Blockhouses

by Gary M. Petrichick 

The Potomac Valley, a boundary between the Union and Confederacy, was also home to the Chesapeake & Ohio Canal and the Baltimore & Ohio Railroad, key transport links for the Union Army and the District of Columbia.  The Federal government worked both to protect those links and defend against Confederate raids across the Potomac. A new element in defensive warfare at the time was the blockhouse — a miniature fortress placed at strategic points housing small troop units.

Festus Summers in The Baltimore and Ohio In The Civil War  cites a Federal officer’s description of the blockhouses:

They were barns, constructed out of the largest and longest logs that could be obtained, each barn being from forty to fifty feet square and ten to twelve feet in height. They were timbered on the top to keep the shells out and were notched through the sides for the purpose of shooting through above the earthwork. In building them stone is first piled around the proposed foundation, about four feet high, then a deep ditch, four or five feet wide, is dug around the stone pile, the earth from the ditch being thrown upon the stones to a height of six or seven feet to protect the inmates of the house from shot and shell. The whole is surrounded with an abatis; we had no barbed wire in those days and the entrance to the building was made in a zigzag fashion.

Another description of blockhouses is in the History of the Nineteenth Regiment Massachusetts Volunteer Infantry which says:

The duties of the regiment, were...three defensive blockhouses to build, 48 feet each way, of the shape of a Greek cross, four feet thick, twelve feet high, with loopholes for infantry arms, roofed with logs three feet thick and covered then with three feet of earth. Two hundred and twenty five officers and men of the regiment were engaged in this work. All the logs used in the construction had to be felled and squared with common axes, hauled to different eminences on which the buildings were to be erected, and then placed, each log being fitted and pinned with treenails.

In December 1861, the 19th Massachusetts Infantry went from Poolesville to a camp at Muddy Branch near Seneca for guarding the Potomac from Great Falls to Seneca Falls and building three blockhouses at Pennyfield Lock (Mi. 19.6), Blockhouse Point (Mi. 21.0), and the Seneca Locks (Mi. 22.1). One source claims that nine blockhouses were placed between Great Falls and the Monocacy River. Following the Battle of Balls Bluff in October 1861, blockhouses were built on the Maryland shore at the middle and northern end of Harrison Island (Mi. 34.0), and at some time in 1864 one was built to protect  Young’s (Selden) Island Ford (Mi. 29.0). Supposedly "one of many," the above six are the only specific locations along the canal I have found. In September 1863, Monocacy Division Superintendent George Spates complained that he was having trouble finding timber for lock gate repairs because the government was cutting all the good trunks for building blockhouses, and again in March 1864, that the government was using his scow for building blockhouses at various fords.

Other blockhouses mentioned in the Potomac Valley were on the B&O RR at the mouth of Patterson Creek, the mouth of the South Branch, and across from Oldtown, Maryland, at Green Spring Depot, all on the West Virginia shore. The last was reportedly burned by Capt. Frank Imboden’s Confederates in June 1863, and must have been promptly rebuilt because the 153rd Ohio National Guard was captured there by retreating Confederates following the Battle of Folcks Mill in August 1864. 

I found nothing on the involvement of blockhouses during the 1862 Antietam and 1863 Gettysburg campaigns, though the Seneca blockhouse should have been in place at the time of Stuart’s crossing there in June 1863. There are references to a Muddy Branch blockhouse being destroyed by Mosby in July 1864. Blockhouses west of Harpers Ferry were built to protect the B&O RR bridges, but along the C&O Canal east of Harpers Ferry, it would appear that the primary concerns were guarding river fords and the defense of Washington.

Resources (on blockhouses):

French, Steve, Imboden’s Brigade in the Gettysberg Campaign

Housely, Don, History of the Camp at Blockhouse Point

Jacobs, Charles T., Civil War Guide to Montgomery County

Scott, Harold, Sr., The Civil War Era in Cumberland, Maryland, and Nearby Keyser, West Virginia

Soderberg, Susan Cooke, A Guide to Civil War Sites in Maryland

Summers, Festus P., The Baltimore and Ohio in the Civil War

Toomey, Daniel Carroll, The Civil War in Maryland

Unrau, Harland D., Chesapeake and Ohio Canal NHP Historic Resource Study – History of the C&O Canal

(This article was published in the December 2009 issue of Along The Towpath, the newsletter of the C&O Canal Association.

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