The Photographic History of The Civil War: in ten volumes, Thousands of Scenes Photographed 1861-65, with Text by many Special Authorities, Volume 5: Forts and Artillery.
Francis Trevelyan Miller, Ed.

[94] eyes were directed to General McClellan, whose successes had already made him a marked man, and under the direction of that able organizer a more secure feeling immediately appeared. He directed the immediate completion of the fortifications of the city, and also bent his energy to organizing the great Army of the Potomac.

Once the positions on the right bank of the Potomac were reasonably secure through the works just mentioned and such additional defenses as Fort Albany, Fort Scott, and various lines of connecting fortifications, attention was given to the Washington side of the river. In the summer and autumn the Potomac is fordable at points not far above Washington, and as the river became lower apprehension increased that the victorious foe, who still rested at Manassas, would avoid the works on the Virginia side, cross above Georgetown, and attack from the Maryland side of the city. To meet the emergency, works were hurriedly thrown up without that careful preliminary study of the topography which the occasion really demanded.

The securing of the roads was the first consideration. The main road which followed the general line of the crest between Rock Creek and the Potomac, branched at Tennallytown, about a mile south of the District line, and entering and leaving the town were other important roads. As this was on fairly high ground it was selected as a proper point for a work, and Fort Pennsylvania (afterward Fort Reno) was placed there. Thus was established one point of the line of works. Fort Stevens, commanding the Seventh Street Road, running north, and Fort Lincoln, commanding the Baltimore turn-pike and the Baltimore and Ohio Railroad, together with Forts Totten and Slocum, between these latter roads and the Seventh Street Road, were all simultaneously started. All these works were on the crest of a somewhat irregular ridge overlooking the valley of Sligo Branch. This carried the general project from Tennallytown, within two miles of the Potomac, around to the north and east of the capital to Anacostia Branch.