Reports of Brig. Army, Chief Engineer Army of the Potomac, of operations during the siege.
CAMP NEAR YORKTOWN, VA., May 6, 1862.
SIR: The accompanying drawing (map No. 2) (All maps referred to in this report will appear in Atlas.) gives with accuracy the outline and armament of the fortifications of Yorktown proper, with the detached works immediately connected therewith (map No. 3). The general outline is almost the same as that of the British works in the Revolution; the trace is somewhat different. The profile is everywhere respectable. The three bastioned forts looking toward our approaches appear to have been earliest built, and have about 15 feet thickness of parapet and 8 to 10 depth of ditch, the width varying much, but never being less at tops of counterscarp than 15 feet, and I should think generally much more. The works extending around the town from the western salient of fronts just mentioned appear to have been finished during the past winter and spring. They have formidable profiles, 18 feet thickness of parapet, and generally 10 feet depth of ditch.
The water batteries had generally 18 feet parapet, the guns en barbette. They were (as well as all the works mentioned) carefully constructed, with well-made sod revetments. There were numerous traverses between the guns and ample magazines, how sufficient in bomb-proof qualities I am unable to say. The water batteries were as follows: No. 1, five 8-inch columbiads; No. 2, four 8-inch columbiads; No. 3, three 32s and one 32 navy; No. 4, three 32s (1827); No. 5, two :32s; No. 6, three 9-inch Dahlgrens and one 10-inch Dahlgren; No. 7, on beach, three 8-inch columbiads and one 64-pounder, besides a 42-pounder carronade, intended to sweep the shore.
The first two guns of the works on the heights bore upon the water as well as the land, and were of heavy caliber. The guns in position on the fronts of attack (the first two of which bore on the water) were as follows, commencing on the left. (See the list herewith, which gives all the guns in position or for which there were emplacements.) The emplacements were all occupied before the evacuation by siege guns, rifled, 4 1/2 -inch 24-pounders and 18-pounders.
In Fort Magruder (the first exterior work) there were found one 8- inch columbiad, one 42-pounder, and one 8-inch siege howitzer, the two former en barbette. The sketch will show the emplacements for guns on field or siege carriages, making, I think, with the foregoing,  twenty-two. Two of these were placed behind traverses, with embrasures covered by blindages. The two external redoubts with the connecting parapets formed a re-entrant with the fronts of attack, and all the guns bore on our approaches. It will be seen, therefore, that our approaches were swept by the fire of at least forty-nine guns, nearly all of which were heavy, and many of them the most formidable guns known; besides that, two-thirds of the guns of the water batteries and all the guns of Gloucester bore on our right batteries, though under disadvantageous circumstances. Besides the above there were emplacements for four or five guns in the entrenchments running from Yorktown toward Fort Magruder. The guns on barbette carriages had not any protection, except in a few cases sand bags had been piled up. It is supposed that they awaited further indications as to the localities of our batteries before constructing merlons. For the guns on ship or siege carriages some arrangements had been made for protection by building up sodded merlons, or by sand bags and cotton bales, but as they were they would have been very inefficient against our fire.
The ravine behind which the left of the Yorktown fronts of attack was placed was not very difficult, and its head formed depressions in front of their left imperfectly seen by their fires, and from which access could be had to the ditches, but we could not be sure of this fact before the evacuation. The enemy held, by means of a slight breastwork and rifle trenches, a position in advance of the heads of these ravines as far forward as the burned house. Our own rifle trenches were advanced to within 60 yards of the burned house—a point from which the day before the evacuation I made my last reconnaissance. Owing, however, to the fact that the enemy’s riflemen were better concealed by shrubbery, &c., than our own, our men, who had just constructed their trench the night before, did not dare to show their heads or use their rifles, and I was unable to examine the grounds in front.
The ravines which head between the Yorktown fortifications and exterior works are deep and intricate. They were tolerably well seen, however, by the works which run westwardly from the Yorktown works, and which were too numerous and complicated to be traced on paper.
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Official Records of the Rebellion: Volume Eleven, Chapter 23, Part 1: Peninsular Campaign: Reports, pp.316-317
web page Rickard, J (4 February 2007)