Pillow lavas are often considered important when trying to decipher old rock sequences because they indicate the presence of water. However, you have to be very careful to make sure that you are not looking at regular old pahoehoe toes, which of course, indicate dry land. Many of the features that supposedly can be used to tell the difference between the two don't always work. The only absolute way to know that you are looking at true pillow lavas is to find water-lain sediments between the individual pillows. Actually, when lava is flowing under water, there is a lot of sediment generated as pieces of the lava fall off during the rapid collapsing of the pillows as the quickly chill.
Pillow lavas are essentially the underwater equivalent of pahoehoe.
They form from low effusion-rate eruptions of fluid basalt lava. Nevertheless, it is often difficult to tell the two types of lava apart in exposures.