the park area is characterized by collision tectonics: over the past millions of years, exotic terranes in the pacific ocean have been moving toward the north american landmass and accreting, or attaching, to the area that now makes up alaska. the oldest rocks in the park are part of the yukon-tanana terrane. they originated from ocean sediments deposited between 400 million and 1 billion years ago. the original rocks have been affected by the processes of regional metamorphism, folding, and faulting to form rocks such as schist, quartzite, phyllite, slate, marble, and limestone. the next oldest group of rocks is the farewell terrane. it is composed of rocks from the paleozoic era (250-500 million years old). the sediments that make up these rocks were deposited in a variety of marine environments, ranging from deep ocean basins to continental shelf areas. the abundant marine fossils are evidence that around 380 million years ago, this area had a warm, tropical climate. the pingston, mckinley, and chulitna terranes are the next oldest; they were deposited in the mesozoic era. the rock types include marble, chert, limestone, shale, and sandstone. there are intrusions of igneous rocks, such as gabbro, diabase, and diorite. special features include pillow basalts, which are formed when molten lava flows into water and a hard outer crust forms, making a puffy, pillow shaped feature; as well as an ophiolite sequence, which is a distinct sequence of rocks indicating that a section of oceanic crust has been uplifted and thrust onto a continental area.