Mortars are Starfleet infantry support weapon used in TOS (and possibly later). They are designed to fire photon grenades.
Spock reads "living creatures (...) azimuth 93 degrees, range 1570 yards". 1570 yards is 1435.6 meters. Soon, disruptor bombardment begins (althought it sounds and appears like mortar shells). However, before firing, Kirk asks tactical officer, Kelowitz, where he thinks Gorn are.
Kirk: "An evaluation, Mr. Kelowitz. Where do you think they are?"
Kelowitz: "If I were them, I'd go to the high ground on the right. I'd make it 1,200 yards, azimuth 87. It's pretty close for one of these little jewels, Captain."
Kirk: "It'll be a lot closer to them. Stand clear."
1200 yards is 1097 meters, and from that, and Kelowitz' comment, we can conclude that mortar is designed to operate on far greater distances, probably 5000 to 10000 yards (4572 to 9144 m). However, we see heavy smoke when it fires, so it probably operates similar to modern mortars. Positioning of mortar itself, which is positioned at ~70 degrees, argues that this range is not maximum. Short-range artillery has longest range at 45 degree elevation, while long-range artillery has highest range at 55 degree elevation, owing to lessening of air resistance in upper parts of the atmosphere.
Dust is being knocked up even behind Kirk & co. So, we can take this calculator and, assuming 0.1 to 1 psi, get yield from 5 to 800 tons of TNT.
It seems to be high-yield long-range lightweight infantry support weapon for both direct and indirect fire support (and it does not require spotter, since tricoder can easily locate targets).
