Category: "Ground ice & patterns"

Raindrop Hillocks and Ground Ice

February 16th, 2020

Ever see these small centimeter-scale hillocks in dirt or sand, usually topped by a small pebble or twig? To me, they look like a miniature mountain landscape. A brief reflection on their appearance suggests erosion by raindrops: The drops fall down on and near the larger grain (e.g., pebble), pushing the smaller grains down, leaving the larger ones to stick up above. In this way, a scene of tiny hillocks emerge. I call them "raindrop hillocks".


Raindrop Hillocks and Ground Ice

Click on any image to enlarge it.


Soft, easily compactable soil seems necessary to their formation. Just toss some loose dirt into a pile, then come back after a heavy rain and you are likely to see something similar. But further reflection may present some difficulties. For example, some hillocks occur where the soil should not be loose. The above image presents one such case: here the soil was on a well used trail where the soil had long been compacted. Clearly such soil could not be so easily carved by tiny raindrops. The case pictured below is on a level sandbar after a river shifted course. Where did the sand go that was up near the peaks? It seems that the sand must have originally been very loose. How could this be?
Raindrop Hillocks and Ground Ice

Read more »