Prior to the twentieth century, geologists had deduced that the interior of the Earth had to be yielding, in order to accommodate the uplift of mountains. Some geologists assumed the interior was molten, but others acknowledged that a yielding solid would be sufficient. The clearest expression of the latter viewpoint is in the famous 1855 paper by Airy [35] in which he proposed an explanation for mountains being approximately in isostatic balance, namely that mountains have thickened crust beneath them. However, Airy also cited a key observation to support the idea of a deformable, solid interior [35]:

There had been debate about whether the interior is presently fluid or whether it had only been fluid when the Earth was forming. It was Hall [36] in 1859 who established that continuing deformation was required, through his observations that sediments throughout thick sedimentary formations had all been deposited in shallow water.