I just sent the following to a rockhound who will be out here next week. It’s about the different rock deposits at the South Larremore ranch, and I thought it might be of interest:
On the Larremore it looks to me as if there are alluvial deposits that came from the area south of the ranch as well as from the north. These deposits from the south are typified by a surface layer of dark brown and white rock: the white being small, angular pieces of calciferous mud or not-quite-limestone, and the brown being a variety of sedimentary rocks (and flint and agate) with a desert varnish. Many of these deposits stick up above the surrounding soil by anywhere from a few inches to several feet, and at the edges show evidence of going down several feet into the soil. Found in these piles are petrified wood (identified by the presence of bark), flint and/or chert, and a variety of nodules similar to those that appear near Needle Peak. In general, these nodules are oval, flat on the top and bottom, with an outside shell of flint/chert/agate and an inside of calciferous mud often hiding fossils. These guys are weird but distinctive. Trey Woodward called them “gargoyles”, but I call them Terlingua Nodules, because they’re found all over the Terlingua area. This is the farthest north that I’ve seen them in any quantity. Interestingly enough, there’s often agate pieces in these deposits, as well: lots of white/blue/grey chalcedony ones, and some pink and red, including red plume, and lots of the yellow/brown moss that’s all over the Big Bend. I have found saginitic agate there, but not pompom as of yet. All of the saginitic agate has been in the red colors, or in the chalcedony nodules.
This is distinctly different from the creek bed material that came from up North via Calamity Creek and Butcherknife Draw. The rock piles from Calamity are at the bottom of the creek bed, generally, and are probably 6 –15 feet below the level of the soil and the deposits mentioned in the previous paragraph. In many places, you can see layers of creek rock in the sides of the draws, down near the bottom of them. These layers are between 1 and 2 feet thick, and the rest of the wall of the draw is soil. These rocks are almost all water-worn, and a mixture of sedimentary and igneous rocks. But in these deposits, there isn’t the calciferous mud or the desert varnished rocks that are in the surface layers, and the rock is just generally different looking, more water-worn and some covered with calciferous mud, but usually not stained. The agate here is often oxidized completely white, but usually you can see a hint of what the color will be inside the stone. There are also very few fossiliferous pieces, and the ones I’ve seen are oyster shell impressions in mudstone. To me, the uneducated observer, these rocks appear to have been deposited long before the other piles appeared, both because of their depth in the soil and because they don’t show the amount of desert varnish that often coats rocks that have been on the surface for a long time.
The creek bed is, of course, a location where these two deposits occasionally overlap. But by and large, each deposit is unique.