Essex 158 (Critchley 50, Leach 4-37) and 59 for 0 trail Somerset 469 (Thomas 110, Nicholls 96, Rew 74*, Abell 65, Cook 4-92) by 252 runs
The prolific young wicketkeeper-batter finished unbeaten on 74 having helped his side extend their first innings total from an overnight 348 for five to 469 all out on a pitch offering assistance to seam and spin. Sam Cook claimed four for 92.
Following on 311 behind, the visitors made a better fist of things and closed on 59 without loss, Dean Elgar (30) and Walter (29) having reduced the deficit to 252.
Essex began the day by bowling better than on the first morning, Cook removing nightwatchman Jake Ball and Leach, while Jamie Porter and Shane Snater sent back Archie Vaughan and Gregory respectively as Somerset fell short of a fourth batting bonus point.
At 397 for nine, the hosts had added just 49 for the loss of four wickets. But Migael Pretorius then joined Rew in a last-wicket stand of 72 that frustrated the Essex attack and led to a delayed lunch.
Rew, unbeaten on 21 overnight, blossomed from a testing start and gradually found his timing. An edged four to third man took him to 40 and the 4,000-run milestone, which has included 12 centuries.
His fifty occupied 89 balls and featured 8 fours. Pretorius was positive from the outset of his innings and by the time he fell leg before for 48, playing no shot to a Critchley googly, Somerset had reestablished a position of considerable strength.
Their grip tightened when Gregory began the afternoon session by pinning Elgar lbw and having Tom Westley caught behind chasing a wide one in the first over of the Essex innings. When he followed up by bowling Charlie Allison with the second ball of his second over, the visitors were two for three.
Critchley counter-attacked with some flowing shots as Somerset’s bowlers began to err in line and length. He and Walter brought up a half-century stand in just 52 balls and had taken the total to 76 when the ball went out of shape and was replaced.
Almost immediately, Walter, on 34, was dropped at fine leg by Lewis Goldsworthy, having top-edged a pull shot off Ball. It looked like proving an expensive mistake, but with the score advanced to 104, Walter played another poor shot and clipped a good length delivery from Ball straight to Leach at mid-wicket.
Critchley had swung Leach for six over deep square, but the wily left-arm spinner responded by trapping Noah Thain lbw on the back foot for ten and at tea Essex were 129 for five. Off-spinner Vaughan then struck with the fourth ball after the interval, gaining another leg before verdict to dismiss Critchley, who had impressed for 88 balls, sweeping.
Leach quickly added Simon Harmer to the list of batting casualties and when Snater tried to attack he edged the former England spinner through to wicketkeeper Rew. It was 146 for nine when Leach bowled Cook for a duck and Sam Pepper was last man out for a fighting 35, caught at mid-off to give Vaughan a second wicket.
Somerset unsurprisingly enforced the follow-on, despite Gregory being unable to bowl after falling on the ball while fielding and damaging a hip. Elgar, on 12, survived a caught and bowled chance to Ball, but was a rare moment of alarm as he and Walter saw out the 19 overs to the close.
