Lias refers to the end of the Early Jurassic period. The Jurassic period consisted of the Early Jurassic Period, the Middle Jurassic Period, and the Late Jurassic Period. The Early Jurassic Period consisted of Hettanigan stage, the Sinemurian Stage, the Plienbachian stage, and the Toarcian stage. The last two stages of the Early Jurassic Period comprise the Lias era. The geologists are of the opinion that the period existed between 195,000,000 and 185,000,000 years.

Most of the UK especially the areas around the Yorkshire was either a lagoon or a delta scorched under the tropical heat of the sun, during those times. It is believed that the rocks in that period were sedimentary in nature, arranged in the form of layers of mudstones and sandstones.

It is believed that the word “lias” has been originated from the word “layer.” The major aspect of this archaeological period was the sediments which are still seen to be maintained in distinct layers.