Before Joshua Tree became a national monument (and, later, expanded and turned in to a national Park) much of the land was subject to mining and/or grazing. The remnants of that past are on display in many parts of the park. One such place is here, at Contact Mine.
This turned out to be true. At the exhibit at the parking area (about 1/2 mile inside the North Entrance of the Park), only mention of the California Riding and Hiking Trail was made. That one starts on the east side of the road, then loops along the northern portion of the park, ending near the Black Rock area, far to the northwest. I had hiked around that section during my first 100 hike year, when I went to Eureka and Warren Peaks.
[Note: That's no longer the case, today. The Contact Mine trailhead has clear signage, now]
After 1/4 mile or so, I saw some poles and barrier material on the hill, far above and to my right. Obviously some mining ruins there, but that was not the main goal, so I continued along the well-defined trail.
As noted previously, this was about 3.6 miles roundtrip. Modest altitude gain. I would guestimate no more than 200 feet. Never far from the road, but this area did not appear to get a lot of foot traffic. Nice little hike.