On the 7th of May the British undertook an expedition against the galleys and other shipping which had escaped up the Delaware after the reduction of Mud Island, and destroyed upward of forty vessels and some stores and provisions. The undisputed superiority of the British naval force and the consequent command of the Delaware gave them great facilities in directing a suitable armament against any particular point, and the movements of the militia, on whom Congress chiefly depended for repelling sudden predatory incursions and for guarding the roads to Philadelphia, were often tardy and inefficient. The roads were ill guarded, and the British frequently accomplished their foraging and returned to camp before an adequate force could be assembled to oppose them. To remedy these evils--to annoy the rear of the British troops in case they evacuated Philadelphia, which it was now suspected they intended to do, and also to form an advanced guard of the main army--Lafayette, with upward of 2,000 chosen men and six pieces of artillery, was ordered to the east of the Schuylkill, and took post on Barren Hill, seven or eight miles in advance of the army at Valley Forge. Sir William Howe immediately got notice of his position and formed a plan to surprise and cut him off. For that purpose a detachment of 5,000 of the best troops of the British army, under General Grant, marched from Philadelphia on the night of the 20th of May and took the road which runs along the Delaware and consequently does not lead directly to Barren Hill. But after advancing a few miles the detachment turned to the left, and proceeding by White Marsh passed at no great distance from Lafayette's left flank and about sunrise reached a point in his rear where two roads diverged, one leading to the camp of the marquis, the other to Matson's ford, each about a mile distant. There General Grant's detachment was first observed by the Americans, and the British perceived by the rapid movements of some hostile horsemen that they were seen. Both Lafayette's camp and the road leading from it to Matson's ford were concealed from the British troops by intervening woods and high grounds. General Grant spent some time in making dispositions for the intended attack. That interval was actively improved by Lafayette, who, although not apprised of the full extent of his danger, acted with promptitude and decision. He marched rapidly to Matson's ford, from which he was somewhat more distant than the British detachment, and reached it while General Grant was advancing against Barren Hill in the belief that Lafayette was still there. The Americans hurried through the ford leaving their artillery behind, but on discovering they were not closely pursued some of them returned and dragged the field pieces across the river; a small party was also sent into the woods to retard the progress of the British advanced guard, if it should approach while the artillery was in the ford.