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Llano and Burnet counties have extensive outcrops of granite, which
may be why Isoetes lithophyllum is found there exclusively. However,
limestone is common in Texas, as the area was a shallow sea several
hundred million years ago. Marine fossils are everywhere.
The type of granite found there is pink, which is due to potassium
feldspar. Pools formed by rainfall would be very soft, while
spring-fed pools would be rather hard.
Rainfall is best described as erratic, with no distinct wet or dry
season. Thus, "vernal" pools can be formed any time of the year, and
we have a 300 day growing season. We have just emerged from a severe
drought, and I am sure most pools and springs with Isoetes dried up.
Droughts occur about every 5-6 years.