City Creek Nature Notes – Salt Lake City

December 24, 2016

December 24th

Filed under: Geology — canopus56 @ 8:03 pm

Natural Bridge

External Link to Image

1:00 p.m. Today, it rained in the morning and then was overcast. It is a Friday before the Christmas weekend and the canyon is full of walking and strolling friends and family.

The red bridge is about a quarter-mile below milepost 1.0. This steel bridge protects the main water line take-off that splits a portion of the supply that climbs the hillside to Morris Reservoir above 13th Avenue. The Morris Reservoir is the primary water source for the Avenues, and historically, the aqueduct and now pipeline at the bridge was called the “Twentieth Ward takeoff”. Thirty feet up canyon from the steel crossing is a natural bridge over the City Creek stream, and it was first discovered by Euro-Americans in the late 1800’s. There are not many archived historical photographs of one of northern Utah’s natural bridges. Some can be found in the Utah Heritage Arts archives at the University of Utah, including an 1894 photograph of Ms. Margaus Glade and Ms. Winifred Laville in Victorian garments perched on the boulders during a summer outing. Another appeared in the July 26, 1903 Salt Lake Tribune. A commercial photographer, Shipley, took a photograph of the bridge during a 1920’s outing. These pictures show that the bridge is different now from when it was discovered. There used to be a large boulder on the near bank, still present, that leaned against a smaller boulder on the far bank, now gone. The two leaning boulders formed the bridge.

At some point, probably the City removed smaller block on the far bank. The removal may have been related to the construction of the Twentieth Ward takeoff gate or as a result of the 1983 severe floods. But the real cause of the change is lost to history. The broken remains of the smaller block appear to be stacked just down canyon of the bridge as a small grouping of four stones about three feet high.

But the canyon would not be denied its bridge. The stream rerouted under the larger stone, and carved another five foot tall bridge through the center of the rock. Thus, a natural bridge remains at this site.

The larger stone is not made of a sturdy material, and it consists of the eroded remains of a great north-south running mountain range, equal to the modern Rockies, that went through central Nevada about sixty million years ago. As the mountains eroded into a shallow sea that covered the canyon at that time, successive layers of an aggregate of sand, mud, and rounded stream rocks were laid down where the red bridge now stands. Van Horn and Crittenden’s 1987 geologic map describe the layers and the large stone as “Tertiary Conglomerate No. 2,” a “[c]onglomerate and sandstone and sandstone, pale-brown to medium-gray, poorly consolidated to well-cemented. . . . The conglomerate appears to be about 2.6 km thick” (id). Layers formed that compressed the rock into the horizontal sandstone layers seen on the high canyon cliff walls that frame the first mile of the canyon. Above and to the southeast of the red bridge and the natural arch is a prominent feature that I believe was called “Chimney Rock” by early miners (see Salt Lake Tribune Jan. 19, 1875). In those high walls and chimney rock on the south side near the red bridge, numerous wind scoured arches are trying to form, like those that make southern Utah famous. But unlike the Jurassic sandstone of southern Utah, these layers are too friable, and any future proto-arch will crumble.

The top of the larger boulder has also changed. The boulder tilted, and the more horizontal surface collected wind blown soil. Small trees sprouted. Although feet from the road, the top of the boulder is not visible behind spring’s foliage nor is it easily climbed. In summer’s heat, this is a favored spot for a nap or a meditative rest.

In Thoreau’s “Journal” on December 24th, 1850, he observes a shrike that has captured, killed and is eating a small bird. On December 24th, 1853, he finds more moth or butterfly cocoons on trees. On December 24th, 1854, he observes how a winter rain storm has glazed trees with ice. On December 24th, 1859, he measures the dimensions of a large blueberry bush. By counting the rings in one bush, he estimates it to be sixty years old.

On December 24th, 1914, C. G. Patterson urged the City to not buy 200 acres of land near the State Capitol for the purpose of a public park on the grounds that adequate open space already existed in City Creek Canyon (Salt Lake Telegram).

Photocredits:

Left Panel – Salt Lake Tribune 1903
Center Panel – University of Utah, Utah Heritage Archives
Right Panel – Kurt A. Fisher 2016

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