This is what I hoped being on the board of the Glendale Historical Society would involve: exploring mysterious, unknown ruins in the local mountains.
Leading up to the mysterious site is a canyon with the foundations of three structures that were apparently washed out during the 1934 flood. This canyon leads to a debris catch basin. Around the other side of the catch basin, up another canyon, lie the stone foundations of a camp, approximately a dozen structures. At the center of the "camp" is a large flagpole with a concrete base.
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Unlike the very accessible Brand Cemetery, unless you are an experienced bushwhacker I can't recommend you explore the site yourself. It requires trespassing and avoiding rattlesnakes, bees, and dense thickets of poisonous sumac. Here are two rattlesnakes I had a close encounter with:
My best guess at the moment is that the site was a Civilian Conservation Corps camp - I believe they worked on the nearby catch basin, so that would make sense. The only thing that feels odd about that hypothesis is the permanence of the rock foundations - it feels overbuilt for a temporary camp, and I haven't found references to a CCC camp near Glendale other than in Griffith Park and Tuna Canyon. Perhaps a base for the Army Corps of Engineers working on nearby flood control projects? Or a private ranch? A Boy Scout Camp? If you have any ideas, let me know.*
*Update: Mystery solved (by research in an actual library). Camp Clarence Edwards, 1933-1938.