Ice expansion ramparts 2 mi S. of Plover Point on South Arm of Yellowstone Lake were formed by expansive force of lake ice during intense thaws. Width of the Arm (1 1/2 mi) is within limits necessary for ice to maintain competence for shoreward thrusting. Wind-induced ice thrust forces are unlikely here, but were observed May 20, 1960, forming ramparts on windward northern shores of the lake which were exposed to optimum fetch. The South Arm ramparts are sinuous in plan and located both above normal summer strand lines and in drowned positions as much as 60 ft out from them. The latter stand 6 ft above rock-veneered bottoms and formed during winter low-water stage. Simplicity and singularity of the ramparts suggests that conditions for their formation have not often occurred, Following Hobbs, an expectable genetic sequence is: 1) Freezing of lake ice; continued chilling with concomitant formation of tension cracks; widely fluctuating temperatures. 2) Water freezes in cracks increasing potential ice volume; 3) Sudden marked warming is communicated to bare ice expanding it to new and greater volume; ice thrusts onto shore if competent.