City Creek Nature Notes – Salt Lake City

January 4, 2017

January 3rd

Filed under: Geology, mile 1.2, picnic site 8, red bridge — canopus56 @ 12:51 am

Erosion

5:30 p.m. In the last twenty-four hours, about a one-foot of snow has fallen in the canyon. I only go for a short run to milepost 1.0 and the red bridge. Up canyon from the red bridge to mile 1.1 and below picnic site 7, as noted in part on December 24th, the canyon contains three types of rock. First, on the south east canyon wall and running down to the far (south) side of the stream are shelves of “Tertiary Conglomerate No. 2,” a “[c]onglomerate and sandstone and sandstone, pale-brown to medium-gray, poorly consolidated to well-cemented. . . .” (Van Horn and Crittenden’s 1987). The effect of erosion in the canyon, by both wind and water, are best seen here. The water-drilled natural arch at the red bridge (Dec. 24th) is made of Van Horn’s red sandstone conglomerate. Above on the south canyon wall is Chimney Rock. Chimney Rock is eroded by many air and freeze-thaw water hollows and proto-arches. Second, above the natural arch at the red bridge and on the south-east canyon side of the stream are layers of Tertiary Conglomerate No. 2 of the Van Horn’s medium-gray color type. Both these layers formed between 35 and 37 million years ago when the canyon and Utah were part of an inland sea at the foot of a mountain range to the west. The lower gray layers were deposited when the sea was deeper, and the upper sandstone as the mountain ranges were nearly worn down and the sea was filling in. Between the red bridge and below picnic site 7, large grey-green conglomerate boulders stick out of the hill-side and are being undercut by stream. In one extreme case, an overhanging fifteen foot ledge has been undercut by the water, but it is being held in place by tree growing on its top side. In another unusual example, a stone the size of a house is partially undercut by the stream (described on Dec. 31st as providing ice-forming shade). In the future, it this undercut outcrop will fall into the stream and perhaps form a new natural arch. Third, on the west side of the stream and extending down to Guardhouse Gate are, according to Van Horn and Crittenden, “Tt – Tuffaceous deposits (Tertiary) – Siltstone, sandstone, and limestone containing abundant volcanic shards . . .” This layer is also between 35 and 37 million years old. Three hundred feet upstream from the red bridge, a twelve foot tall boulder of Tertiary Conglomerate No. 2 has rolled into the middle of the stream. It is an anomaly, in that it survives undercutting, but has been carved by water into an an egg shape. I call it the “zen rock”.

In Thoreau’s “Journal” on January 3rd, 1853, he identifies nature with the female aspect and humans with the male aspect.

On January 3rd, 1911, geology professor Fred J. Pack gave a lecture on the Wasatch Fault, and described how the line of the fault extends from Box Elder County in the north, travels through City Creek Canyon, and then on to Nephi in the south. Dr. Pack described a 100 foot escarpment caused by an earthquake that is associated with the fault near the Beck Street Hot Springs (Salt Lake Tribune).

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