|
|
By Edwin D. McKee, Park Naturalist IN MANY of the sedimentary formations of the Grand Canyon are found peculiar worm-like ridges of varying shapes and sizes whose origins have been a matter of considerable speculation. Some of them are known locally as "worms". Geologists call them "fucoides" because of their resemblance in shape to certain seaweeds, especially those of the family Fucaceae. As a matter of fact some of the so-called fucoides may have been formed from the impressions of such plants, but many of them very definitely had other origins. The term now serves admirably for a "loop-hole" in referring to any structure of that general nature regardless of its origin. Some of the most interesting but least understood of the fucoides found in Grand Canyon come from the bright red shales of the Dox formation (Algonkian age).
As with most fucoides they represent the natural casts of depressions occuring on the old mud surfaces. They average about one inch in length, taper to a point at each end and curve, often in two directions. They are found associated with the casts of ripple marks, the square moulds of salt crystals and the casts of shrinkage cracks in mud. Because of the possibility that they may be of organic origin and because of the remarkable scarcity of fossil plant life together with the entire lack of traces of animal life in rocks of Algonkian age, they are of especial interest. Possibly, however, they formed from the impressions of some mineral crystals. Our commonest mineral, ice, is known to leave such impressions (though not curving) and certain fucoides found in the Portage shale of New York state are believed* to have been formed in that manner. Further investigation may disclose some valuable information regarding these peculiar and somewhat similar structures of the ancient Algonkian age.
Among the green shales on the lower part of the Tonto Platform of Grand Canyon are found some very large and spectacular fucoides. They vary extremely in size and shape but for the most part are about as large as a person's finger and are curving and bending. Dr. Charles Walcott of the Smithsonian Institution, who collected many of these from the Chuar Valley of Grand Canyon, considered** that they were, for the most part, the natural casts of annelid worm trails. A few varieties which occurred in pairs, however, he attributed to burrows made by Trilobites. Dr. Walcott presents a very interesting word picture of the ancient Cambrian beach on which these were formed -- where the Trilobites pushed their way through the soft sand seeking their prey, the annelid worms. He also points out the great variations in the tracks of the same animal caused by creeping, running, or wallowing in sediments of varying material, hardness and consistency, and he gives some interesting comparisons with the tracks of modern creatures. Most of the fucoides recently collected from the Tapeats sandstone of Grand Canyon suggest by their character that Dr. Walcott interpreted them correctly. Those attributed to Trilobites occasionally even show delicate, nearly parallel ridges crossing both fucoides in the pair and representing the depressions of the animal's appendages. Those of the annelid worm type often occur in great masses - individuals overlapping one another and branching. Their sides are smooth and their widths are remarkably uniform over distances of six inches and more -- good evidence that they were formed as burrows.
Other interesting fucoides are found abundantly in the Muav limestone. There they are small and often rod-like. They vary extremely in size, shape and character, however, and deserve to be given careful consideration and study. At present it seems unsafe to venture any conclusions concerning their origin. Some of the most peculiar fucoides or fucoid-like structures found in the Grand Canyon occur on the inclined laminae of the Coconino sandstone near the uppermost fossil footprint horizon in Hermit Basin. The individuals found here range in length between a few inches and nearly a foot. In width they average about one-half inch, but they vary considerably and often taper. A few of them appear to lie on top of others but they do not branch. Nearly all have a lumpy appearance. Several dozen of this type occur in a very limited area -- perhaps not more than ten square yards. Regarding the origin of the fucoid-like structures in the Coconino sandstone, worm borings are disproved by specimens which branch but never pass through each other, by tapering ends and by the lack of any individuals cutting through the inclined laminae. Inorganic materials such as mud curls or balls resulting from shrinkage due to sun's heat seem unsatisfactory in explanation since the Coconino is so free from clay and other fine clastics. Plant origin also seems unlikely because of the general lumpy appearance and because of the probable conditions under which they were formed. Coprolites (fossil excrement), on the other hand, seem to satisfactorily explain the peculiar shapes and to be very logically expected considering the association with the abundant footprints of vertebrate animals. In certain sandy phases of the Kaibab limestone are found very long, branching forms of fucoides with diameters of half an inch or less. Because of their shape, and tendency to taper they are considered to have probably originated from the impressions of seaweeds.
In this short article a complete survey and description of Grand Canyon fucoides is not attempted; nor have conclusions been reached in every case concerning, the origin or significance of those described. Nevertheless, because of the puzzling nature and the general interest of the structures to all observers -- laymen and scientists alike -- it is thought worthwhile to briefly state a few of the problems and to give the conclusions reached by some detailed studies made by the writer. | ||||||
| <<< Previous | > Cover < | Next >>> |
vol7-8d.htm
14-Oct-2011
