When the Army changed from the Model 1840 mortars to the Model 1861 patterns, the entire weapon system underwent a redesign. Since the mortars dispensed with the wooden quoin elevation wedge, centered the trunnions for zero preponderance, and used elevating sockets on the breech, the old mortar beds just would not work. So a new mortar bed design went with the new mortars. The examples at Fort McNair sit atop original mortar beds, allowing for examination.
When Major Theodore T. S. Laidley wrote the 1862 edition of the Ordnance Manual, these beds were still new. He described them rather ambiguously, “The mortar-beds for the new-model mortars are made of wrought iron. Their details are not determined with sufficient accuracy to be inserted at this time.”
Two decades later, Colonel John C. Tidball provided a bit more descriptive examination of the beds in his Manual of Heavy Artillery:
[Mortar carriages] are constructed and put together in a manner similar to the top-carriage for guns. At the ends of each cheek are projections, called front and rear notches, underneath which the cannoneers embar with their handspikes to move the carriage. On those for siege mortars there are also two front and rear maneuvering bolts for the same purpose. The bottom part of each cheek, resting on the platform, is called the shoe; the front and rear ends being designated the toe and heel, respectively.
Here’s Tidball’s drawing of the mortar on its “carriage” as he called it:
The bed pictured above matches that description. The cheeks are connected with two cross members and a transom. On that transom is the eye for the elevating bar. The mortars at Fort Macon, North Carolina are a bit better prepared for public viewing in that regard:
From the side, the mortars feature a rough triangle shape.
As Tidball described, there are “toes” and “heels” with the “shoe” resting on the ground. The “toes” and “heels” provided clearance as the crew wedged the mortar up to adjust the azimuth. Notice the maneuvering bolt sticking out at the front. The surviving examples I’ve seen lack the rear bolts Tidball described.
Tidball put the weight of the 10-inch mortar’s wrought iron bed at 1313 pounds. That of the 8-inch mortar weighed 900 pounds. This put the weight of the mortar and bed at 3213 and 1910 pounds, respectively. Mortar wagons could carry three of the 8-inch mortars, but only one of the 10-inch. Eight horses pulled that load.
But that was not the complete system. This wartime photo of Fort C.F. Smith shows one of the 8-inch Mortars on a wooden platform. Even has the bars maneuvering hadspikes laying on the side.
The platform was a necessary component of the mortar system. The wooden platform prevented the shoe from digging into the earth upon firing. It also afforded a relatively smooth surface on which the crew could adjust the mortar’s placement. Tidball provided details of the platform’s materials:
The sleepers rested on ground cleared and leveled by the crew. The deck-planks lay on top of those, just above the ground to ensure drainage. As you can see from the chart, this added another 1400 pounds, give or take, to the weight of the entire system. Just more stuff to haul around the battlefield. But necessary stuff.
The mortars at Fort Macon have a reconstructed platforms offering visitors a chance to see the entire system as it would have looked when employed during the war.
The only other piece of equipment I’d like to see there would be a mortar wagon and limber. But I’m sure *someone* who handles things down that way has already put those on the wish list.
To the Sound of the Guns 
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