It's was and is limestone. How solid or whatever hasn't been discussed.
Your argument is with this story and the references for it, not me.
Significance for Geologists:
The 2002 flood at Canyon Lake and subsequent rapid formation of Canyon Lake Gorge presented a unique opportunity to study the geomorphological power of rapidly moving water and to better understand the process of canyon formation.
In their 2010 study, Michael Lamb of the California Institute of Technology and Mark Fonstad of Texas State Universitydocumented the dramatic transformation of a section of the Guadalupe River Valley landscape into a steep-walled bedrock canyon in just three days.[6]
The scientists documented the excavation of bedrock limestone to an average depth of over 20 feet and average width of 130–200 feet for a distance of over one mile.[7] The “plucking” and transport of massive boulders from the site resulted in the formation of several waterfalls, inner channels, and bedrock terraces. The abrasion of rock by sediment-loaded water sculpted walls and created plunge pools and teardrop-shaped “streamlined islands”.[8]
Although some of the geological formations present in the gorge are known to be associated with rapidly flowing flood water (such as the streamlined islands), other formations (such as the inner channels, knickpoints and terraces) have traditionally been interpreted through the “long ago and very slow” paradigm of geologic time in response to shifting climate or tectonic forcing.[6]
Typically, a steep-walled narrow gorge is inferred to represent slow persistent erosion,[6] but because many of the geological formations of Canyon Lake Gorge are virtually indistinguishable from other formations which have been attributed to long term (slower) processes, the data collected from Canyon Lake Gorge lends further credence to the hypothesis that some of the most spectacular canyons on Earth may have been carved rapidly during ancient megaflood events.[6]Additionally, because the flood conditions under which the gorge was formed are known, Canyon Lake Gorge provides a means of developing improved computer modelreconstructions of pre-historic floods to determine water volume, flood duration and erosion rates.[8]
Canyon Lake Gorge - Wikipedia