been used as a geographic marker for African American slaves escaping North along the Underground Railroad. There was a suggestion that the brook was man made and not available as a marker during the years that the UGRR functioned.
With our library somewhat inaccessible it took a little extra searching to find the following. 
Click on the images to the left to enlarge each map. If closer examination by younger eyes reaches a different conclusion, please let me know.
Joseph Mitlof is founder of the Historic Underground Railroad Society.