The stunning geology of the north shore is hard to ignore! So, my beach walking has led me to a rock collecting hobby, in order to learn more about the brilliant colors & textures along the shoreline. I’m interested in finding pipe vesicles, which form at the base of a lava flow. It’s kind of rare to find small rocks containing them, but I’ve been to a natural area to see them where collecting is prohibited. I would like to find a piece I can add to my coffee table collection though, so I can show visitors alongside the rock guide I set out!
There are detailed quadrangle bedrock maps produced by the Minnesota Geological Survey, so I turned to the French River & Lakewood shoreline map for an interesting area to search near me. I found some map units of interest:
- nbd: “Overall, the flows exhibit textures typical of highly fluid pahoehoe type (for example, Green, 1989), with 5–20 centimeters long, Y-shaped pipe amygdules at the flow base overlain by a 10–30 centimeter massive zone that gives way upward to a zone of thin vesicle cylinders.”
- nba: “Pipe vesicles locally at flow bases; upper parts of flows contain abundant stretched amygdules of laumontite, as well as quartz, calcite, and chlorite.”
I love the descriptions of the rock types in these bedrock maps, but I wanted to go beyond a static map. So, I matched up the static map to the current shape file map labels. To do so, I looked at the online map! (If you want to make your own map, the quadrangles shape file will be useful.) Thus, I was able to make this dictionary for my map units of interest.
Key between old geologic map and published shape file
- nbd/nlb = Mnd
- nba = Mna
- nbo/nsb = Mno
Stay tuned for part 2: sub-setting the shape file and getting it onto your phone for navigation!
Literature Cited
Green 1989, Physical volcanology of mid-Proterozoic plateau lavas—the Keweenawan
North Shore Volcanic Group, Minnesota: Geological Society of America, Bulletin,
v. 101, p. 486–500.