Scientists employ a number of tools and concepts to study paleoclimate (past climates and climate changes). These tools include sediment cores, ocean cores, ice cores, geologic features and stratigraphy, cave deposits (speleothems), tree rings (dendrochronology), and fossil coral. Each of these different tools contains useful proxies, which provide clues about different aspects of past climates. Geologists analyze stratigraphic layers in sedimentary rock, with each layer representing millions to hundreds of millions of years. Similarly, lacustrine deposits are sedimentary rock formations which formed in the bottom of ancient lake. Lake sediments are a very good indicator for the changes in climate, as the climate is responsible for the deposition of sediment in lakes.