Allerlei - The Rifle Meteorite Alleged Impact Site

The 'Impact' Site.

A large meteorite was found near Rifle, Colorado in 1948. At one time it was thought to be a fragment from the Barringer Meteor (crater is near Winslow, Arizona) but this has since been proven otherwise. Above the find of the Rifle Meteorite lies a rugged hillside with a barren scree slope feature. Local information indicated that this was the impact site of the Rifle Meteorite. Various studies were done to assess that claim. One such study was undertaken by myself and another amateur astronomer. The results were included in a geology field guide to be used on a southwest field trip. The following writeup is not the one included in the field guide. That writeup was a bit more professional.

Here is the laid back travel log version:

STUDY OF THE RIFLE 'IMPACT SITE'

By Jim Fuchs and Dave Miller

The Rifle Impact Site has been so named because of a theory that the area has been formed by a meteor impacting the steep hillside.

In the late 1980's the Rifle Impact Site had come to the attention of Dave Miller, a Grand Junction amateur astronomer:

"There was a book in the Mesa County Library I looked at probably 10-12 years ago (I think it has since grown legs. It was coded as missing when you and I first talked about the site). I think the book was written in the 50's or 60's. It listed known impact sites in the States. I looked up Colorado and Rifle was listed. The text said part of the meteorite was in Denver at the Museum and part had been sent to a New York museum. I don't recall anything questioning the authenticity of it.

The second occurrence was around 1990 on one of my business trips to Craig. I stopped in McDonald's in Rifle to get coffee and picked up a brochure on Rifle. It was one of those cartoon-type Chamber of Commerce maps showing touristy things in the area. On it was the meteor impact site.

I went to the Chamber of Commerce but no one there knew anything about the site. They sent me to the museum where a lady on duty recalled something about the meteor story but that was it. She referred me to the Forest Service office on the West Rifle bypass highway.

I stopped in their office and a woman pointed out the site from the front door. On a later trip to Craig I took my 35 mm camera and a telephoto lens. I took several photos of the site from the west side of Rifle."

Dave asked Jim Fuchs to join him in researching the history of the impact site and possibly follow up with some field work. We had both been to the Barringer Meteorite Crater and the idea of an impact site almost in our back yard was exciting.

A visit to the Rifle library gave us scant information but we did notice the barren site in an old photo which hung in the Rifle visitor center off of Interstate 70. The attendant at the visitor center was unable to give us any information abut the photo but he related that a fragment of the Rifle Meteorite was supposed to be in the Denver Museum of Natural History (now called the Museum of Nature and Science). Jim contacted museum geologist Jack Murphy and asked what he knew about it. Jack said that there had been a little research already done at the site but would welcome more data.

In November, 1999 Dave and Jim finally were able to hike up to the site, which is on BLM land but was formerly part of the Naval Oil Shale Reserve. At that time of year the best approach for us seemed to be from the bottom up, hiking through Yellow Slide Gulch, northwest of the town of Rifle.

The approach to the gulch took us through sagebrush/pinyon flats and steeper terrain. The walk up Yellow Slide Gulch was easy, however we had difficulty maintaining our orientation relative to the 'impact site' since the view from the deeply entrenched streambed was very limited. We planned on walking up an unnamed small side drainage which entered Yellow Slide Gulch near the west 1/2 of section 31, T.5 S., R.93 W. Unfortunately we did not find it and instead went farther up Yellow Slide Gulch until we were forced to scramble up an extremely steep hillside.

Using tree trunks and limbs to pull ourselves up we finally topped out in thick growth of oakbrush and pinyon pine. Unable to see over or through the scrub oak we wandered aimlessly uphill through the thickets for a half hour. When Dave stopped to check compass bearings he looked down and spotted a U.S.G.S. survey marker cemented into a large rock. Looking at our map we realized we were just downhill from the site. Quite tired, we finally made it the 'impact site', however, the delay in reaching the site combined with the short days of November left us with little time.

With shovels, trowels, and magnets in hand we began our investigation. Jim's experience as a soil scientist for the Soil Conservation Service (now Natural Resource Conservation Service) enabled him to describe the soil profile. We also collected samples to send off to the Denver Museum for further analysis.

Inspection of Rifle 'Impact Site'

Location: Rifle 'Impact Site', SW of Yellow Slide Gulch in the SW 1/4 of the NE 1/4 of Section 36, T.5 S., R. 94 W., Anvil Point Quadrangle; UTM 13S 251980mE 4384340mN. The site is an oval-shaped barren side slope, south facing, about 750 feet by 300 feet in size.

Soils: The site itself has very shallow soil and is mostly fragmented buff colored breccia-like shale and sandstone:
0-1 inch: very skeletal loamy sand
1-15 inches: breccia-like sandstone and shale; often cemented by silica and/or calcium carbonate. Sand and loamy sand is interstitial to these rocks.
15+ inches: fractured sandy shale.

The area surrounding the site has relatively deep soil with surface rocks of primarily buff colored sandstone intermixed with some shale.

