In 1881, Madeleine Vinton Dahlgren, wife of Rear Admiral John A. Dahlgren, began construction of a private chapel across from her manor house known as South Mountain House. No expense was spared in the construction. The chapel was completed in 1884 and consecrated as "The Chapel of Saint Joseph of the Sacred Heart of Jesus" by Archbishop James Gibbons of the Archdiocese of Baltimore.
The area is very rugged, rocky and underdeveloped. The Appalachian Trail runs right next to the church, goes over Route 40, an continues right into the woods; it is a really neat site to see. The church sits in the Turner's and Fox's Gaps Historic District but is not a contributing structure as it was built after the Civil War. There is also an historical marker at the site as well. That marker reads: This chapel was built around 1881 by Madeline Vinton Dahlgren, widow of Admiral John A. Dahlgren, USN, inventor of the Dahlgren gun, the armament used by the USS Monitor against the CSS Virginia, formerly the steam frigate USS Merrimack. SOURCE
Naturally, there is an America Guide Series entry as well:
The Little Stone Church (R) was built in 1881 by Mrs. Madeleine Dahlgren, widow of Admiral John A.B. Dahlgren, expert on naval ordnance. The admiral's grave is inside the church, which is no longer used. --- Maryland: a Guide to the Old Line State, 1940; page 334
From the onset of construction, Madeleine Dahlgren’s private family chapel atop South Mountain would take nearly three years to complete. The stone would be quarried behind South Mountain House, and the walnut timber cut from stands on the estate. Records show that work progressed over the next year with the bell being ordered from McShane Bell Foundry, Baltimore, MD on January 17th and delivered at a cost of $28.25 on June 23, 1882. The bell, cast in manure and mud, reputedly weighed 400 lbs. SOURCE
Upon its completion, the English Gothic Revival style St. Joseph’s Chapel of the Sacred Heart of Jesus, would boast 18” thick stone walls and buttresses, slate roof, hand-painted and stained glass windows in the Nave, as well as a large rose window in the gallery accessed through the bell tower. It would measure approximately 68’ x 24’ with the attached bell tower approximately 40’ high. Above the marble floors and wainscoting, the interior walls and ceiling were sheathed in native walnut paneling culminating in bracketed and trussed walnut arches supporting the roof.
The chapel was consecrated by Archbishop Gibbons on July 29, 1884 with six additional priests assisting. The chapel, referred to by Mrs. Dahlgren as “the South Mountain Mission”, was presided over by clergy from St. Augustine Church, Williamsport and St. Mary Church, Hagerstown, MD with regular services being held from May to October. Her reference to the private chapel as a mission is interesting as local remembrance holds that Mrs. Dahlgren actively invited and encouraged local citizens to attend mass at the chapel. SOURCE
I also was able to retrieve some interesting information from Wikipedia. The chapel was built for Sarah Madeleine Vinton Dahlgren, daughter of Congressman Samuel Finley Vinton, who had married Admiral John A. Dahlgren, inventor of the Dahlgren gun, in 1865. Admiral Dahlgren died in 1870. Mrs. Dahlgren purchased a former tavern on South Mountain at Turner's Gap as a summer retreat, naming it Dahlgren Manor. The chapel was built across the National Road from the house. When Mrs. Dahlgren died in 1898, she was interred in the chapel's family crypt.
After a period under the ownership of the Sisters of the Holy Cross from 1922 to 1925 the chapel returned to the Dahlgren family. It was purchased in 1960 by Richard G. Griffin, who undertook a restoration. The property was acquired by the Central Maryland Heritage League in 1996. The chapel is included in the Turner's and Fox's Gaps Historic District, but as a non-contributing structure, owing to its post-Civil War construction, which places it outside the historic district's time of historic emphasis. SOURCE