Greatly strengthening the Confederate position was the irregular topography which resulted from the peculiar characteristics of the region’s loess soil. Possessing an unusual tenacity, except when eroded by the action of running water, the loess had over the centuries been cut into deep gullies and ravines with abrupt faces separated by narrow, twisting ridges. This resulted in a broken and complicated terrain which would seriously obstruct the Union movement. To permit a clear field of fire and to hinder advancing troops, all the trees fronting the Confederate line were cut down. Several hundred yards away from the Confederate position and roughly parallel to it was a ridge system not so continuous and more broken than that occupied by Pemberton’s Army. Along this line, the Union Army took position and began its siege operations. On the scattered natural bridges of high ground, which spanned the ravines and provided approaches to Vicksburg, were located the six roads and one railroad leading into that city. Nine forts had been constructed overlooking each of these routes into Vicksburg, their guns completely commanding the approaches—Fort Hill on the river north of the city, Stockade Redan, Third Louisiana Redan, Great Redoubt, Second Texas Lunette, Railroad Redoubt, Fort Garrott (also known as Square Fort), Salient Works, and South Fort on the river below Vicksburg. (All but two of these works are well preserved today.) The Confederate divisions, left to right, were commanded by Maj. Gen. M. L. Smith, General Bowen, Maj. Gen. John H. Forney, and General Stevenson. The Army of Vicksburg, at the beginning of the siege, numbered about 31,000 men, of which Pemberton listed 18,500 effectives as available to man his defense line. Grant gave his strength, shortly after the siege began, as 50,000 effectives; his army was steadily enlarged during the siege by reinforcements from Memphis. THE ASSAULT OF MAY 19. By midday of May 19, Grant had completed his investment of the city. In the north, Sherman’s Corps was in position opposite the Confederate left from the river (at the present location of the national cemetery) to the Graveyard Road, at an average distance of about 500 yards. McPherson’s Corps took position on Sherman’s left from the Graveyard Road to near the Baldwin’s Ferry Road; the front of McClernand’s Corps extended from the Baldwin’s Ferry Road southward.