Wednesday, March 18, 2015

Geo 1095: March 18, Day 807: Perched Water

Visible here are the same two units we saw at the cut ~100 yards westward: a lower, apparently homogenous and more or less massive finer layer, and an upper layer of poorly sorted, but overall, much coarser material. The upward grading seen in that previous exposure, if it exists at all, is not as evident here. However, there is an immediately obvious and interesting phenomenon illustrated in this photo: the lower unit is clearly not as permeable to water as the upper one. So water percolates down through the upper unit until it reaches its lower contact, then flows along that surface until it reaches the exposure, where its reliable presence has allowed a small colony of shrubs to become established. If you look below and to the right of the largest clump of plants, you can see a darkened area where the surface is much damper than its surroundings. This was a late, dry fall; we normally get some rain in September, but I don't think we'd seen any rain since June or July over this summer. So the presence of significant moisture this much later in the season, and at this high an elevation (we're near 5000 feet here), indicates that it really is a very reliable source of moisture.

Funny... I knew I had posted an analogous photo toward the end of last year, and intended to link it. Turns out, I had given that post the same title I gave this one.

Photo unmodified. October 9, 2012. FlashEarth Location.

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