A few darker colored rock fragments were found on the lower slope of the site itself and were collected for further analysis.

Paramagnetic Properties: Materials only slightly attracted to a strong magnet are said to be paramagnetic. None of the sampled material deflected a compass needle but some particles were attracted to a strong magnet - they do not however seem to be magnetic themselves. These particles are buff colored only. Fragments of the darker rocks found on the lower slope of the site were not attracted by a magnet.

The paramagnetic particles were found on the surface and to at least 12 inches deep. As we left the site and descended down the slope we continued to find more shale and sandstone fragments that were paramagnetic. These were found at least 2 miles away but were not as abundant as at the site itself. At 3 miles from the site no paramagnetic particles were found.

Vegetation: Lack of soil and the dry southern exposure makes it difficult for plants to establish themselves at the site. A few very scattered forbs were found in the site as well as one small pinyon tree which seemed to be about 10-15 years old (it was about 15 inches tall). Soil outside of the site is a skeletal sandy loam and does support various grasses, forbs, pinyon, and gambel oak trees.

Conclusions: There is nothing obvious enough for us to say that this is a meteor impact site. From a soils point of view it would seem that the parent material at the site may be an inclusion in the local geology. It is noted that the breccia-like material at the site was also found up to 1/2 mile away although it was much less abundant and was buried a few inches deeper under the overlying soil.

Paramagnetic rock fragments were found up to 2 miles away near the surface and also in anthill mounds. It should be noted that when we tested particles for paramagnetic properties the 2 mile distance was primarily down the slope from the site. We were in a hurry to beat sundown so we did not stay in the same geologic rock layer but instead headed down to Yellow Slide Gulch.

Lack of soil certainly makes any natural revegetation almost impossible at this time. Where did the soil go? If the site is a remnant of a rock outcrop then soil may never have had much of a chance to form yet. Wind erosion and possibly a slow rock creep down the hill could also be factors keeping vegetation from growing.

Final Notes

In March 2000 Jim Fuchs researched the "Soil Survey of the Rifle Area," issued by the Soil Conservation Service, USDA, May 1985. The delineation of the study site area consists of a soil series named Irigul:
Description of the Irigul soil is a surface layer of 6 inches thick channery loam; underlain by 11 inches of extremely channery loam; with hard fractured sandstone at a depth of 17 inches. Irigul is found on 50-70% slopes. Inclusions that can be found in Irigul are rock outcrops and soils that are deeper. Channery soil, as defined by the Soil Survey, is a soil with greater than 15% thin, flat fragments of sandstone, shale, slate, limestone, or schist with a maximum length of 6 inches along the longest axis.

Time did not allow for detailed transects away from the site as we were in a rush to get back to our vehicle before nightfall. The paramagnetic rock fragments we did find were located on steep as well as almost flat terrain. They were found in the Irigul soil, in Ildefonso stony loam (usually derived from basalt alluvium), and also in soils which were partially derived from Mesa Verde Sandstone and Wasatch Shale and which were relatively unweathered.

We are not convinced that this is a meteor impact site. Further analysis would be beneficial. For instance, finding out exactly what is cementing the breccia; taking a heavy spud bar up to the site and trying to go deeper; doing some plots on the paramagnetic properties of the local rock fragments; finding out what metals are contributing to the paramagnetic properties; checking downslope from the site and into the unnamed side gully to look for meteor fragments; etc. It would be interesting, though time consuming, to find out if this barren site is changing in size through aeolian deposition or erosion - what could that indicate about a time frame for the exposed site? Some other relevent questions are: was the Rifle Meteorite really found below the site and what did local papers say about the find at that time?

Often, a seeemingly unrelated discipline can shed light on another discipline's mystery. One of Jim's hobbies is pottery. The high temperatures used in firing pots can change hematite in the clay to magnetite. A reducing atmosphere (one which restricts oxygen) also plays a role in that change. The black and sometimes iridescent sheen of certain southwest pots is attributed to this transformation. Could a similar process be responsible for the paramagnetic properties of the collected rock fragments from the site?

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Geologist Jack Murphy gave a presentation about the Rifle Meteorite Impact Site at the 2002 Denver Annual Meeting of the Geological Society of America. The conclusion was that the Rifle Meteorite, a 102.7 kg (recovered weight) iron (coarse octahedrite) object, did not produce the surface feature located at the study site near Yellow Slide Gulch. This link goes to an abstract of that presentation:
GSA Confex Abstract.

For some great information on meteorites, including a catalogue of known meteorites, see this site from the Natural History Museum in London: Meteorite Info

Addendum: You may have noticed the use of meteor impact site and meteorite impact site in the forgoing material. I have seen both phrases used in books and journals. The reader may ponder which is 'correct'!

Illustrations

Map of area

Rifle area.

Jim at the site.

At the site.

Map of the area

'Impact Site' area.

breccia

Breccia in natural light.

breccia

Same breccia, reverse side, with flash illumination.

black rocks

Dark colored rocks from lower slope.

paramagnetic samples

Paramagnetic rock fragments.