Geologic wonders uncovered

There's a place in Big Cottonwood Canyon where you can watch thetides come and go -- a billion years in the past.

Half a mile up City Creek Canyon from 11th Avenue, a concretebarrier braces the toe of an active landslide.

A massive limestone ridge is peppered with oceanic fossils, only5.5 miles above the fee station near the mouth of Mill Creek Canyon.The fossils include shells and bits of coral from an ocean thatwashed Utah during the Triassic period, 248 million to 206 millionyears ago. At that time, on land, the earliest dinosaurs appeared.

These are only three of scores of geological features in the SaltLake City area canyons, each with its own story. They are pinpointedand described in a fascinating new

booklet by the Utah Geological Survey, "Geologic Guide to theCentral Wasatch Front Canyons: Salt Lake County, Utah." It'savailable for $3.95 at the Utah Department of Natural Resources Map &Bookstore, 1594 W. North Temple.

The 30-page booklet, with color photos and foldout maps, also canbe ordered through the Internet, http://mapstore.utah.gov, or bycontacting the store at 801-537-3320.

Accounts of roadside geology, with miles indicated from landmarks,cover the vehicle-accessible canyons: City Creek, Emigration,Parleys, Mill Creek, Big Cottonwood and Little Cottonwood.

"We'd actually been getting a lot of requests for such acomprehensive guide to the local canyons," said Sandy Eldredge, whoworked on the project. "And up until now that has not been availablein any form."

The project began about three years ago, with UGS geologistsWilliam F. Case handling Emigration, Parleys and Mill Creek; Mark R.Milligan, City Creek; Christine Wilkerson, Little Cottonwood; andEldredge, Big Cottonwood.

"We each picked the canyons to do the field work and mileage andexplain the geology," Eldredge said.

They took photos and researched topics of interest. Besidesabundant geology references, the booklet points out some historicsites like a camping spot of the Donner Party in Emigration Canyon,the route of the pioneers of 1847, and drill bit marks for the quarryin Little Cottonwood where granite was mined for the Salt Lake LDSTemple.

But the focus is geology. These include the basics, she said, like"glaciers in the mountains, Lake Bonneville in the valley, theWasatch Fault." The booklet also discusses earthquake faulting,periods of Earth history, comings and goings of oceans and rivers andLake Bonneville, and varieties of rocks.

Some of the details are astonishing, like the Precambrian rock inBig Cottonwood Canyon. It is an exceptionally old formation, datingto between 800 million and 1 billion years ago. The canyon isdominated by Precambrian rock for its first six miles.

The booklet points out places where ripples of waves on ancientbeaches are still visible (1.4 miles above Wasatch Boulevard) and mudcracks are preserved in this ancient formation (2.3 miles).

"You still have preserved ripple marks and mud cracks," Eldredgesaid. "It shows the daily and semi-daily ocean tides."

The Big Cottonwood Formation "tidal rhythmite" rocks are of worldimportance, with researchers studying them to learn about changes inthe length of the day and the distance to the moon.

A close-up of the rock and information about it were provided byMarjorie A. Chan, professor and chairwoman of the University ofUtah's department of geology and geophysics.

"In the Big Cottonwood Formation, which is right up Big CottonwoodCanyon, there are some shales and sandstones that everybody drivespast on their way to the ski resorts," Chan told the Deseret MorningNews. "Some of these have very fine rock layering."

Black and white layers are marks left by the twice-daily high andlow tides. "It's so fine, it's sort of like counting tree rings,"Chan said. The dark material is silt and clay while the white is sandfrom the ancient beach.

According to the guidebook, sand was carried in by peak flows,higher tides, while the silt and clay came with weaker waters atchanging tides.

Thick layers indicate where the tides were especially high, whichhappens when the Earth, moon and sun line up. When the deposits arethin, the three spheres were least aligned.

Counting tide layers and using statistics, scientists showed thatEarth, sun and moon were in the same orientation in the Precambrianas they are now. But some things were different.

Knowing the celestial geometry and calculating how many dayspassed between exceptionally high and low tides, scientists madesurprising discoveries.

"You can actually calculate how many days were in a year and youcan estimate how fast the Earth was rotating back in thePrecambrian," she said. The year was closer to 400 days, "and theEarth was spinning faster."

Days were only about 18 hours long. In the eons since the layerswere preserved, Chan said, "the tides have actually probably sloweddown the rotation of the Earth."

E-mail: bau@desnews.com