I pondered whether to use the term "alluvial fan", suspecting I might be going beyond my competency, and thank you for confirming that I chose incorrectly...;-)
Not everyone can get away with the phrase "In my professional opinion", but as always what followed was enlightening. One thing I came away with is affirmation that a planetary surface is complex and formed by multiple interacting processes. I wasn't thinking of how lava tubes would form where prior local features such as fault lines exist, but that's the thing: Whatever we see is the result of a long history of interactions. The rock is here, and not there, because of something that happened in its history.
Michael is thinking in terms of a single phenomenon responsible for everything, presumably at a single point in time, and he wasn't considering the "narrative". Geologists look for the story told by the landscape, and it seems as though narrow objections such as his tend to evaporate when you consider the full story.
Thanks once again, Nick.
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re: "alluvial" fan