According to a dispatch issued by the N.Y. Herarld on April 28, 1863 "...The expedition to the Northern Neck returned last night....A number of prisoners were taken and some important letters found in the houses of promminent secessionists. Among the correspondence captured by Captain Mann was a letter to authorities in Richmond, urging that measures be taken for securing the exchange of leading citizens of that section now in our custody, alleging that their influence was indispensable in averting a negro insurrection and assisting smugglers. After this exploit the rebels fired the bridge over Mattox creek, a large unfordable stream; but the expedition returned before it was consumed, and crossed while the structure was in flames....". Where is Mattox creek located?