Abstract Cambrian strata in Wisconsin compose a sheet of mostly marine sandstone, with minor dolomite, deposited during the fluctuating advance of the North American epeiric sea. Sedimentary features and fossils indicate that deposition took place in both shallower, current-dominated regimes and deeper quiet-water settings swept by episodic storm surges. The sand sheet surrounds inliers of Precambrian rocks in the Baraboo area. The Baraboo inliers are remnants of an elliptical ring of islands in a subtropical shallow sea, which were gradually buried by Cambrian and Ordovician sediments. Spectacular conglomerates composed of red quartzite clasts accumulated around the islands, which were pounded repeatedly by waves that we presume to have been generated by tropical storms. Paleomagnetic evidence places Cambrian Wisconsin in the southern tropics. Boulders up to 1.5 m in diameter are well rounded whereas larger ones (up to 8 m) are not. This suggests the possibility of estimating the magnitude of the Cambrian storm waves using knowledge from modern oceanography and from wave trough experiments by coastal engineers. Such analysis suggests waves necessary to tumble quartzite boulders 1.5 m in diameter were of the order of 7–8 m high at their point of breaking. Such magnitudes are not uncommon today during storms on many modern rocky coasts.