On my first day in Montana this summer, I borrowed Lily's Jeep and set off to look for petrified wood in the Tom Miner Basin, an offshoot of the Paradise Valley (which connects Livingston to Gardiner). Along the way, though, I stopped at Point of Rocks CKCK, and found some nice exposures of the lahar deposits of the Absaroka/Gallatin Volcanic Field. These Eocene-aged extrusions basically consist of a series of lava flows and volcaniclastics interlayered to a substantial thickness.

Here's a map of Point of Rocks:


Here's the view north from Point of Rocks:


Here's some images of the rocks exposed there: poorly-sorted, matrix-supported grey conglomerates that I interpret on the basis of the previous year's field notes to be lahar deposits:




I've got a big fat chunk of this stuff in the NOVA geology lab now -- thanks to Lily for giving that forty pounds of lahar a lift cross-country!

Eventually I made it into the Tom Miner Basin, an area of Forest Service land where there is some petrified wood exposed. There is an interpretive trail where people are specifically asked NOT to collect but of course people collect anyhow, so it's kind of lame, but there are some nice examples of petrified branches and what not, and some nice examples of reverse-graded-bedding in the lahar deposits.

Map of the area where the road ends (at a campsite) and the trail begins:


Reverse graded bedding:


You can climb up above the trail to some exposures of the volcanics which are harder to get to and therefore not picked-over, and with a permit you can collect a fist-sized chunk per person per year.

Here's a couple examples of petrified wood that I saw:




More volcaniclastics, this time showing normal graded bedding (coarse at the bottom, fine at the top):


And on the way out, I saw a nice example of a couple of rugose corals cross-sectioned in a boulder of presumably-Mississippian-aged Madison Group limestone:



It was a nice first day in Montana! More photos to come...

Labels: fossils, igneous, montana, plants, volcano