Cumberland Gap, Tenn., which had been well fortified and occupied by the rebels for the year past, surrendered to the Union forces under the command of General Shackelford, without firing a gun. The garrison consisted of four regiments, namely, Fifty-fifth Georgia, Sixty-fourth Virginia, Sixty-second and Sixty-fourth North-Carolina, a portion of Leyden's artillery, Captain Barnes's company, of Georgia; also Fain's Tennessee battery, commanded by Lieutenant Conner.--A cavalry force belonging to General Herron's army, under Major Montgomery, on a reconnoissance from Morgan's Bend, La., met a party of rebel pickets about three miles from the river and commenced skirmishing with them, continuing all day, the rebels constantly falling back, the Unionists following until the rebels had crossed the Atchafalaya River, twelve miles from the position where the skirmishing commenced. Here the rebels made a stand, and crossing the river being impracticable, the Unionists fell back and encamped for the night, with a loss of one killed and eight wounded.--this evening the monitor Weehawken went aground midway between Forts Sumter and Moultrie, in Charleston harbor. Several attempts were made to get her off, but each proved ineffectual. To. ward evening the Ironsides, with the monitors Nahant, Montauk, Patapsco, and Lehigh commenced a vigorous bombardment of Fort Moultrie, withdrawing at dark.