Lines and Angles: Bomardment of Fort Jackson

Among the primary sources for the Battle of Forts Jackson and St. Philip are several maps drafted to support naval operations. The maps themselves were products of the Coast Survey, working with Army surveys done before the war.  The products of the survey in turn were used for operational planning.  That provides what in my generation of warfighters we’d have called an “overlay,” alluding to days of clear acetate and grease pencil smudges that passed for operational graphics.

Those graphics, at the time of the operation, are a valuable tool for the commander to relate the orders.  A well drawn graphic rises above any ambiguities, perhaps due to misinterpretation of the written word, and places a THERE on the map.  For the historian, the things that are recorded on the map provide some insight into the thoughts, concerns, or preferences of the commander and his staff.

A copy of both the survey team’s draft and the finished copy are part of Fold3’s free Civil War map collection.  Here’s the final:

The caption on the map reads:

Reconnoissance of the Mississippi River below Forts Jackson and St. Philip made previous to the reduction by the U.S. Fleet under the command of Flag officer D.G. Farragut, U.S.N.  By the party under the direction of F.H. Gerdes, Asst. U.S. Coast Survey, A.D. Bache, Supdt.

NOTE – O1, O2, O3 &c. H1, H2, H3 &c are points established by triangulation.  A, B, C, D &c are points on the left bank and 1, 2, 3, 4 &c. points on the right bank of the River, established for placing the Gunboats and Mortarboats in position.  The position of the Mortar Flotilla on the first day of the bombardment, April 18th was as follows – 6 mortars on the left bank between C & J, distance to Fort Jackson 3900 to 4500 yards — 14 mortars on the right bank from 1 to 5; distance to Fort Jackson 2830 to 3490 yards.  On the 19th, the 2nd day of the bombardment, they were all on the right bank and 20 mortars were placed distant from Fort Jackson 3010 to 4100 yards.  They remained on the 3rd and 4th days nearly in the same position.  All the large armed steamers and Gunboats were placed from ¼ to 1 ¾ miles below the lowest mortar vessel.  On the first day the small steam sloops and the Gunboats went up to abreast of the Smokestack, where they engaged the forts and the enemy’s steamers.

Here’s a close up view of the left edge of the map, showing the location of the forts and marked positions for the bombarding boats.

A set of lines show the surveyors paid careful attention to the layout of the forts.  One line shows the “lower limit of casemate tier” of Fort Jackson’s down river facing battery.  Another set of lines indicates the “sector without casemate fire.”   The river facing bastion created a blind spot that guns within the casemates could not cover.  Of course, guns on the barbette tier could cover that angle.  But the mortars were supposed to make those positions untenable, if not dismounting the guns entirely.  Notice what lays within that uncovered angle.  Yes, the hulks and chains obstructing the river.  Go figure….

Another line indicates the upper limit of the casemate guns, reaching across to Fort St. Philip.  While the arc of coverage from that fort’s casemates is not depicted, simple extrapolation demonstrates the “cross fire” from the forts lay upstream of the obstructions.   So if the mortars could suppress the open barbette tier and the exterior water batteries, the casemate guns would have difficulty covering the channel.

The maps pertaining to the “battle of the forts” also includes a survey of damage done to Fort Jackson:

A close up view shows large sections, including two bastions and the central citadel, burned.  But the most serious damage was to the glacis facing downriver.  Not only did this open the moat and interior of the fort to flooding, it also exposed the casemate walls.

With the masonry exposed, the attackers could then begin battering work… in due time…. But as related yesterday, Admiral Farragut didn’t want to prosecute a formal siege.  He wanted to get up river to his assigned target.  The forts were just impediments to be bypassed if need be.  Damaged, flooded, and demoralized, the forts alone could not hold the West Gulf Blockading Squadron at bay.


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