I figure that during and after rainstorms there is probably going to be a stream running just east of the 1967 (and 1975, presumably) western boundary of the nursing home, that will empty out into the storm drain on Drumm. I believe this is the same boundary that nowadays separates the Drumm Court subdivision from the Starner Court subdivision. It is quite possible or even likely that the path that the Lyon sisters took ran near this boundary, perhaps in the conservation easement that exists now in the western part of the Drumm Court subdivision. Near the middle of this
easement there is a sewer easement dating from 1947, which I suppose has a sewer in it. Sewers tend to be built near places where creeks might be, since sewers need to be downhill from most of the surrounding land (as creeks are), and houses, etc., can't well be built near streams without having flooding problems. And conservation easements (which prohibit the cutting of trees except for extraordinary circumstances) tend to be designated near streams or gullies from temporary streams, where erosion is especially possible. It is obvious looking at topographical maps that there is something of a valley running about where the sewage easement is, even though there is no stream marked. This sewage easement starts at the end of Jennings Pl (the stub of a road that was never extended that the Washington Times article suggests is where the path through the wood started) and empties out into the clearing south of Hobson (eventually ending up near the storm drain, but probably not going into it since Montgomery County reasonably tries to separate sewage from storm drain runoff to make sewage treatment less costly). One possibility is that there was a path on or next to the sewer for maintenance purposes, which would have been on nursing home land, though.
A possibility is that a gully existed in the conservation easement, especially in the clearing south of Hobson (where there were no tree roots to protect from deep erosion), and that the path ran just west of this gully, more along the border line. When there is a sudden drop in a stream, there can be a great deal of undercutting taking place downstream from the drop. There is a place like that in the forest near our house here in North Carolina, where an intermittent stream goes over a large tree root that blocks the stream, producing a sudden drop. The left bank below is so scoured out underneath that the bank overhangs the stream by several feet, which we avoid walking on and which would be downright scary to be underneath. Now, if there were an overhanging bank overhanging (cantilevering) a stream, underneath would probably be the same scary, muddy, messy wet dirty area that exists in the stream bank here--I can't imagine the Lyon sisters would have gone into such a place (even to look at frogs that tend to hang out in such places at right time of year).
Suppose though that, say, right where the gully exited from the woods into the clearing, where it might have dropped precipitously, there had been scouring resulting in extremely overhanging banks, and that there had been partial (gradual?) cantilever failure of beam type (section 2.1.4 of this
excellent article from New Zealand) on one bank that was stopped by the other bank, forming an arched bridge of sorts across the stream. The day previous, March 24, was a very rainy day in some places across the Washington area. There was a thunderstorm that went through the previous day around 4-5pm that hit National Airport (though probably missed Dulles). According to
Weather Underground, National (DCA) got 1.35 inches for the day, about an inch of which was from the late afternoon storm. BWI got .77 inches for the day, while Dulles (IAD) supposedly got .17 inches (though the precipitation for Dulles isn't broken down by hour, so maybe there is something sketchy about the data there). It may have been that going south along the eastern edge of the clearing southish of Hobson was a standard shortcut from the mall, and that this would involve crossing the gully. The whole gully may have been a muddy, puddle-ridden mess except for where the natural bridge existed. The girls may have been walking together across the bridge to keep their shoes clean, perhaps not even realizing it was hollow underneath (and perhaps especially scoured by yesterday's storm), when the arch failed at the top, dropping them downward into a washed out area below. This would turn the remains of the arch into two cantilevers, each of which may have been too weak (quite possible since at least one had already collapsed partially) to be held without the arch. Thereupon, both stream banks may have collapsed on top of them, each a cantilever collapse of the beam type (the most common type of cantilever collapse), burying them under feet of mud and changing the stream bed from looking (upon close inspection) bizarre into looking like a fairly normal stream bed. The cantilever failure may have continued downstream a good ways, rendering the particular spot not especially unusual looking. But I am no expert on geology, much less stream bank geology, so maybe such bridges are essentially impossible, or maybe some other stream bank failure be responsible, or maybe there was no real gully there. Or indeed as SteveP120 suggested, a murderer may have buried them in the stream bed (or they are somewhere totally different--gullies and streams may have nothing to do with anything). Perhaps SteveP120 or someone else who lives nearby could drive or jog by the storm drain on Drumm after a storm and see how much of a stream is running into it? Maybe the stream SteveP120 was referring to was a stream in the place I indicated that he has seen there in his jogging?
