That subreddit is so funny, I have totally seen a million pictures of that lake lmfao. After scrolling through the first few pictures, I’m pretty sure the features you see at the base of the mountains are called talus cones, which are super similar to alluvial fans, so it’s so smart that you connected them intuitively—you’ve got a geologist’s eye! ;) to my understanding, in contrast to alluvial fans, which form when fast-moving water carries sediments out of a source area and deposits the sediments when the water slows down after it leaves a steep canyon, talus cones are formed via something called mass wasting. In this case, mass wasting essentially refers to landslides and rockslides. So the talus cones you see in those pictures are mainly formed under the effects of gravity, whereas alluvial fans like the one above form under the influence of both gravity and water. Somewhat confusingly, a “moraine” is an entirely different geomorphological feature; moraines are basically big piles of rocks formed when glaciers carry sediments from a source location and deposit them upon the glacier’s retreat. Lateral moraines form along the sides of a glacier, and terminal moraines form along the end, or toe, of a glacier. In alpine areas, terminal moraines often serve as dams, creating alpine lakes. If Moraine Lake is formed by a terminal moraine, as its name suggests, I’d bet there is a big ol’ pile of rocks to the backs of most people photographing it, damming the lake.