I have very, very little information on the Bly Granite Quarry as of now but I promise to dig further. A friend did find this information for me though: "The two quarries in the area were worked for dark gray gabbro in the 1920’s. Only one of these could be located during the present investigation: this was the quarry of the Bly Stone Company which was worked from 1921 to 1924. The other quarry, that of W. E. Vandeventer, was worked from 1921 to 1925.” This quarry was most likely used to build the dam nearby. This is a very beautiful hike in the Lake Hodges region and this spot is easy to miss as it's hidden from the main trail.