Volcanic episodes happened frequently in the state at this time. Volcanism continued into the Jurassic as Alaska experienced a period of relative geologic upheaval. Areas of the state remained inundated by the sea. This sea was home to ammonites and crinoids. In the middle Jurassic most of the mountain ranges characterizing modern Alaska began to form. Alaska's Middle Jurassic Callovian deposits are part of a large geologic region spreading down through Canada and even into the Lower 48 states including Montana, Idaho, North Dakota, Utah and New Mexico. From the mid to late Jurassic, the area now occupied by Snug Harbor was home to a great diversity of marine invertebrates, which left behind a plethora of fossils. Among these were ammonites. Others include belemnites, the gastropod "Amberlya", the pelecypods "Lima", "Oxytoma", and possibly "Astarte" and "Isocyprina". Cretaceous Alaska gained additional landmass due to collisions with other tectonic plates. Local mountain building resulted in the formation of the Brooks Range and other topographic features. Some areas of Alaska were covered by the sea and others were dry land. There were at least 5 species of "Inoceramus" in Alaska during the Cretaceous period. This was a widespread genus in Alaska and its fossil remains have been discovered in hundreds of different places. Other Cretaceous shellfish were preserved at what is now Umiat Mountain. More than 235 species of plants are known to have grown in Alaska during the Cretaceous, most of which were cycads. Their remains are scattered across hundreds of sites. Among the finds were algae, "Ampelopsis", conifers, elm, "Ficus", a great diversity of hepaticae, laurel, magnolia, oaks, "Pinus", "Platanus", and sequoias. Invertebrate remains were also found with the plants. Pieces of Cretaceous amber have been found on the shore of Nelson Island, which is located in the Bering Sea. Dinosaurs lived in Alaska during the Cretaceous. Alaska remained tectonically active into the Cenozoic era. Volcanism produced the Aleutian Islands. During the Eocene, Alaska's plants resembled those today growing in the temperate, subtropical and tropical regions of earth today. Their remains were preserved in locations such as the Alaska Peninsula, Awik, the Cook Inlet's shoreline, Eagle City, Unga Island.